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PERLFAQ4(1)	       Perl Programmers	Reference Guide		   PERLFAQ4(1)

NAME
       perlfaq4	- Data Manipulation

VERSION
       version 5.021011

DESCRIPTION
       This section of the FAQ answers questions related to manipulating
       numbers,	dates, strings,	arrays,	hashes,	and miscellaneous data issues.

Data: Numbers
   Why am I getting long decimals (eg, 19.9499999999999) instead of the
       numbers I should	be getting (eg,	19.95)?
       For the long explanation, see David Goldberg's "What Every Computer
       Scientist Should	Know About Floating-Point Arithmetic"
       (<http://web.cse.msu.edu/~cse320/Documents/FloatingPoint.pdf>).

       Internally, your	computer represents floating-point numbers in binary.
       Digital (as in powers of	two) computers cannot store all	numbers
       exactly.	Some real numbers lose precision in the	process. This is a
       problem with how	computers store	numbers	and affects all	computer
       languages, not just Perl.

       perlnumber shows	the gory details of number representations and
       conversions.

       To limit	the number of decimal places in	your numbers, you can use the
       "printf"	or "sprintf" function. See "Floating-point Arithmetic" in
       perlop for more details.

	   printf "%.2f", 10/3;

	   my $number =	sprintf	"%.2f",	10/3;

   Why is int()	broken?
       Your "int()" is most probably working just fine.	It's the numbers that
       aren't quite what you think.

       First, see the answer to	"Why am	I getting long decimals	(eg,
       19.9499999999999) instead of the	numbers	I should be getting (eg,
       19.95)?".

       For example, this

	   print int(0.6/0.2-2), "\n";

       will in most computers print 0, not 1, because even such	simple numbers
       as 0.6 and 0.2 cannot be	presented exactly by floating-point numbers.
       What you	think in the above as 'three' is really	more like
       2.9999999999999995559.

   Why isn't my	octal data interpreted correctly?
       (contributed by brian d foy)

       You're probably trying to convert a string to a number, which Perl only
       converts	as a decimal number. When Perl converts	a string to a number,
       it ignores leading spaces and zeroes, then assumes the rest of the
       digits are in base 10:

	   my $string =	'0644';

	   print $string + 0;  # prints	644

	   print $string + 44; # prints	688, certainly not octal!

       This problem usually involves one of the	Perl built-ins that has	the
       same name a Unix	command	that uses octal	numbers	as arguments on	the
       command line. In	this example, "chmod" on the command line knows	that
       its first argument is octal because that's what it does:

	   %prompt> chmod 644 file

       If you want to use the same literal digits (644)	in Perl, you have to
       tell Perl to treat them as octal	numbers	either by prefixing the	digits
       with a 0	or using "oct":

	   chmod(     0644, $filename );  # right, has leading zero
	   chmod( oct(644), $filename );  # also correct

       The problem comes in when you take your numbers from something that
       Perl thinks is a	string,	such as	a command line argument	in @ARGV:

	   chmod( $ARGV[0],	 $filename );  # wrong,	even if	"0644"

	   chmod( oct($ARGV[0]), $filename );  # correct, treat	string as octal

       You can always check the	value you're using by printing it in octal
       notation	to ensure it matches what you think it should be. Print	it in
       octal  and decimal format:

	   printf "0%o %d", $number, $number;

   Does	Perl have a round() function? What about ceil()	and floor()? Trig
       functions?
       Remember	that "int()" merely truncates toward 0.	For rounding to	a
       certain number of digits, "sprintf()" or	"printf()" is usually the
       easiest route.

	   printf("%.3f", 3.1415926535);   # prints 3.142

       The POSIX module	(part of the standard Perl distribution) implements
       "ceil()", "floor()", and	a number of other mathematical and
       trigonometric functions.

	   use POSIX;
	   my $ceil   =	ceil(3.5);   # 4
	   my $floor  =	floor(3.5);  # 3

       In 5.000	to 5.003 perls,	trigonometry was done in the Math::Complex
       module. With 5.004, the Math::Trig module (part of the standard Perl
       distribution) implements	the trigonometric functions. Internally	it
       uses the	Math::Complex module and some functions	can break out from the
       real axis into the complex plane, for example the inverse sine of 2.

       Rounding	in financial applications can have serious implications, and
       the rounding method used	should be specified precisely. In these	cases,
       it probably pays	not to trust whichever system of rounding is being
       used by Perl, but instead to implement the rounding function you	need
       yourself.

       To see why, notice how you'll still have	an issue on half-way-point
       alternation:

	   for (my $i =	0; $i <	1.01; $i += 0.05) { printf "%.1f ",$i}

	   0.0 0.1 0.1 0.2 0.2 0.2 0.3 0.3 0.4 0.4 0.5 0.5 0.6 0.7 0.7
	   0.8 0.8 0.9 0.9 1.0 1.0

       Don't blame Perl. It's the same as in C.	IEEE says we have to do	this.
       Perl numbers whose absolute values are integers under 2**31 (on 32-bit
       machines) will work pretty much like mathematical integers.  Other
       numbers are not guaranteed.

   How do I convert between numeric representations/bases/radixes?
       As always with Perl there is more than one way to do it.	Below are a
       few examples of approaches to making common conversions between number
       representations.	This is	intended to be representational	rather than
       exhaustive.

       Some of the examples later in perlfaq4 use the Bit::Vector module from
       CPAN. The reason	you might choose Bit::Vector over the perl built-in
       functions is that it works with numbers of ANY size, that it is
       optimized for speed on some operations, and for at least	some
       programmers the notation	might be familiar.

       How do I	convert	hexadecimal into decimal
	   Using perl's	built in conversion of "0x" notation:

	       my $dec = 0xDEADBEEF;

	   Using the "hex" function:

	       my $dec = hex("DEADBEEF");

	   Using "pack":

	       my $dec = unpack("N", pack("H8",	substr("0" x 8 . "DEADBEEF", -8)));

	   Using the CPAN module "Bit::Vector":

	       use Bit::Vector;
	       my $vec = Bit::Vector->new_Hex(32, "DEADBEEF");
	       my $dec = $vec->to_Dec();

       How do I	convert	from decimal to	hexadecimal
	   Using "sprintf":

	       my $hex = sprintf("%X", 3735928559); # upper case A-F
	       my $hex = sprintf("%x", 3735928559); # lower case a-f

	   Using "unpack":

	       my $hex = unpack("H*", pack("N",	3735928559));

	   Using Bit::Vector:

	       use Bit::Vector;
	       my $vec = Bit::Vector->new_Dec(32, -559038737);
	       my $hex = $vec->to_Hex();

	   And Bit::Vector supports odd	bit counts:

	       use Bit::Vector;
	       my $vec = Bit::Vector->new_Dec(33, 3735928559);
	       $vec->Resize(32); # suppress leading 0 if unwanted
	       my $hex = $vec->to_Hex();

       How do I	convert	from octal to decimal
	   Using Perl's	built in conversion of numbers with leading zeros:

	       my $dec = 033653337357; # note the leading 0!

	   Using the "oct" function:

	       my $dec = oct("33653337357");

	   Using Bit::Vector:

	       use Bit::Vector;
	       my $vec = Bit::Vector->new(32);
	       $vec->Chunk_List_Store(3, split(//, reverse "33653337357"));
	       my $dec = $vec->to_Dec();

       How do I	convert	from decimal to	octal
	   Using "sprintf":

	       my $oct = sprintf("%o", 3735928559);

	   Using Bit::Vector:

	       use Bit::Vector;
	       my $vec = Bit::Vector->new_Dec(32, -559038737);
	       my $oct = reverse join('', $vec->Chunk_List_Read(3));

       How do I	convert	from binary to decimal
	   Perl	5.6 lets you write binary numbers directly with	the "0b"
	   notation:

	       my $number = 0b10110110;

	   Using "oct":

	       my $input = "10110110";
	       my $decimal = oct( "0b$input" );

	   Using "pack"	and "ord":

	       my $decimal = ord(pack('B8', '10110110'));

	   Using "pack"	and "unpack" for larger	strings:

	       my $int = unpack("N", pack("B32",
	       substr("0" x 32 . "11110101011011011111011101111", -32)));
	       my $dec = sprintf("%d", $int);

	       # substr() is used to left-pad a	32-character string with zeros.

	   Using Bit::Vector:

	       my $vec = Bit::Vector->new_Bin(32, "11011110101011011011111011101111");
	       my $dec = $vec->to_Dec();

       How do I	convert	from decimal to	binary
	   Using "sprintf" (perl 5.6+):

	       my $bin = sprintf("%b", 3735928559);

	   Using "unpack":

	       my $bin = unpack("B*", pack("N",	3735928559));

	   Using Bit::Vector:

	       use Bit::Vector;
	       my $vec = Bit::Vector->new_Dec(32, -559038737);
	       my $bin = $vec->to_Bin();

	   The remaining transformations (e.g. hex -> oct, bin -> hex, etc.)
	   are left as an exercise to the inclined reader.

   Why doesn't & work the way I	want it	to?
       The behavior of binary arithmetic operators depends on whether they're
       used on numbers or strings. The operators treat a string	as a series of
       bits and	work with that (the string "3" is the bit pattern 00110011).
       The operators work with the binary form of a number (the	number 3 is
       treated as the bit pattern 00000011).

       So, saying "11 &	3" performs the	"and" operation	on numbers (yielding
       3). Saying "11" & "3" performs the "and"	operation on strings (yielding
       "1").

       Most problems with "&" and "|" arise because the	programmer thinks they
       have a number but really	it's a string or vice versa. To	avoid this,
       stringify the arguments explicitly (using "" or "qq()") or convert them
       to numbers explicitly (using "0+$arg"). The rest	arise because the
       programmer says:

	   if ("\020\020" & "\101\101")	{
	       # ...
	   }

       but a string consisting of two null bytes (the result of	"\020\020" &
       "\101\101") is not a false value	in Perl. You need:

	   if (	("\020\020" & "\101\101") !~ /[^\000]/)	{
	       # ...
	   }

   How do I multiply matrices?
       Use the Math::Matrix or Math::MatrixReal	modules	(available from	CPAN)
       or the PDL extension (also available from CPAN).

   How do I perform an operation on a series of	integers?
       To call a function on each element in an	array, and collect the
       results,	use:

	   my @results = map { my_func($_) } @array;

       For example:

	   my @triple =	map { 3	* $_ } @single;

       To call a function on each element of an	array, but ignore the results:

	   foreach my $iterator	(@array) {
	       some_func($iterator);
	   }

       To call a function on each integer in a (small) range, you can use:

	   my @results = map { some_func($_) } (5 .. 25);

       but you should be aware that in this form, the ".." operator creates a
       list of all integers in the range, which	can take a lot of memory for
       large ranges. However, the problem does not occur when using ".."
       within a	"for" loop, because in that case the range operator is
       optimized to iterate over the range, without creating the entire	list.
       So

	   my @results = ();
	   for my $i (5	.. 500_005) {
	       push(@results, some_func($i));
	   }

       or even

	  push(@results, some_func($_))	for 5 .. 500_005;

       will not	create an intermediate list of 500,000 integers.

