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PERLFAQ4(1)							   PERLFAQ4(1)

NAME    [Toc]    [Back]

     perlfaq4 -	Data Manipulation ($Revision: 1.19 $, $Date: 1997/04/24
     22:43:57 $)

DESCRIPTION    [Toc]    [Back]

     The section of the	FAQ answers question related to	the manipulation of
     data as numbers, dates, strings, arrays, hashes, and miscellaneous	data

Data: Numbers
     Why am I getting long decimals (eg, 19.9499999999999) instead of the
     numbers I should be getting (eg, 19.95)?

     Internally, your computer represents floating-point numbers in binary.
     Floating-point numbers read in from a file, or appearing as literals in
     your program, are converted from their decimal floating-point
     representation (eg, 19.95)	to the internal	binary representation.

     However, 19.95 can't be precisely represented as a	binary floating-point
     number, just like 1/3 can't be exactly represented	as a decimal
     floating-point number.  The computer's binary representation of 19.95,
     therefore,	isn't exactly 19.95.

     When a floating-point number gets printed,	the binary floating-point
     representation is converted back to decimal.  These decimal numbers are
     displayed in either the format you	specify	with printf(), or the current
     output format for numbers (see the	section	on $# in the perlvar manpage
     if	you use	print.	$# has a different default value in Perl5 than it did
     in	Perl4.	Changing $# yourself is	deprecated.

     This affects all computer languages that represent	decimal	floating-point
     numbers in	binary,	not just Perl.	Perl provides arbitrary-precision
     decimal numbers with the Math::BigFloat module (part of the standard Perl
     distribution), but	mathematical operations	are consequently slower.

     To	get rid	of the superfluous digits, just	use a format (eg,
     printf("%.2f", 19.95)) to get the required	precision.

     Why isn't my octal	data interpreted correctly?

     Perl only understands octal and hex numbers as such when they occur as
     literals in your program.	If they	are read in from somewhere and
     assigned, no automatic conversion takes place.  You must explicitly use
     oct() or hex() if you want	the values converted.  oct() interprets	both
     hex ("0x350") numbers and octal ones ("0350" or even without the leading
     "0", like "377"), while hex() only	converts hexadecimal ones, with	or
     without a leading "0x", like "0x255", "3A", "ff", or "deadbeef".

     This problem shows	up most	often when people try using chmod(), mkdir(),
     umask(), or sysopen(), which all want permissions in octal.

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PERLFAQ4(1)							   PERLFAQ4(1)

	 chmod(644,  $file); # WRONG --	perl -w	catches	this
	 chmod(0644, $file); # right

     Does perl have a round function?  What about ceil() and floor()? Trig

     For rounding to a certain number of digits, sprintf() or printf() is
     usually the easiest route.

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

     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 rounding is being used	by
     Perl, but to instead implement the	rounding function you need yourself.

     How do I convert bits into	ints?

     To	turn a string of 1s and	0s like	'10110110' into	a scalar containing
     its binary	value, use the pack() function (documented in the section on
     pack in the perlfunc manpage):

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

     Here's an example of going	the other way:

	 $binary_string	= join('', unpack('B*',	"\x29"));

     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,

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

     For example:

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PERLFAQ4(1)							   PERLFAQ4(1)

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

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

	 foreach $iterator (@array) {

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

	 @results = map	{ &my_func($_) } (5 .. 25);

     but you should be aware that the .. operator creates an array of all
     integers in the range.  This can take a lot of memory for large ranges.
     Instead use:

	 @results = ();
	 for ($i=5; $i < 500_005; $i++)	{
	     push(@results, &my_func($i));

     How can I output Roman numerals?

     Get the http://www.perl.com/CPAN/modules/by-module/Roman module.

     Why aren't	my random numbers random?

     The short explanation is that you're getting pseudorandom numbers,	not
     random ones, because that's how these things work.	 A longer explanation
     is	available on http://www.perl.com/CPAN/doc/FMTEYEWTK/random, courtesy
     of	Tom Phoenix.

     You should	also check out the Math::TrulyRandom module from CPAN.

Data: Dates
     How do I find the week-of-the-year/day-of-the-year?

     The day of	the year is in the array returned by localtime() (see the
     section on	localtime in the perlfunc manpage):

	 $day_of_year =	(localtime(time()))[7];

     or	more legibly (in 5.004 or higher):

	 use Time::localtime;
	 $day_of_year =	localtime(time())->yday;

     You can find the week of the year by dividing this	by 7:

									Page 3

PERLFAQ4(1)							   PERLFAQ4(1)

	 $week_of_year = int($day_of_year / 7);

     Of	course,	this believes that weeks start at zero.

     How can I compare two date	strings?

     Use the Date::Manip or Date::DateCalc modules from	CPAN.

     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 one of the Date
     modules from CPAN.

     How can I find the	Julian Day?