   How can I output Roman numerals?
       Get the <http://www.cpan.org/modules/by-module/Roman> module.

   Why aren't my random	numbers	random?
       If you're using a version of Perl before	5.004, you must	call "srand"
       once at the start of your program to seed the random number generator.

	    BEGIN { srand() if $] < 5.004 }

       5.004 and later automatically call "srand" at the beginning. Don't call
       "srand" more than once--you make	your numbers less random, rather than
       more.

       Computers are good at being predictable and bad at being	random
       (despite	appearances caused by bugs in your programs :-). The random
       article in the "Far More	Than You Ever Wanted To	Know" collection in
       <http://www.cpan.org/misc/olddoc/FMTEYEWTK.tgz>,	courtesy of Tom
       Phoenix,	talks more about this. John von	Neumann	said, "Anyone who
       attempts	to generate random numbers by deterministic means is, of
       course, living in a state of sin."

       Perl relies on the underlying system for	the implementation of "rand"
       and "srand"; on some systems, the generated numbers are not random
       enough (especially on Windows : see
       <http://www.perlmonks.org/?node_id=803632>).  Several CPAN modules in
       the "Math" namespace implement better pseudorandom generators; see for
       example Math::Random::MT	("Mersenne Twister", fast), or
       Math::TrulyRandom (uses the imperfections in the	system's timer to
       generate	random numbers,	which is rather	slow).	More algorithms	for
       random numbers are described in "Numerical Recipes in C"	at
       <http://www.nr.com/>

   How do I get	a random number	between	X and Y?
       To get a	random number between two values, you can use the "rand()"
       built-in	to get a random	number between 0 and 1.	From there, you	shift
       that into the range that	you want.

       "rand($x)" returns a number such	that "0	<= rand($x) < $x". Thus	what
       you want	to have	perl figure out	is a random number in the range	from 0
       to the difference between your X	and Y.

       That is,	to get a number	between	10 and 15, inclusive, you want a
       random number between 0 and 5 that you can then add to 10.

	   my $number =	10 + int rand( 15-10+1 ); # ( 10,11,12,13,14, or 15 )

       Hence you derive	the following simple function to abstract that.	It
       selects a random	integer	between	the two	given integers (inclusive).
       For example: "random_int_between(50,120)".

	   sub random_int_between {
	       my($min,	$max) =	@_;
	       # Assumes that the two arguments	are integers themselves!
	       return $min if $min == $max;
	       ($min, $max) = ($max, $min)  if	$min > $max;
	       return $min + int rand(1	+ $max - $min);
	   }

Data: Dates
   How do I find the day or week of the	year?
       The day of the year is in the list returned by the "localtime"
       function. Without an argument "localtime" uses the current time.

	   my $day_of_year = (localtime)[7];

       The POSIX module	can also format	a date as the day of the year or week
       of the year.

	   use POSIX qw/strftime/;
	   my $day_of_year  = strftime "%j", localtime;
	   my $week_of_year = strftime "%W", localtime;

       To get the day of year for any date, use	POSIX's	"mktime" to get	a time
       in epoch	seconds	for the	argument to "localtime".

	   use POSIX qw/mktime strftime/;
	   my $week_of_year = strftime "%W",
	       localtime( mktime( 0, 0,	0, 18, 11, 87 )	);

       You can also use	Time::Piece, which comes with Perl and provides	a
       "localtime" that	returns	an object:

	   use Time::Piece;
	   my $day_of_year  = localtime->yday;
	   my $week_of_year = localtime->week;

       The Date::Calc module provides two functions to calculate these,	too:

	   use Date::Calc;
	   my $day_of_year  = Day_of_Year(  1987, 12, 18 );
	   my $week_of_year = Week_of_Year( 1987, 12, 18 );

   How do I find the current century or	millennium?
       Use the following simple	functions:

	   sub get_century    {
	       return int((((localtime(shift ||	time))[5] + 1999))/100);
	   }

	   sub get_millennium {
	       return 1+int((((localtime(shift || time))[5] + 1899))/1000);
	   }

       On some systems,	the POSIX module's "strftime()"	function has been
       extended	in a non-standard way to use a %C format, which	they sometimes
       claim is	the "century". It isn't, because on most such systems, this is
       only the	first two digits of the	four-digit year, and thus cannot be
       used to determine reliably the current century or millennium.

   How can I compare two dates and find	the difference?
       (contributed by brian d foy)

       You could just store all	your dates as a	number and then	subtract.
       Life isn't always that simple though.

       The Time::Piece module, which comes with	Perl, replaces localtime with
       a version that returns an object. It also overloads the comparison
       operators so you	can compare them directly:

	   use Time::Piece;
	   my $date1 = localtime( $some_time );
	   my $date2 = localtime( $some_other_time );

	   if( $date1 <	$date2 ) {
	       print "The date was in the past\n";
	   }

       You can also get	differences with a subtraction,	which returns a
       Time::Seconds object:

	   my $date_diff = $date1 - $date2;
	   print "The difference is ", $date_diff->days, " days\n";

       If you want to work with	formatted dates, the Date::Manip, Date::Calc,
       or DateTime modules can help you.

   How can I take a string and turn it into epoch seconds?
       If it's a regular enough	string that it always has the same format, you
       can split it up and pass	the parts to "timelocal" in the	standard
       Time::Local module. Otherwise, you should look into the Date::Calc,
       Date::Parse, and	Date::Manip modules from CPAN.

   How can I find the Julian Day?
       (contributed by brian d foy and Dave Cross)

       You can use the Time::Piece module, part	of the Standard	Library, which
       can convert a date/time to a Julian Day:

	   $ perl -MTime::Piece	-le 'print localtime->julian_day'
	   2455607.7959375

       Or the modified Julian Day:

	   $ perl -MTime::Piece	-le 'print localtime->mjd'
	   55607.2961226851

       Or even the day of the year (which is what some people think of as a
       Julian day):

	   $ perl -MTime::Piece	-le 'print localtime->yday'
	   45

       You can also do the same	things with the	DateTime module:

	   $ perl -MDateTime -le'print DateTime->today->jd'
	   2453401.5
	   $ perl -MDateTime -le'print DateTime->today->mjd'
	   53401
	   $ perl -MDateTime -le'print DateTime->today->doy'
	   31

       You can use the Time::JulianDay module available	on CPAN. Ensure	that
       you really want to find a Julian	day, though, as	many people have
       different ideas about Julian days (see
       <http://www.hermetic.ch/cal_stud/jdn.htm> for instance):

	   $  perl -MTime::JulianDay -le 'print	local_julian_day( time )'
	   55608

   How do I find yesterday's date?
       (contributed by brian d foy)

       To do it	correctly, you can use one of the "Date" modules since they
       work with calendars instead of times. The DateTime module makes it
       simple, and give	you the	same time of day, only the day before, despite
       daylight	saving time changes:

	   use DateTime;

	   my $yesterday = DateTime->now->subtract( days => 1 );

	   print "Yesterday was	$yesterday\n";

       You can also use	the Date::Calc module using its	"Today_and_Now"
       function.

	   use Date::Calc qw( Today_and_Now Add_Delta_DHMS );

	   my @date_time = Add_Delta_DHMS( Today_and_Now(), -1,	0, 0, 0	);

	   print "@date_time\n";

       Most people try to use the time rather than the calendar	to figure out
       dates, but that assumes that days are twenty-four hours each. For most
       people, there are two days a year when they aren't: the switch to and
       from summer time	throws this off. For example, the rest of the
       suggestions will	be wrong sometimes:

       Starting	with Perl 5.10,	Time::Piece and	Time::Seconds are part of the
       standard	distribution, so you might think that you could	do something
       like this:

	   use Time::Piece;
	   use Time::Seconds;

	   my $yesterday = localtime() - ONE_DAY; # WRONG
	   print "Yesterday was	$yesterday\n";

       The Time::Piece module exports a	new "localtime"	that returns an
       object, and Time::Seconds exports the "ONE_DAY" constant	that is	a set
       number of seconds. This means that it always gives the time 24 hours
       ago, which is not always	yesterday. This	can cause problems around the
       end of daylight saving time when	there's	one day	that is	25 hours long.

       You have	the same problem with Time::Local, which will give the wrong
       answer for those	same special cases:

	   # contributed by Gunnar Hjalmarsson
	    use	Time::Local;
	    my $today =	timelocal 0, 0,	12, ( localtime	)[3..5];
	    my ($d, $m,	$y) = (	localtime $today-86400 )[3..5];	# WRONG
	    printf "Yesterday: %d-%02d-%02d\n",	$y+1900, $m+1, $d;

   Does	Perl have a Year 2000 or 2038 problem? Is Perl Y2K compliant?
       (contributed by brian d foy)

       Perl itself never had a Y2K problem, although that never	stopped	people
       from creating Y2K problems on their own.	See the	documentation for
       "localtime" for its proper use.

       Starting	with Perl 5.12,	"localtime" and	"gmtime" can handle dates past
       03:14:08	January	19, 2038, when a 32-bit	based time would overflow. You
       still might get a warning on a 32-bit "perl":

	   % perl5.12 -E 'say scalar localtime(	0x9FFF_FFFFFFFF	)'
	   Integer overflow in hexadecimal number at -e	line 1.
	   Wed Nov  1 19:42:39 5576711

       On a 64-bit "perl", you can get even larger dates for those really long
       running projects:

	   % perl5.12 -E 'say scalar gmtime( 0x9FFF_FFFFFFFF )'
	   Thu Nov  2 00:42:39 5576711

       You're still out	of luck	if you need to keep track of decaying protons
       though.

Data: Strings
   How do I validate input?
       (contributed by brian d foy)

       There are many ways to ensure that values are what you expect or	want
       to accept. Besides the specific examples	that we	cover in the perlfaq,
       you can also look at the	modules	with "Assert" and "Validate" in	their
       names, along with other modules such as Regexp::Common.

       Some modules have validation for	particular types of input, such	as
       Business::ISBN, Business::CreditCard, Email::Valid, and
       Data::Validate::IP.

   How do I unescape a string?
       It depends just what you	mean by	"escape". URL escapes are dealt	with
       in perlfaq9. Shell escapes with the backslash ("\") character are
       removed with

	   s/\\(.)/$1/g;

       This won't expand "\n" or "\t" or any other special escapes.

   How do I remove consecutive pairs of	characters?
       (contributed by brian d foy)

       You can use the substitution operator to	find pairs of characters (or
       runs of characters) and replace them with a single instance. In this
       substitution, we	find a character in "(.)". The memory parentheses
       store the matched character in the back-reference "\g1" and we use that
       to require that the same	thing immediately follow it. We	replace	that
       part of the string with the character in	$1.

	   s/(.)\g1/$1/g;

       We can also use the transliteration operator, "tr///". In this example,
       the search list side of our "tr///" contains nothing, but the "c"
       option complements that so it contains everything. The replacement list
       also contains nothing, so the transliteration is	almost a no-op since
       it won't	do any replacements (or	more exactly, replace the character
       with itself). However, the "s" option squashes duplicated and
       consecutive characters in the string so a character does	not show up
       next to itself

	   my $str = 'Haarlem';	  # in the Netherlands
	   $str	=~ tr///cs;	  # Now	Harlem,	like in	New York

   How do I expand function calls in a string?
       (contributed by brian d foy)

       This is documented in perlref, and although it's	not the	easiest	thing
       to read,	it does	work. In each of these examples, we call the function
       inside the braces used to dereference a reference. If we	have more than
       one return value, we can	construct and dereference an anonymous array.
       In this case, we	call the function in list context.

	   print "The time values are @{ [localtime] }.\n";

       If we want to call the function in scalar context, we have to do	a bit
       more work. We can really	have any code we like inside the braces, so we
       simply have to end with the scalar reference, although how you do that
       is up to	you, and you can use code inside the braces. Note that the use
       of parens creates a list	context, so we need "scalar" to	force the
       scalar context on the function:

	   print "The time is ${\(scalar localtime)}.\n"

	   print "The time is ${ my $x = localtime; \$x	}.\n";

       If your function	already	returns	a reference, you don't need to create
       the reference yourself.

	   sub timestamp { my $t = localtime; \$t }

	   print "The time is ${ timestamp() }.\n";

       The "Interpolation" module can also do a	lot of magic for you. You can
       specify a variable name,	in this	case "E", to set up a tied hash	that
       does the	interpolation for you. It has several other methods to do this
       as well.

	   use Interpolation E => 'eval';
	   print "The time values are $E{localtime()}.\n";

       In most cases, it is probably easier to simply use string
       concatenation, which also forces	scalar context.

	   print "The time is "	. localtime() .	".\n";

   How do I find matching/nesting anything?
       To find something between two single characters,	a pattern like
       "/x([^x]*)x/" will get the intervening bits in $1. For multiple ones,
       then something more like	"/alpha(.*?)omega/" would be needed. For
       nested patterns and/or balanced expressions, see	the so-called (?PARNO)
       construct (available since perl 5.10).  The CPAN	module Regexp::Common
       can help	to build such regular expressions (see in particular
       Regexp::Common::balanced	and Regexp::Common::delimited).

       More complex cases will require to write	a parser, probably using a
       parsing module from CPAN, like Regexp::Grammars,	Parse::RecDescent,
       Parse::Yapp, Text::Balanced, or Marpa::R2.

   How do I reverse a string?
       Use "reverse()" in scalar context, as documented	in "reverse" in
       perlfunc.

	   my $reversed	= reverse $string;

   How do I expand tabs	in a string?
       You can do it yourself:

	   1 while $string =~ s/\t+/' '	x (length($&) *	8 - length($`) % 8)/e;

       Or you can just use the Text::Tabs module (part of the standard Perl
       distribution).

	   use Text::Tabs;
	   my @expanded_lines =	expand(@lines_with_tabs);

   How do I reformat a paragraph?
       Use Text::Wrap (part of the standard Perl distribution):

	   use Text::Wrap;
	   print wrap("\t", '  ', @paragraphs);

       The paragraphs you give to Text::Wrap should not	contain	embedded
       newlines. Text::Wrap doesn't justify the	lines (flush-right).

       Or use the CPAN module Text::Autoformat.	Formatting files can be	easily
       done by making a	shell alias, like so:

	   alias fmt="perl -i -MText::Autoformat -n0777	\
	       -e 'print autoformat $_,	{all=>1}' $*"

       See the documentation for Text::Autoformat to appreciate	its many
       capabilities.

   How can I access or change N	characters of a	string?
       You can access the first	characters of a	string with substr().  To get
       the first character, for	example, start at position 0 and grab the
       string of length	1.

	   my $string =	"Just another Perl Hacker";
	   my $first_char = substr( $string, 0,	1 );  #	 'J'

       To change part of a string, you can use the optional fourth argument
       which is	the replacement	string.

	   substr( $string, 13,	4, "Perl 5.8.0"	);

       You can also use	substr() as an lvalue.

	   substr( $string, 13,	4 ) =  "Perl 5.8.0";

   How do I change the Nth occurrence of something?
       You have	to keep	track of N yourself. For example, let's	say you	want
       to change the fifth occurrence of "whoever" or "whomever" into
       "whosoever" or "whomsoever", case insensitively.	These all assume that
       $_ contains the string to be altered.

	   $count = 0;
	   s{((whom?)ever)}{
	   ++$count == 5       # is it the 5th?
	       ? "${2}soever"  # yes, swap
	       : $1	       # renege	and leave it there
	       }ige;

       In the more general case, you can use the "/g" modifier in a "while"
       loop, keeping count of matches.

	   $WANT = 3;
	   $count = 0;
	   $_ =	"One fish two fish red fish blue fish";
	   while (/(\w+)\s+fish\b/gi) {
	       if (++$count == $WANT) {
		   print "The third fish is a $1 one.\n";
	       }
	   }

       That prints out:	"The third fish	is a red one."	You can	also use a
       repetition count	and repeated pattern like this:

	   /(?:\w+\s+fish\s+){2}(\w+)\s+fish/i;

   How can I count the number of occurrences of	a substring within a string?
       There are a number of ways, with	varying	efficiency. If you want	a
       count of	a certain single character (X) within a	string,	you can	use
       the "tr///" function like so:

	   my $string =	"ThisXlineXhasXsomeXx'sXinXit";
	   my $count = ($string	=~ tr/X//);
	   print "There	are $count X characters	in the string";

       This is fine if you are just looking for	a single character. However,
       if you are trying to count multiple character substrings	within a
       larger string, "tr///" won't work. What you can do is wrap a while()
       loop around a global pattern match. For example,	let's count negative
       integers:

	   my $string =	"-9 55 48 -2 23	-76 4 14 -44";
	   my $count = 0;
	   while ($string =~ /-\d+/g) {	$count++ }
	   print "There	are $count negative numbers in the string";

       Another version uses a global match in list context, then assigns the
       result to a scalar, producing a count of	the number of matches.

	   my $count = () = $string =~ /-\d+/g;

   How do I capitalize all the words on	one line?
       (contributed by brian d foy)

       Damian Conway's Text::Autoformat	handles	all of the thinking for	you.

	   use Text::Autoformat;
	   my $x = "Dr.	Strangelove or:	How I Learned to Stop ".
	     "Worrying and Love	the Bomb";

	   print $x, "\n";
	   for my $style (qw( sentence title highlight )) {
	       print autoformat($x, { case => $style }), "\n";
	   }

       How do you want to capitalize those words?

	   FRED	AND BARNEY'S LODGE	  # all	uppercase
	   Fred	And Barney's Lodge	  # title case
	   Fred	and Barney's Lodge	  # highlight case

       It's not	as easy	a problem as it	looks. How many	words do you think are
       in there? Wait for it...	wait for it....	If you answered	5 you're
       right. Perl words are groups of "\w+", but that's not what you want to
       capitalize. How is Perl supposed	to know	not to capitalize that "s"
       after the apostrophe? You could try a regular expression:

	   $string =~ s/ (
			(^\w)	 #at the beginning of the line
			  |	 # or
			(\s\w)	 #preceded by whitespace
			  )
		       /\U$1/xg;

	   $string =~ s/([\w']+)/\u\L$1/g;

       Now, what if you	don't want to capitalize that "and"? Just use
       Text::Autoformat	and get	on with	the next problem. :)

   How can I split a [character]-delimited string except when inside
       [character]?
       Several modules can handle this sort of parsing--Text::Balanced,
       Text::CSV, Text::CSV_XS,	and Text::ParseWords, among others.

       Take the	example	case of	trying to split	a string that is comma-
       separated into its different fields. You	can't use "split(/,/)" because
       you shouldn't split if the comma	is inside quotes. For example, take a
       data line like this:

	   SAR001,"","Cimetrix,	Inc","Bob Smith","CAM",N,8,1,0,7,"Error, Core Dumped"

       Due to the restriction of the quotes, this is a fairly complex problem.
       Thankfully, we have Jeffrey Friedl, author of Mastering Regular
       Expressions, to handle these for	us. He suggests	(assuming your string
       is contained in $text):

	    my @new = ();
	    push(@new, $+) while $text =~ m{
		"([^\"\\]*(?:\\.[^\"\\]*)*)",? # groups	the phrase inside the quotes
	       | ([^,]+),?
	       | ,
	    }gx;
	    push(@new, undef) if substr($text,-1,1) eq ',';

       If you want to represent	quotation marks	inside a quotation-mark-
       delimited field,	escape them with backslashes (eg, "like	\"this\"".

       Alternatively, the Text::ParseWords module (part	of the standard	Perl
       distribution) lets you say:

	   use Text::ParseWords;
	   @new	= quotewords(",", 0, $text);

       For parsing or generating CSV, though, using Text::CSV rather than
       implementing it yourself	is highly recommended; you'll save yourself
       odd bugs	popping	up later by just using code which has already been
       tried and tested	in production for years.

   How do I strip blank	space from the beginning/end of	a string?
       (contributed by brian d foy)

       A substitution can do this for you. For a single	line, you want to
       replace all the leading or trailing whitespace with nothing. You	can do
       that with a pair	of substitutions:

	   s/^\s+//;
	   s/\s+$//;

       You can also write that as a single substitution, although it turns out
       the combined statement is slower	than the separate ones.	That might not
       matter to you, though:

	   s/^\s+|\s+$//g;

       In this regular expression, the alternation matches either at the
       beginning or the	end of the string since	the anchors have a lower
       precedence than the alternation.	With the "/g" flag, the	substitution
       makes all possible matches, so it gets both. Remember, the trailing
       newline matches the "\s+", and  the "$" anchor can match	to the
       absolute	end of the string, so the newline disappears too. Just add the
       newline to the output, which has	the added benefit of preserving
       "blank" (consisting entirely of whitespace) lines which the "^\s+"
       would remove all	by itself:

	   while( <> ) {
	       s/^\s+|\s+$//g;
	       print "$_\n";
	   }

       For a multi-line	string,	you can	apply the regular expression to	each
       logical line in the string by adding the	"/m" flag (for "multi-line").
       With the	"/m" flag, the "$" matches before an embedded newline, so it
       doesn't remove it. This pattern still removes the newline at the	end of
       the string:

	   $string =~ s/^\s+|\s+$//gm;

       Remember	that lines consisting entirely of whitespace will disappear,
       since the first part of the alternation can match the entire string and
       replace it with nothing.	If you need to keep embedded blank lines, you
       have to do a little more	work. Instead of matching any whitespace
       (since that includes a newline),	just match the other whitespace:

	   $string =~ s/^[\t\f ]+|[\t\f	]+$//mg;

   How do I pad	a string with blanks or	pad a number with zeroes?
       In the following	examples, $pad_len is the length to which you wish to
       pad the string, $text or	$num contains the string to be padded, and
       $pad_char contains the padding character. You can use a single
       character string	constant instead of the	$pad_char variable if you know
       what it is in advance. And in the same way you can use an integer in
       place of	$pad_len if you	know the pad length in advance.

       The simplest method uses	the "sprintf" function.	It can pad on the left
       or right	with blanks and	on the left with zeroes	and it will not
       truncate	the result. The	"pack" function	can only pad strings on	the
       right with blanks and it	will truncate the result to a maximum length
       of $pad_len.

	   # Left padding a string with	blanks (no truncation):
	   my $padded =	sprintf("%${pad_len}s",	$text);
	   my $padded =	sprintf("%*s", $pad_len, $text);  # same thing

	   # Right padding a string with blanks	(no truncation):
	   my $padded =	sprintf("%-${pad_len}s", $text);
	   my $padded =	sprintf("%-*s",	$pad_len, $text); # same thing

	   # Left padding a number with	0 (no truncation):
	   my $padded =	sprintf("%0${pad_len}d", $num);
	   my $padded =	sprintf("%0*d",	$pad_len, $num); # same	thing

	   # Right padding a string with blanks	using pack (will truncate):
	   my $padded =	pack("A$pad_len",$text);

       If you need to pad with a character other than blank or zero you	can
       use one of the following	methods. They all generate a pad string	with
       the "x" operator	and combine that with $text. These methods do not
       truncate	$text.

       Left and	right padding with any character, creating a new string:

	   my $padded =	$pad_char x ( $pad_len - length( $text ) ) . $text;
	   my $padded =	$text .	$pad_char x ( $pad_len - length( $text ) );

       Left and	right padding with any character, modifying $text directly:

	   substr( $text, 0, 0 ) = $pad_char x ( $pad_len - length( $text ) );
	   $text .= $pad_char x	( $pad_len - length( $text ) );

   How do I extract selected columns from a string?
       (contributed by brian d foy)

       If you know the columns that contain the	data, you can use "substr" to
       extract a single	column.

	   my $column =	substr(	$line, $start_column, $length );

       You can use "split" if the columns are separated	by whitespace or some
       other delimiter,	as long	as whitespace or the delimiter cannot appear
       as part of the data.

	   my $line    = ' fred	barney	 betty	 ';
	   my @columns = split /\s+/, $line;
	       # ( '', 'fred', 'barney', 'betty' );

	   my $line    = 'fred||barney||betty';
	   my @columns = split /\|/, $line;
	       # ( 'fred', '', 'barney', '', 'betty' );

       If you want to work with	comma-separated	values,	don't do this since
       that format is a	bit more complicated. Use one of the modules that
       handle that format, such	as Text::CSV, Text::CSV_XS, or Text::CSV_PP.

       If you want to break apart an entire line of fixed columns, you can use
       "unpack"	with the A (ASCII) format. By using a number after the format
       specifier, you can denote the column width. See the "pack" and "unpack"
       entries in perlfunc for more details.

	   my @fields =	unpack(	$line, "A8 A8 A8 A16 A4" );

       Note that spaces	in the format argument to "unpack" do not denote
       literal spaces. If you have space separated data, you may want "split"
       instead.

   How do I find the soundex value of a	string?
       (contributed by brian d foy)

       You can use the "Text::Soundex" module. If you want to do fuzzy or
       close matching, you might also try the String::Approx, and
       Text::Metaphone,	and Text::DoubleMetaphone modules.

   How can I expand variables in text strings?
       (contributed by brian d foy)

       If you can avoid	it, don't, or if you can use a templating system, such
       as Text::Template or Template Toolkit, do that instead. You might even
       be able to get the job done with	"sprintf" or "printf":

	   my $string =	sprintf	'Say hello to %s and %s', $foo,	$bar;

       However,	for the	one-off	simple case where I don't want to pull out a
       full templating system, I'll use	a string that has two Perl scalar
       variables in it.	In this	example, I want	to expand $foo and $bar	to
       their variable's	values:

	   my $foo = 'Fred';
	   my $bar = 'Barney';
	   $string = 'Say hello	to $foo	and $bar';

       One way I can do	this involves the substitution operator	and a double
       "/e" flag. The first "/e" evaluates $1 on the replacement side and
       turns it	into $foo. The second /e starts	with $foo and replaces it with
       its value. $foo,	then, turns into 'Fred', and that's finally what's
       left in the string:

	   $string =~ s/(\$\w+)/$1/eeg;	# 'Say hello to	Fred and Barney'

       The "/e"	will also silently ignore violations of	strict,	replacing
       undefined variable names	with the empty string. Since I'm using the
       "/e" flag (twice	even!),	I have all of the same security	problems I
       have with "eval"	in its string form. If there's something odd in	$foo,
       perhaps something like "@{[ system "rm -rf /" ]}", then I could get
       myself in trouble.

       To get around the security problem, I could also	pull the values	from a
       hash instead of evaluating variable names. Using	a single "/e", I can
       check the hash to ensure	the value exists, and if it doesn't, I can
       replace the missing value with a	marker,	in this	case "???" to signal
       that I missed something:

	   my $string =	'This has $foo and $bar';

	   my %Replacements = (
	       foo  => 'Fred',
	       );

	   # $string =~	s/\$(\w+)/$Replacements{$1}/g;
	   $string =~ s/\$(\w+)/
	       exists $Replacements{$1}	? $Replacements{$1} : '???'
	       /eg;

	   print $string;

   What's wrong	with always quoting "$vars"?
       The problem is that those double-quotes force stringification--coercing
       numbers and references into strings--even when you don't	want them to
       be strings. Think of it this way: double-quote expansion	is used	to
       produce new strings. If you already have	a string, why do you need
       more?

       If you get used to writing odd things like these:

	   print "$var";       # BAD
	   my $new = "$old";	   # BAD
	   somefunc("$var");	# BAD

       You'll be in trouble. Those should (in 99.8% of the cases) be the
       simpler and more	direct:

	   print $var;
	   my $new = $old;
	   somefunc($var);

       Otherwise, besides slowing you down, you're going to break code when
       the thing in the	scalar is actually neither a string nor	a number, but
       a reference:

	   func(\@array);
	   sub func {
	       my $aref	= shift;
	       my $oref	= "$aref";  # WRONG
	   }

       You can also get	into subtle problems on	those few operations in	Perl
       that actually do	care about the difference between a string and a
       number, such as the magical "++"	autoincrement operator or the
       syscall() function.

       Stringification also destroys arrays.

	   my @lines = `command`;
	   print "@lines";     # WRONG - extra blanks
	   print @lines;       # right

   Why don't my	<<HERE documents work?
       Here documents are found	in perlop. Check for these three things:

       There must be no	space after the	<< part.
       There (probably)	should be a semicolon at the end of the	opening	token
       You can't (easily) have any space in front of the tag.
       There needs to be at least a line separator after the end token.

       If you want to indent the text in the here document, you	can do this:

	   # all in one
	   (my $VAR = <<HERE_TARGET) =~	s/^\s+//gm;
	       your text
	       goes here
	   HERE_TARGET

       But the HERE_TARGET must	still be flush against the margin.  If you
       want that indented also,	you'll have to quote in	the indentation.

	   (my $quote =	<<'    FINIS') =~ s/^\s+//gm;
		   ...we will have peace, when you and all your	works have
		   perished--and the works of your dark	master to whom you
		   would deliver us. You are a liar, Saruman, and a corrupter
		   of men's hearts. --Theoden in /usr/src/perl/taint.c
	       FINIS
	   $quote =~ s/\s+--/\n--/;

       A nice general-purpose fixer-upper function for indented	here documents
       follows.	It expects to be called	with a here document as	its argument.
       It looks	to see whether each line begins	with a common substring, and
       if so, strips that substring off. Otherwise, it takes the amount	of
       leading whitespace found	on the first line and removes that much	off
       each subsequent line.

	   sub fix {
	       local $_	= shift;
	       my ($white, $leader);  #	common whitespace and common leading string
	       if (/^\s*(?:([^\w\s]+)(\s*).*\n)(?:\s*\g1\g2?.*\n)+$/) {
		   ($white, $leader) = ($2, quotemeta($1));
	       } else {
		   ($white, $leader) = (/^(\s+)/, '');
	       }
	       s/^\s*?$leader(?:$white)?//gm;
	       return $_;
	   }

       This works with leading special strings,	dynamically determined:

	   my $remember_the_main = fix<<'    MAIN_INTERPRETER_LOOP';
	   @@@ int
	   @@@ runops()	{
	   @@@	   SAVEI32(runlevel);
	   @@@	   runlevel++;
	   @@@	   while ( op =	(*op->op_ppaddr)() );
	   @@@	   TAINT_NOT;
	   @@@	   return 0;
	   @@@ }
	   MAIN_INTERPRETER_LOOP

       Or with a fixed amount of leading whitespace, with remaining
       indentation correctly preserved:

	   my $poem = fix<<EVER_ON_AND_ON;
	      Now far ahead the	Road has gone,
	     And I must	follow,	if I can,
	      Pursuing it with eager feet,
	     Until it joins some larger	way
	      Where many paths and errands meet.
	     And whither then? I cannot	say.
	       --Bilbo in /usr/src/perl/pp_ctl.c
	   EVER_ON_AND_ON

Data: Arrays
   What	is the difference between a list and an	array?
       (contributed by brian d foy)

       A list is a fixed collection of scalars.	An array is a variable that
       holds a variable	collection of scalars. An array	can supply its
       collection for list operations, so list operations also work on arrays:

	   # slices
	   ( 'dog', 'cat', 'bird' )[2,3];
	   @animals[2,3];

	   # iteration
	   foreach ( qw( dog cat bird )	) { ...	}
	   foreach ( @animals )	{ ... }

	   my @three = grep { length ==	3 } qw(	dog cat	bird );
	   my @three = grep { length ==	3 } @animals;

	   # supply an argument	list
	   wash_animals( qw( dog cat bird ) );
	   wash_animals( @animals );

       Array operations, which change the scalars, rearrange them, or add or
       subtract	some scalars, only work	on arrays. These can't work on a list,
       which is	fixed. Array operations	include	"shift", "unshift", "push",
       "pop", and "splice".

       An array	can also change	its length:

	   $#animals = 1;  # truncate to two elements
	   $#animals = 10000; #	pre-extend to 10,001 elements

       You can change an array element,	but you	can't change a list element:

	   $animals[0] = 'Rottweiler';
	   qw( dog cat bird )[0] = 'Rottweiler'; # syntax error!

	   foreach ( @animals )	{
	       s/^d/fr/;  # works fine
	   }

	   foreach ( qw( dog cat bird )	) {
	       s/^d/fr/;  # Error! Modification	of read	only value!
	   }

       However,	if the list element is itself a	variable, it appears that you
       can change a list element. However, the list element is the variable,
       not the data. You're not	changing the list element, but something the
       list element refers to. The list	element	itself doesn't change: it's
       still the same variable.

       You also	have to	be careful about context. You can assign an array to a
       scalar to get the number	of elements in the array. This only works for
       arrays, though:

	   my $count = @animals;  # only works with arrays

       If you try to do	the same thing with what you think is a	list, you get
       a quite different result. Although it looks like	you have a list	on the
       righthand side, Perl actually sees a bunch of scalars separated by a
       comma:

	   my $scalar =	( 'dog', 'cat',	'bird' );  # $scalar gets bird

       Since you're assigning to a scalar, the righthand side is in scalar
       context.	The comma operator (yes, it's an operator!) in scalar context
       evaluates its lefthand side, throws away	the result, and	evaluates it's
       righthand side and returns the result. In effect, that list-lookalike
       assigns to $scalar it's rightmost value.	Many people mess this up
       because they choose a list-lookalike whose last element is also the
       count they expect:

	   my $scalar =	( 1, 2,	3 );  #	$scalar	gets 3,	accidentally

   What	is the difference between $array[1] and	@array[1]?
       (contributed by brian d foy)

       The difference is the sigil, that special character in front of the
       array name. The "$" sigil means "exactly	one item", while the "@" sigil
       means "zero or more items". The "$" gets	you a single scalar, while the
       "@" gets	you a list.

       The confusion arises because people incorrectly assume that the sigil
       denotes the variable type.

       The $array[1] is	a single-element access	to the array. It's going to
       return the item in index	1 (or undef if there is	no item	there).	 If
       you intend to get exactly one element from the array, this is the form
       you should use.

       The @array[1] is	an array slice,	although it has	only one index.	 You
       can pull	out multiple elements simultaneously by	specifying additional
       indices as a list, like @array[1,4,3,0].

       Using a slice on	the lefthand side of the assignment supplies list
       context to the righthand	side. This can lead to unexpected results.
       For instance, if	you want to read a single line from a filehandle,
       assigning to a scalar value is fine:

	   $array[1] = <STDIN>;

       However,	in list	context, the line input	operator returns all of	the
       lines as	a list.	The first line goes into @array[1] and the rest	of the
       lines mysteriously disappear:

	   @array[1] = <STDIN>;	 # most	likely not what	you want

       Either the "use warnings" pragma	or the -w flag will warn you when you
       use an array slice with a single	index.

   How can I remove duplicate elements from a list or array?
       (contributed by brian d foy)

       Use a hash. When	you think the words "unique" or	"duplicated", think
       "hash keys".

       If you don't care about the order of the	elements, you could just
       create the hash then extract the	keys. It's not important how you
       create that hash: just that you use "keys" to get the unique elements.

	   my %hash   =	map { $_, 1 } @array;
	   # or	a hash slice: @hash{ @array } =	();
	   # or	a foreach: $hash{$_} = 1 foreach ( @array );

	   my @unique =	keys %hash;

       If you want to use a module, try	the "uniq" function from
       List::MoreUtils.	In list	context	it returns the unique elements,
       preserving their	order in the list. In scalar context, it returns the
       number of unique	elements.

	   use List::MoreUtils qw(uniq);

	   my @unique =	uniq( 1, 2, 3, 4, 4, 5,	6, 5, 7	); # 1,2,3,4,5,6,7
	   my $unique =	uniq( 1, 2, 3, 4, 4, 5,	6, 5, 7	); # 7

       You can also go through each element and	skip the ones you've seen
       before. Use a hash to keep track. The first time	the loop sees an
       element,	that element has no key	in %Seen. The "next" statement creates
       the key and immediately uses its	value, which is	"undef", so the	loop
       continues to the	"push" and increments the value	for that key. The next
       time the	loop sees that same element, its key exists in the hash	and
       the value for that key is true (since it's not 0	or "undef"), so	the
       next skips that iteration and the loop goes to the next element.

	   my @unique =	();
	   my %seen   =	();

	   foreach my $elem ( @array ) {
	       next if $seen{ $elem }++;
	       push @unique, $elem;
	   }

       You can write this more briefly using a grep, which does	the same
       thing.

	   my %seen = ();
	   my @unique =	grep { ! $seen{	$_ }++ } @array;

   How can I tell whether a certain element is contained in a list or array?
       (portions of this answer	contributed by Anno Siegel and brian d foy)

       Hearing the word	"in" is	an indication that you probably	should have
       used a hash, not	a list or array, to store your data. Hashes are
       designed	to answer this question	quickly	and efficiently. Arrays
       aren't.

       That being said,	there are several ways to approach this. In Perl 5.10
       and later, you can use the smart	match operator to check	that an	item
       is contained in an array	or a hash:

	   use 5.010;

	   if( $item ~~	@array ) {
	       say "The	array contains $item"
	   }

	   if( $item ~~	%hash )	{
	       say "The	hash contains $item"
	   }

       With earlier versions of	Perl, you have to do a bit more	work. If you
       are going to make this query many times over arbitrary string values,
       the fastest way is probably to invert the original array	and maintain a
       hash whose keys are the first array's values:

	   my @blues = qw/azure	cerulean teal turquoise	lapis-lazuli/;
	   my %is_blue = ();
	   for (@blues)	{ $is_blue{$_} = 1 }

       Now you can check whether $is_blue{$some_color}.	It might have been a
       good idea to keep the blues all in a hash in the	first place.

       If the values are all small integers, you could use a simple indexed
       array. This kind	of an array will take up less space:

	   my @primes =	(2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31);
	   my @is_tiny_prime = ();
	   for (@primes) { $is_tiny_prime[$_] =	1 }
	   # or	simply	@istiny_prime[@primes] = (1) x @primes;

       Now you check whether $is_tiny_prime[$some_number].

       If the values in	question are integers instead of strings, you can save
       quite a lot of space by using bit strings instead:

	   my @articles	= ( 1..10, 150..2000, 2017 );
	   undef $read;
	   for (@articles) { vec($read,$_,1) = 1 }

       Now check whether "vec($read,$n,1)" is true for some $n.

       These methods guarantee fast individual tests but require a re-
       organization of the original list or array. They	only pay off if	you
       have to test multiple values against the	same array.

       If you are testing only once, the standard module List::Util exports
       the function "first" for	this purpose. It works by stopping once	it
       finds the element. It's written in C for	speed, and its Perl equivalent
       looks like this subroutine:

	   sub first (&@) {
	       my $code	= shift;
	       foreach (@_) {
		   return $_ if	&{$code}();
	       }
	       undef;
	   }

       If speed	is of little concern, the common idiom uses grep in scalar
       context (which returns the number of items that passed its condition)
       to traverse the entire list. This does have the benefit of telling you
       how many	matches	it found, though.

	   my $is_there	= grep $_ eq $whatever,	@array;

       If you want to actually extract the matching elements, simply use grep
       in list context.

	   my @matches = grep $_ eq $whatever, @array;

   How do I compute the	difference of two arrays? How do I compute the
       intersection of two arrays?
       Use a hash. Here's code to do both and more. It assumes that each
       element is unique in a given array:

	   my (@union, @intersection, @difference);
	   my %count = ();
	   foreach my $element (@array1, @array2) { $count{$element}++ }
	   foreach my $element (keys %count) {
	       push @union, $element;
	       push @{ $count{$element}	> 1 ? \@intersection : \@difference }, $element;
	   }

       Note that this is the symmetric difference, that	is, all	elements in
       either A	or in B	but not	in both. Think of it as	an xor operation.

   How do I test whether two arrays or hashes are equal?
       With Perl 5.10 and later, the smart match operator can give you the
       answer with the least amount of work:

	   use 5.010;

	   if( @array1 ~~ @array2 ) {
	       say "The	arrays are the same";
	   }

	   if( %hash1 ~~ %hash2	) # doesn't check values!  {
	       say "The	hash keys are the same";
	   }

       The following code works	for single-level arrays. It uses a stringwise
       comparison, and does not	distinguish defined versus undefined empty
       strings.	Modify if you have other needs.

	   $are_equal =	compare_arrays(\@frogs,	\@toads);

	   sub compare_arrays {
	       my ($first, $second) = @_;
	       no warnings;  # silence spurious	-w undef complaints
	       return 0	unless @$first == @$second;
	       for (my $i = 0; $i < @$first; $i++) {
		   return 0 if $first->[$i] ne $second->[$i];
	       }
	       return 1;
	   }

       For multilevel structures, you may wish to use an approach more like
       this one. It uses the CPAN module FreezeThaw:

	   use FreezeThaw qw(cmpStr);
	   my @a = my @b = ( "this", "that", [ "more", "stuff" ] );

	   printf "a and b contain %s arrays\n",
	       cmpStr(\@a, \@b)	== 0
	       ? "the same"
	       : "different";

       This approach also works	for comparing hashes. Here we'll demonstrate
       two different answers:

	   use FreezeThaw qw(cmpStr cmpStrHard);

	   my %a = my %b = ( "this" => "that", "extra" => [ "more", "stuff" ] );
	   $a{EXTRA} = \%b;
	   $b{EXTRA} = \%a;

	   printf "a and b contain %s hashes\n",
	   cmpStr(\%a, \%b) == 0 ? "the	same" :	"different";

	   printf "a and b contain %s hashes\n",
	   cmpStrHard(\%a, \%b)	== 0 ? "the same" : "different";

       The first reports that both those the hashes contain the	same data,
       while the second	reports	that they do not. Which	you prefer is left as
       an exercise to the reader.

   How do I find the first array element for which a condition is true?
       To find the first array element which satisfies a condition, you	can
       use the "first()" function in the List::Util module, which comes	with
       Perl 5.8. This example finds the	first element that contains "Perl".

	   use List::Util qw(first);

	   my $element = first { /Perl/	} @array;

       If you cannot use List::Util, you can make your own loop	to do the same
       thing. Once you find the	element, you stop the loop with	last.

	   my $found;
	   foreach ( @array ) {
	       if( /Perl/ ) { $found = $_; last	}
	   }

       If you want the array index, use	the "firstidx()" function from
       "List::MoreUtils":

	   use List::MoreUtils qw(firstidx);
	   my $index = firstidx	{ /Perl/ } @array;

       Or write	it yourself, iterating through the indices and checking	the
       array element at	each index until you find one that satisfies the
       condition:

	   my( $found, $index )	= ( undef, -1 );
	   for(	$i = 0;	$i < @array; $i++ ) {
	       if( $array[$i] =~ /Perl/	) {
		   $found = $array[$i];
		   $index = $i;
		   last;
	       }
	   }

   How do I handle linked lists?
       (contributed by brian d foy)

       Perl's arrays do	not have a fixed size, so you don't need linked	lists
       if you just want	to add or remove items.	You can	use array operations
       such as "push", "pop", "shift", "unshift", or "splice" to do that.

       Sometimes, however, linked lists	can be useful in situations where you
       want to "shard" an array	so you have many small arrays instead of a
       single big array. You can keep arrays longer than Perl's	largest	array
       index, lock smaller arrays separately in	threaded programs, reallocate
       less memory, or quickly insert elements in the middle of	the chain.

       Steve Lembark goes through the details in his YAPC::NA 2009 talk	"Perly
       Linked Lists" ( <http://www.slideshare.net/lembark/perly-linked-lists>
       ), although you can just	use his	LinkedList::Single module.

   How do I handle circular lists?
       (contributed by brian d foy)

       If you want to cycle through an array endlessly,	you can	increment the
       index modulo the	number of elements in the array:

	   my @array = qw( a b c );
	   my $i = 0;

	   while( 1 ) {
	       print $array[ $i++ % @array ], "\n";
	       last if $i > 20;
	   }

       You can also use	Tie::Cycle to use a scalar that	always has the next
       element of the circular array:

	   use Tie::Cycle;

	   tie my $cycle, 'Tie::Cycle',	[ qw( FFFFFF 000000 FFFF00 ) ];

	   print $cycle; # FFFFFF
	   print $cycle; # 000000
	   print $cycle; # FFFF00

       The Array::Iterator::Circular creates an	iterator object	for circular
       arrays:

	   use Array::Iterator::Circular;

	   my $color_iterator =	Array::Iterator::Circular->new(
	       qw(red green blue orange)
	       );

	   foreach ( 1 .. 20 ) {
	       print $color_iterator->next, "\n";
	   }

   How do I shuffle an array randomly?
       If you either have Perl 5.8.0 or	later installed, or if you have
       Scalar-List-Utils 1.03 or later installed, you can say:

	   use List::Util 'shuffle';

	   @shuffled = shuffle(@list);

       If not, you can use a Fisher-Yates shuffle.

	   sub fisher_yates_shuffle {
	       my $deck	= shift;  # $deck is a reference to an array
	       return unless @$deck; # must not	be empty!

	       my $i = @$deck;
	       while (--$i) {
		   my $j = int rand ($i+1);
		   @$deck[$i,$j] = @$deck[$j,$i];
	       }
	   }

	   # shuffle my	mpeg collection
	   #
	   my @mpeg = <audio/*/*.mp3>;
	   fisher_yates_shuffle( \@mpeg	);    #	randomize @mpeg	in place
	   print @mpeg;

       Note that the above implementation shuffles an array in place, unlike
       the "List::Util::shuffle()" which takes a list and returns a new
       shuffled	list.

       You've probably seen shuffling algorithms that work using splice,
       randomly	picking	another	element	to swap	the current element with

	   srand;
	   @new	= ();
	   @old	= 1 .. 10;  # just a demo
	   while (@old)	{
	       push(@new, splice(@old, rand @old, 1));
	   }

       This is bad because splice is already O(N), and since you do it N
       times, you just invented	a quadratic algorithm; that is,	O(N**2).  This
       does not	scale, although	Perl is	so efficient that you probably won't
       notice this until you have rather largish arrays.

   How do I process/modify each	element	of an array?
       Use "for"/"foreach":

	   for (@lines)	{
	       s/foo/bar/;    #	change that word
	       tr/XZ/ZX/;    # swap those letters
	   }

       Here's another; let's compute spherical volumes:

	   my @volumes = @radii;
	   for (@volumes) {   #	@volumes has changed parts
	       $_ **= 3;
	       $_ *= (4/3) * 3.14159;  # this will be constant folded
	   }

       which can also be done with "map()" which is made to transform one list
       into another:

	   my @volumes = map {$_ ** 3 *	(4/3) *	3.14159} @radii;

       If you want to do the same thing	to modify the values of	the hash, you
       can use the "values" function. As of Perl 5.6 the values	are not
       copied, so if you modify	$orbit (in this	case), you modify the value.

	   for my $orbit ( values %orbits ) {
	       ($orbit **= 3) *= (4/3) * 3.14159;
	   }

       Prior to	perl 5.6 "values" returned copies of the values, so older perl
       code often contains constructions such as @orbits{keys %orbits} instead
       of "values %orbits" where the hash is to	be modified.

   How do I select a random element from an array?
       Use the "rand()"	function (see "rand" in	perlfunc):

	   my $index   = rand @array;
	   my $element = $array[$index];

       Or, simply:

	   my $element = $array[ rand @array ];

   How do I permute N elements of a list?
       Use the List::Permutor module on	CPAN. If the list is actually an
       array, try the Algorithm::Permute module	(also on CPAN).	It's written
       in XS code and is very efficient:

	   use Algorithm::Permute;

	   my @array = 'a'..'d';
	   my $p_iterator = Algorithm::Permute->new ( \@array );

	   while (my @perm = $p_iterator->next)	{
	      print "next permutation: (@perm)\n";
	   }

       For even	faster execution, you could do:

	   use Algorithm::Permute;

	   my @array = 'a'..'d';

	   Algorithm::Permute::permute {
	       print "next permutation:	(@array)\n";
	   } @array;

       Here's a	little program that generates all permutations of all the
       words on	each line of input. The	algorithm embodied in the "permute()"
       function	is discussed in	Volume 4 (still	unpublished) of	Knuth's	The
       Art of Computer Programming and will work on any	list:

	   #!/usr/bin/perl -n
	   # Fischer-Krause ordered permutation	generator

	   sub permute (&@) {
	       my $code	= shift;
	       my @idx = 0..$#_;
	       while ( $code->(@_[@idx]) ) {
		   my $p = $#idx;
		   --$p	while $idx[$p-1] > $idx[$p];
		   my $q = $p or return;
		   push	@idx, reverse splice @idx, $p;
		   ++$q	while $idx[$p-1] > $idx[$q];
		   @idx[$p-1,$q]=@idx[$q,$p-1];
	       }
	   }

	   permute { print "@_\n" } split;

       The Algorithm::Loops module also	provides the "NextPermute" and
       "NextPermuteNum"	functions which	efficiently find all unique
       permutations of an array, even if it contains duplicate values,
       modifying it in-place: if its elements are in reverse-sorted order then
       the array is reversed, making it	sorted,	and it returns false;
       otherwise the next permutation is returned.

       "NextPermute" uses string order and "NextPermuteNum" numeric order, so
       you can enumerate all the permutations of 0..9 like this:

	   use Algorithm::Loops	qw(NextPermuteNum);

	   my @list= 0..9;
	   do {	print "@list\n"	} while	NextPermuteNum @list;

   How do I sort an array by (anything)?
       Supply a	comparison function to sort() (described in "sort" in
       perlfunc):

	   @list = sort	{ $a <=> $b } @list;

       The default sort	function is cmp, string	comparison, which would	sort
       "(1, 2, 10)" into "(1, 10, 2)". "<=>", used above, is the numerical
       comparison operator.

       If you have a complicated function needed to pull out the part you want
       to sort on, then	don't do it inside the sort function. Pull it out
       first, because the sort BLOCK can be called many	times for the same
       element.	Here's an example of how to pull out the first word after the
       first number on each item, and then sort	those words case-
       insensitively.

	   my @idx;
	   for (@data) {
	       my $item;
	       ($item) = /\d+\s*(\S+)/;
	       push @idx, uc($item);
	   }
	   my @sorted =	@data[ sort { $idx[$a] cmp $idx[$b] } 0	.. $#idx ];

       which could also	be written this	way, using a trick that's come to be
       known as	the Schwartzian	Transform:

	   my @sorted =	map  { $_->[0] }
	       sort { $a->[1] cmp $b->[1] }
	       map  { [	$_, uc(	(/\d+\s*(\S+)/)[0]) ] }	@data;

       If you need to sort on several fields, the following paradigm is
       useful.

	   my @sorted =	sort {
	       field1($a) <=> field1($b) ||
	       field2($a) cmp field2($b) ||
	       field3($a) cmp field3($b)
	   } @data;

       This can	be conveniently	combined with precalculation of	keys as	given
       above.

       See the sort article in the "Far	More Than You Ever Wanted To Know"
       collection in <http://www.cpan.org/misc/olddoc/FMTEYEWTK.tgz> for more
       about this approach.

       See also	the question later in perlfaq4 on sorting hashes.

   How do I manipulate arrays of bits?
       Use "pack()" and	"unpack()", or else "vec()" and	the bitwise
       operations.

       For example, you	don't have to store individual bits in an array	(which
       would mean that you're wasting a	lot of space). To convert an array of
       bits to a string, use "vec()" to	set the	right bits. This sets $vec to
       have bit	N set only if $ints[N] was set:

	   my @ints = (...); # array of	bits, e.g. ( 1,	0, 0, 1, 1, 0 ... )
	   my $vec = '';
	   foreach( 0 .. $#ints	) {
	       vec($vec,$_,1) =	1 if $ints[$_];
	   }

       The string $vec only takes up as	many bits as it	needs. For instance,
       if you had 16 entries in	@ints, $vec only needs two bytes to store them
       (not counting the scalar	variable overhead).

       Here's how, given a vector in $vec, you can get those bits into your
       @ints array:

	   sub bitvec_to_list {
	       my $vec = shift;
	       my @ints;
	       # Find null-byte	density	then select best algorithm
	       if ($vec	=~ tr/\0// / length $vec > 0.95) {
		   use integer;
		   my $i;

		   # This method is faster with	mostly null-bytes
		   while($vec =~ /[^\0]/g ) {
		       $i = -9 + 8 * pos $vec;
		       push @ints, $i if vec($vec, ++$i, 1);
		       push @ints, $i if vec($vec, ++$i, 1);
		       push @ints, $i if vec($vec, ++$i, 1);
		       push @ints, $i if vec($vec, ++$i, 1);
		       push @ints, $i if vec($vec, ++$i, 1);
		       push @ints, $i if vec($vec, ++$i, 1);
		       push @ints, $i if vec($vec, ++$i, 1);
		       push @ints, $i if vec($vec, ++$i, 1);
		   }
	       }
	       else {
		   # This method is a fast general algorithm
		   use integer;
		   my $bits = unpack "b*", $vec;
		   push	@ints, 0 if $bits =~ s/^(\d)// && $1;
		   push	@ints, pos $bits while($bits =~	/1/g);
	       }

	       return \@ints;
	   }

       This method gets	faster the more	sparse the bit vector is.  (Courtesy
       of Tim Bunce and	Winfried Koenig.)

       You can make the	while loop a lot shorter with this suggestion from
       Benjamin	Goldberg:

	   while($vec =~ /[^\0]+/g ) {
	       push @ints, grep	vec($vec, $_, 1), $-[0]	* 8 .. $+[0] * 8;
	   }

       Or use the CPAN module Bit::Vector:

	   my $vector =	Bit::Vector->new($num_of_bits);
	   $vector->Index_List_Store(@ints);
	   my @ints = $vector->Index_List_Read();

       Bit::Vector provides efficient methods for bit vector, sets of small
       integers	and "big int" math.

       Here's a	more extensive illustration using vec():

	   # vec demo
	   my $vector =	"\xff\x0f\xef\xfe";
	   print "Ilya's string	\\xff\\x0f\\xef\\xfe represents	the number ",
	   unpack("N", $vector), "\n";
	   my $is_set =	vec($vector, 23, 1);
	   print "Its 23rd bit is ", $is_set ? "set" : "clear",	".\n";
	   pvec($vector);

	   set_vec(1,1,1);
	   set_vec(3,1,1);
	   set_vec(23,1,1);

	   set_vec(3,1,3);
	   set_vec(3,2,3);
	   set_vec(3,4,3);
	   set_vec(3,4,7);
	   set_vec(3,8,3);
	   set_vec(3,8,7);

	   set_vec(0,32,17);
	   set_vec(1,32,17);

	   sub set_vec {
	       my ($offset, $width, $value) = @_;
	       my $vector = '';
	       vec($vector, $offset, $width) = $value;
	       print "offset=$offset width=$width value=$value\n";
	       pvec($vector);
	   }

	   sub pvec {
	       my $vector = shift;
	       my $bits	= unpack("b*", $vector);
	       my $i = 0;
	       my $BASE	= 8;

	       print "vector length in bytes: ", length($vector), "\n";
	       @bytes =	unpack("A8" x length($vector), $bits);
	       print "bits are:	@bytes\n\n";
	   }

   Why does defined() return true on empty arrays and hashes?
       The short story is that you should probably only	use defined on scalars
       or functions, not on aggregates (arrays and hashes). See	"defined" in
       perlfunc	in the 5.004 release or	later of Perl for more detail.

Data: Hashes (Associative Arrays)
   How do I process an entire hash?
       (contributed by brian d foy)

       There are a couple of ways that you can process an entire hash. You can
       get a list of keys, then	go through each	key, or	grab a one key-value
       pair at a time.

       To go through all of the	keys, use the "keys" function. This extracts
       all of the keys of the hash and gives them back to you as a list. You
       can then	get the	value through the particular key you're	processing:

	   foreach my $key ( keys %hash	) {
	       my $value = $hash{$key}
	       ...
	   }

       Once you	have the list of keys, you can process that list before	you
       process the hash	elements. For instance,	you can	sort the keys so you
       can process them	in lexical order:

	   foreach my $key ( sort keys %hash ) {
	       my $value = $hash{$key}
	       ...
	   }

       Or, you might want to only process some of the items. If	you only want
       to deal with the	keys that start	with "text:", you can select just
       those using "grep":

	   foreach my $key ( grep /^text:/, keys %hash ) {
	       my $value = $hash{$key}
	       ...
	   }

       If the hash is very large, you might not	want to	create a long list of
       keys. To	save some memory, you can grab one key-value pair at a time
       using "each()", which returns a pair you	haven't	seen yet:

	   while( my( $key, $value ) = each( %hash ) ) {
	       ...
	   }

       The "each" operator returns the pairs in	apparently random order, so if
       ordering	matters	to you,	you'll have to stick with the "keys" method.

       The "each()" operator can be a bit tricky though. You can't add or
       delete keys of the hash while you're using it without possibly skipping
       or re-processing	some pairs after Perl internally rehashes all of the
       elements. Additionally, a hash has only one iterator, so	if you mix
       "keys", "values", or "each" on the same hash, you risk resetting	the
       iterator	and messing up your processing.	See the	"each" entry in
       perlfunc	for more details.

   How do I merge two hashes?
       (contributed by brian d foy)

       Before you decide to merge two hashes, you have to decide what to do if
       both hashes contain keys	that are the same and if you want to leave the
       original	hashes as they were.

       If you want to preserve the original hashes, copy one hash (%hash1) to
       a new hash (%new_hash), then add	the keys from the other	hash (%hash2
       to the new hash.	Checking that the key already exists in	%new_hash
       gives you a chance to decide what to do with the	duplicates:

	   my %new_hash	= %hash1; # make a copy; leave %hash1 alone

	   foreach my $key2 ( keys %hash2 ) {
	       if( exists $new_hash{$key2} ) {
		   warn	"Key [$key2] is	in both	hashes!";
		   # handle the	duplicate (perhaps only	warning)
		   ...
		   next;
	       }
	       else {
		   $new_hash{$key2} = $hash2{$key2};
	       }
	   }

       If you don't want to create a new hash, you can still use this looping
       technique; just change the %new_hash to %hash1.

	   foreach my $key2 ( keys %hash2 ) {
	       if( exists $hash1{$key2}	) {
		   warn	"Key [$key2] is	in both	hashes!";
		   # handle the	duplicate (perhaps only	warning)
		   ...
		   next;
	       }
	       else {
		   $hash1{$key2} = $hash2{$key2};
	       }
	     }

       If you don't care that one hash overwrites keys and values from the
       other, you could	just use a hash	slice to add one hash to another. In
       this case, values from %hash2 replace values from %hash1	when they have
       keys in common:

	   @hash1{ keys	%hash2 } = values %hash2;

   What	happens	if I add or remove keys	from a hash while iterating over it?
       (contributed by brian d foy)

       The easy	answer is "Don't do that!"

       If you iterate through the hash with each(), you	can delete the key
       most recently returned without worrying about it. If you	delete or add
       other keys, the iterator	may skip or double up on them since perl may
       rearrange the hash table. See the entry for "each()" in perlfunc.

   How do I look up a hash element by value?
       Create a	reverse	hash:

	   my %by_value	= reverse %by_key;
	   my $key = $by_value{$value};

       That's not particularly efficient. It would be more space-efficient to
       use:

	   while (my ($key, $value) = each %by_key) {
	       $by_value{$value} = $key;
	   }

       If your hash could have repeated	values,	the methods above will only
       find one	of the associated keys.	 This may or may not worry you.	If it
       does worry you, you can always reverse the hash into a hash of arrays
       instead:

	   while (my ($key, $value) = each %by_key) {
		push @{$key_list_by_value{$value}}, $key;
	   }

   How can I know how many entries are in a hash?
       (contributed by brian d foy)

       This is very similar to "How do I process an entire hash?", also	in
       perlfaq4, but a bit simpler in the common cases.

       You can use the "keys()"	built-in function in scalar context to find
       out have	many entries you have in a hash:

	   my $key_count = keys	%hash; # must be scalar	context!

       If you want to find out how many	entries	have a defined value, that's a
       bit different. You have to check	each value. A "grep" is	handy:

	   my $defined_value_count = grep { defined } values %hash;

       You can use that	same structure to count	the entries any	way that you
       like. If	you want the count of the keys with vowels in them, you	just
       test for	that instead:

	   my $vowel_count = grep { /[aeiou]/ }	keys %hash;

       The "grep" in scalar context returns the	count. If you want the list of
       matching	items, just use	it in list context instead:

	   my @defined_values =	grep { defined } values	%hash;

       The "keys()" function also resets the iterator, which means that	you
       may see strange results if you use this between uses of other hash
       operators such as "each()".

   How do I sort a hash	(optionally by value instead of	key)?
       (contributed by brian d foy)

       To sort a hash, start with the keys. In this example, we	give the list
       of keys to the sort function which then compares	them ASCIIbetically
       (which might be affected	by your	locale settings). The output list has
       the keys	in ASCIIbetical	order. Once we have the	keys, we can go
       through them to create a	report which lists the keys in ASCIIbetical
       order.

	   my @keys = sort { $a	cmp $b } keys %hash;

	   foreach my $key ( @keys ) {
	       printf "%-20s %6d\n", $key, $hash{$key};
	   }

       We could	get more fancy in the "sort()" block though. Instead of
       comparing the keys, we can compute a value with them and	use that value
       as the comparison.

       For instance, to	make our report	order case-insensitive,	we use "lc" to
       lowercase the keys before comparing them:

	   my @keys = sort { lc	$a cmp lc $b } keys %hash;

       Note: if	the computation	is expensive or	the hash has many elements,
       you may want to look at the Schwartzian Transform to cache the
       computation results.

       If we want to sort by the hash value instead, we	use the	hash key to
       look it up. We still get	out a list of keys, but	this time they are
       ordered by their	value.

	   my @keys = sort { $hash{$a} <=> $hash{$b} } keys %hash;

       From there we can get more complex. If the hash values are the same, we
       can provide a secondary sort on the hash	key.

	   my @keys = sort {
	       $hash{$a} <=> $hash{$b}
		   or
	       "\L$a" cmp "\L$b"
	   } keys %hash;

   How can I always keep my hash sorted?
       You can look into using the "DB_File" module and	"tie()"	using the
       $DB_BTREE hash bindings as documented in	"In Memory Databases" in
       DB_File.	The Tie::IxHash	module from CPAN might also be instructive.
       Although	this does keep your hash sorted, you might not like the
       slowdown	you suffer from	the tie	interface. Are you sure	you need to do
       this? :)

   What's the difference between "delete" and "undef" with hashes?
       Hashes contain pairs of scalars:	the first is the key, the second is
       the value. The key will be coerced to a string, although	the value can
       be any kind of scalar: string, number, or reference. If a key $key is
       present in %hash, "exists($hash{$key})" will return true. The value for
       a given key can be "undef", in which case $hash{$key} will be "undef"
       while "exists $hash{$key}" will return true. This corresponds to	($key,
       "undef")	being in the hash.

       Pictures	help...	Here's the %hash table:

	     keys  values
	   +------+------+
	   |  a	  |  3	 |
	   |  x	  |  7	 |
	   |  d	  |  0	 |
	   |  e	  |  2	 |
	   +------+------+

       And these conditions hold

	   $hash{'a'}			    is true
	   $hash{'d'}			    is false
	   defined $hash{'d'}		    is true
	   defined $hash{'a'}		    is true
	   exists $hash{'a'}		    is true (Perl 5 only)
	   grep	($_ eq 'a', keys %hash)	    is true

       If you now say

	   undef $hash{'a'}

       your table now reads:

	     keys  values
	   +------+------+
	   |  a	  | undef|
	   |  x	  |  7	 |
	   |  d	  |  0	 |
	   |  e	  |  2	 |
	   +------+------+

       and these conditions now	hold; changes in caps:

	   $hash{'a'}			    is FALSE
	   $hash{'d'}			    is false
	   defined $hash{'d'}		    is true
	   defined $hash{'a'}		    is FALSE
	   exists $hash{'a'}		    is true (Perl 5 only)
	   grep	($_ eq 'a', keys %hash)	    is true

       Notice the last two: you	have an	undef value, but a defined key!

       Now, consider this:

	   delete $hash{'a'}

       your table now reads:

	     keys  values
	   +------+------+
	   |  x	  |  7	 |
	   |  d	  |  0	 |
	   |  e	  |  2	 |
	   +------+------+

       and these conditions now	hold; changes in caps:

	   $hash{'a'}			    is false
	   $hash{'d'}			    is false
	   defined $hash{'d'}		    is true
	   defined $hash{'a'}		    is false
	   exists $hash{'a'}		    is FALSE (Perl 5 only)
	   grep	($_ eq 'a', keys %hash)	    is FALSE

       See, the	whole entry is gone!

   Why don't my	tied hashes make the defined/exists distinction?
       This depends on the tied	hash's implementation of EXISTS().  For
       example,	there isn't the	concept	of undef with hashes that are tied to
       DBM* files. It also means that exists() and defined() do	the same thing
       with a DBM* file, and what they end up doing is not what	they do	with
       ordinary	hashes.

   How do I reset an each() operation part-way through?
       (contributed by brian d foy)

       You can use the "keys" or "values" functions to reset "each". To	simply
       reset the iterator used by "each" without doing anything	else, use one
       of them in void context:

	   keys	%hash; # resets	iterator, nothing else.
	   values %hash; # resets iterator, nothing else.

       See the documentation for "each"	in perlfunc.

   How can I get the unique keys from two hashes?
       First you extract the keys from the hashes into lists, then solve the
       "removing duplicates" problem described above. For example:

	   my %seen = ();
	   for my $element (keys(%foo),	keys(%bar)) {
	       $seen{$element}++;
	   }
	   my @uniq = keys %seen;

       Or more succinctly:

	   my @uniq = keys %{{%foo,%bar}};

       Or if you really	want to	save space:

	   my %seen = ();
	   while (defined ($key	= each %foo)) {
	       $seen{$key}++;
	   }
	   while (defined ($key	= each %bar)) {
	       $seen{$key}++;
	   }
	   my @uniq = keys %seen;

   How can I store a multidimensional array in a DBM file?
       Either stringify	the structure yourself (no fun), or else get the MLDBM
       (which uses Data::Dumper) module	from CPAN and layer it on top of
       either DB_File or GDBM_File. You	might also try DBM::Deep, but it can
       be a bit	slow.

   How can I make my hash remember the order I put elements into it?
       Use the Tie::IxHash from	CPAN.

	   use Tie::IxHash;

	   tie my %myhash, 'Tie::IxHash';

	   for (my $i=0; $i<20;	$i++) {
	       $myhash{$i} = 2*$i;
	   }

	   my @keys = keys %myhash;
	   # @keys = (0,1,2,3,...)

   Why does passing a subroutine an undefined element in a hash	create it?
       (contributed by brian d foy)

       Are you using a really old version of Perl?

       Normally, accessing a hash key's	value for a nonexistent	key will not
       create the key.

	   my %hash  = ();
	   my $value = $hash{ 'foo' };
	   print "This won't print\n" if exists	$hash{ 'foo' };

       Passing $hash{ 'foo' } to a subroutine used to be a special case,
       though.	Since you could	assign directly	to $_[0], Perl had to be ready
       to make that assignment so it created the hash key ahead	of time:

	   my_sub( $hash{ 'foo'	} );
	   print "This will print before 5.004\n" if exists $hash{ 'foo' };

	   sub my_sub {
	       # $_[0] = 'bar';	# create hash key in case you do this
	       1;
	   }

       Since Perl 5.004, however, this situation is a special case and Perl
       creates the hash	key only when you make the assignment:

	   my_sub( $hash{ 'foo'	} );
	   print "This will print, even	after 5.004\n" if exists $hash{	'foo' };

	   sub my_sub {
	       $_[0] = 'bar';
	   }

       However,	if you want the	old behavior (and think	carefully about	that
       because it's a weird side effect), you can pass a hash slice instead.
       Perl 5.004 didn't make this a special case:

	   my_sub( @hash{ qw/foo/ } );

   How can I make the Perl equivalent of a C structure/C++ class/hash or array
       of hashes or arrays?
       Usually a hash ref, perhaps like	this:

	   $record = {
	       NAME   => "Jason",
	       EMPNO  => 132,
	       TITLE  => "deputy peon",
	       AGE    => 23,
	       SALARY => 37_000,
	       PALS   => [ "Norbert", "Rhys", "Phineas"],
	   };

       References are documented in perlref and	perlreftut.  Examples of
       complex data structures are given in perldsc and	perllol. Examples of
       structures and object-oriented classes are in perlootut.

   How can I use a reference as	a hash key?
       (contributed by brian d foy and Ben Morrow)

       Hash keys are strings, so you can't really use a	reference as the key.
       When you	try to do that,	perl turns the reference into its stringified
       form (for instance, "HASH(0xDEADBEEF)").	From there you can't get back
       the reference from the stringified form,	at least without doing some
       extra work on your own.

       Remember	that the entry in the hash will	still be there even if the
       referenced variable  goes out of	scope, and that	it is entirely
       possible	for Perl to subsequently allocate a different variable at the
       same address. This will mean a new variable might accidentally be
       associated with the value for an	old.

       If you have Perl	5.10 or	later, and you just want to store a value
       against the reference for lookup	later, you can use the core
       Hash::Util::Fieldhash module. This will also handle renaming the	keys
       if you use multiple threads (which causes all variables to be
       reallocated at new addresses, changing their stringification), and
       garbage-collecting the entries when the referenced variable goes	out of
       scope.

       If you actually need to be able to get a	real reference back from each
       hash entry, you can use the Tie::RefHash	module,	which does the
       required	work for you.

   How can I check if a	key exists in a	multilevel hash?
       (contributed by brian d foy)

       The trick to this problem is avoiding accidental	autovivification. If
       you want	to check three keys deep, you might naievely try this:

	   my %hash;
	   if( exists $hash{key1}{key2}{key3} )	{
	       ...;
	   }

       Even though you started with a completely empty hash, after that	call
       to "exists" you've created the structure	you needed to check for
       "key3":

	   %hash = (
		     'key1' => {
				 'key2'	=> {}
			       }
		   );

       That's autovivification.	You can	get around this	in a few ways. The
       easiest way is to just turn it off. The lexical "autovivification"
       pragma is available on CPAN. Now	you don't add to the hash:

	   {
	       no autovivification;
	       my %hash;
	       if( exists $hash{key1}{key2}{key3} ) {
		   ...;
	       }
	   }

       The Data::Diver module on CPAN can do it	for you	too. Its "Dive"
       subroutine can tell you not only	if the keys exist but also get the
       value:

	   use Data::Diver qw(Dive);

	   my @exists =	Dive( \%hash, qw(key1 key2 key3) );
	   if(	! @exists  ) {
	       ...; # keys do not exist
	   }
	   elsif(  ! defined $exists[0]	 ) {
	       ...; # keys exist but value is undef
	   }

       You can easily do this yourself too by checking each level of the hash
       before you move onto the	next level. This is essentially	what
       Data::Diver does	for you:

	   if( check_hash( \%hash, qw(key1 key2	key3) )	) {
	       ...;
	   }

	   sub check_hash {
	      my( $hash, @keys ) = @_;

	      return unless @keys;

	      foreach my $key (	@keys )	{
		  return unless	eval { exists $hash->{$key} };
		  $hash	= $hash->{$key};
	       }

	      return 1;
	   }

   How can I prevent addition of unwanted keys into a hash?
       Since version 5.8.0, hashes can be restricted to	a fixed	number of
       given keys. Methods for creating	and dealing with restricted hashes are
       exported	by the Hash::Util module.

Data: Misc
   How do I handle binary data correctly?
       Perl is binary-clean, so	it can handle binary data just fine.  On
       Windows or DOS, however,	you have to use	"binmode" for binary files to
       avoid conversions for line endings. In general, you should use
       "binmode" any time you want to work with	binary data.

       Also see	"binmode" in perlfunc or perlopentut.

       If you're concerned about 8-bit textual data then see perllocale.  If
       you want	to deal	with multibyte characters, however, there are some
       gotchas.	See the	section	on Regular Expressions.

   How do I determine whether a	scalar is a number/whole/integer/float?
       Assuming	that you don't care about IEEE notations like "NaN" or
       "Infinity", you probably	just want to use a regular expression (see
       also perlretut and perlre):

	   use 5.010;

	   if (	/\D/ )
	       { say "\thas nondigits";	}
	   if (	/^\d+\z/ )
	       { say "\tis a whole number"; }
	   if (	/^-?\d+\z/ )
	       { say "\tis an integer";	}
	   if (	/^[+-]?\d+\z/ )
	       { say "\tis a +/- integer"; }
	   if (	/^-?(?:\d+\.?|\.\d)\d*\z/ )
	       { say "\tis a real number"; }
	   if (	/^[+-]?(?=\.?\d)\d*\.?\d*(?:e[+-]?\d+)?\z/i )
	       { say "\tis a C float" }

       There are also some commonly used modules for the task.	Scalar::Util
       (distributed with 5.8) provides access to perl's	internal function
       "looks_like_number" for determining whether a variable looks like a
       number. Data::Types exports functions that validate data	types using
       both the	above and other	regular	expressions. Thirdly, there is
       Regexp::Common which has	regular	expressions to match various types of
       numbers.	Those three modules are	available from the CPAN.

       If you're on a POSIX system, Perl supports the "POSIX::strtod" function
       for converting strings to doubles (and also "POSIX::strtol" for longs).
       Its semantics are somewhat cumbersome, so here's	a "getnum" wrapper
       function	for more convenient access. This function takes	a string and
       returns the number it found, or "undef" for input that isn't a C	float.
       The "is_numeric"	function is a front end	to "getnum" if you just	want
       to say, "Is this	a float?"

	   sub getnum {
	       use POSIX qw(strtod);
	       my $str = shift;
	       $str =~ s/^\s+//;
	       $str =~ s/\s+$//;
	       $! = 0;
	       my($num,	$unparsed) = strtod($str);
	       if (($str eq '')	|| ($unparsed != 0) || $!) {
		       return undef;
	       }
	       else {
		   return $num;
	       }
	   }

	   sub is_numeric { defined getnum($_[0]) }

       Or you could check out the String::Scanf	module on the CPAN instead.

   How do I keep persistent data across	program	calls?
       For some	specific applications, you can use one of the DBM modules.
       See AnyDBM_File.	More generically, you should consult the FreezeThaw or
       Storable	modules	from CPAN. Starting from Perl 5.8, Storable is part of
       the standard distribution. Here's one example using Storable's "store"
       and "retrieve" functions:

	   use Storable;
	   store(\%hash, "filename");

	   # later on...
	   $href = retrieve("filename");	# by ref
	   %hash = %{ retrieve("filename") };	# direct to hash

   How do I print out or copy a	recursive data structure?
       The Data::Dumper	module on CPAN (or the 5.005 release of	Perl) is great
       for printing out	data structures. The Storable module on	CPAN (or the
       5.8 release of Perl), provides a	function called	"dclone" that
       recursively copies its argument.

	   use Storable	qw(dclone);
	   $r2 = dclone($r1);

       Where $r1 can be	a reference to any kind	of data	structure you'd	like.
       It will be deeply copied. Because "dclone" takes	and returns
       references, you'd have to add extra punctuation if you had a hash of
       arrays that you wanted to copy.

	   %newhash = %{ dclone(\%oldhash) };

   How do I define methods for every class/object?
       (contributed by Ben Morrow)

       You can use the "UNIVERSAL" class (see UNIVERSAL). However, please be
       very careful to consider	the consequences of doing this:	adding methods
       to every	object is very likely to have unintended consequences. If
       possible, it would be better to have all	your object inherit from some
       common base class, or to	use an object system like Moose	that supports
       roles.

   How do I verify a credit card checksum?
       Get the Business::CreditCard module from	CPAN.

   How do I pack arrays	of doubles or floats for XS code?
       The arrays.h/arrays.c code in the PGPLOT	module on CPAN does just this.
       If you're doing a lot of	float or double	processing, consider using the
       PDL module from CPAN instead--it	makes number-crunching easy.

       See <http://search.cpan.org/dist/PGPLOT>	for the	code.

AUTHOR AND COPYRIGHT
       Copyright (c) 1997-2010 Tom Christiansen, Nathan	Torkington, and	other
       authors as noted. All rights reserved.

       This documentation is free; you can redistribute	it and/or modify it
       under the same terms as Perl itself.

       Irrespective of its distribution, all code examples in this file	are
       hereby placed into the public domain. You are permitted and encouraged
       to use this code	in your	own programs for fun or	for profit as you see
       fit. A simple comment in	the code giving	credit would be	courteous but
       is not required.

perl v5.28.3			  2020-05-14			   PERLFAQ4(1)

NAME | VERSION | DESCRIPTION | Data: Numbers | Data: Dates | Data: Strings | Data: Arrays | Data: Hashes (Associative Arrays) | Data: Misc | AUTHOR AND COPYRIGHT

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