     Neither Date::Manip nor Date::DateCalc deal with Julian days.  Instead,
     there is an example of Julian date	calculation in
     which should help.

     Does Perl have a year 2000	problem?

     Not unless	you use	Perl to	create one. The	date and time functions
     supplied with perl	(gmtime	and localtime) supply adequate information to
     determine the year	well beyond 2000 (2038 is when trouble strikes).  The
     year returned by these functions when used	in an array context is the
     year minus	1900. For years	between	1910 and 1999 this happens to be a 2-
     digit decimal number. To avoid the	year 2000 problem simply do not	treat
     the year as a 2-digit number.  It isn't.

     When gmtime() and localtime() are used in a scalar	context	they return a
     timestamp string that contains a fully-expanded year.  For	example,
     $timestamp	= gmtime(1005613200) sets $timestamp to	"Tue Nov 13 01:00:00
     2001".  There's no	year 2000 problem here.

Data: Strings
     How do I validate input?

     The answer	to this	question is usually a regular expression, perhaps with
     auxiliary logic.  See the more specific questions (numbers, email
     addresses,	etc.) for details.

     How do I unescape a string?

     It	depends	just what you mean by "escape".	 URL escapes are dealt with in
     the perlfaq9 manpage.  Shell escapes with the backslash (\) character are
     removed with:

									Page 4

PERLFAQ4(1)							   PERLFAQ4(1)


     Note that this won't expand \n or \t or any other special escapes.

     How do I remove consecutive pairs of characters?

     To	turn "abbcccd" into "abccd":


     How do I expand function calls in a string?

     This is documented	in the perlref manpage.	 In general, this is fraught
     with quoting and readability problems, but	it is possible.	 To
     interpolate a subroutine call (in a list context) into a string:

	 print "My sub returned	@{[mysub(1,2,3)]} that time.\n";

     If	you prefer scalar context, similar chicanery is	also useful for
     arbitrary expressions:

	 print "That yields ${\($n + 5)} widgets\n";

     See also "How can I expand	variables in text strings?" in this section of
     the FAQ.

     How do I find matching/nesting anything?

     This isn't	something that can be tackled in one regular expression, no
     matter how	complicated.  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.  But none of these	deals with nested patterns, nor	can they.  For
     that you'll have to write a parser.

     How do I reverse a	string?

     Use reverse() in a	scalar context,	as documented in the reverse entry in
     the perlfunc manpage.

	 $reversed = reverse $string;

     How do I expand tabs in a string?

     You can do	it the old-fashioned way:

	 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

									Page 5

PERLFAQ4(1)							   PERLFAQ4(1)

	 use Text::Tabs;
	 @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 may not contain embedded newlines.
     Text::Wrap	doesn't	justify	the lines (flush-right).

     How can I access/change the first N letters of a string?

     There are many ways.  If you just want to grab a copy, use	substr:

	 $first_byte = substr($a, 0, 1);

     If	you want to modify part	of a string, the simplest way is often to use
     substr() as an lvalue:

	 substr($a, 0, 3) = "Tom";

     Although those with a regexp kind of thought process will likely prefer

	 $a =~ s/^.../Tom/;

     How do I change the Nth occurrence	of something?

     You have to keep track.  For example, let's say you want to change	the
     fifth occurrence of "whoever" or "whomever" into "whosoever" or
     "whomsoever", case	insensitively.

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

     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:

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	 $string = "ThisXlineXhasXsomeXx'sXinXit":
	 $count	= ($string =~ tr/X//);
	 print "There are $count X charcters 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:

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

     How do I capitalize all the words on one line?

     To	make the first letter of each word upper case:

	     $line =~ s/\b(\w)/\U$1/g;

     This has the strange effect of turning "don't do it" into "Don'T Do It".
     Sometimes you might want this, instead (Suggested by Brian	Foy

	 $string =~ s/ (
		      (^\w)    #at the beginning of the	line
			|      # or
		      (\s\w)   #preceded by whitespace
	 $string =~ /([\w']+)/\u\L$1/g;

     To	make the whole line upper case:

	     $line = uc($line);

     To	force each word	to be lower case, with the first letter	upper case:

	     $line =~ s/(\w+)/\u\L$1/g;

     How can I split a [character] delimited string except when	inside
     [character]? (Comma-separated files)

     Take the example case of trying to	split a	string that is comma-separated
     into its different	fields.	 (We'll	pretend	you said comma-separated, not
     comma-delimited, which is different and almost never what you mean.) You
     can't use split(/,/) because you shouldn't	split if the comma is inside
     quotes.  For example, take	a data line like this:

									Page 7

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	 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 a highly recommended	book
     on	regular	expressions, to	handle these for us.  He suggests (assuming
     your string is contained in $text):

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

     If	you want to represent quotation	marks inside a quotation-markdelimited
 field, escape them with backslashes (eg,	"like \"this\"".
     Unescaping	them is	a task addressed earlier in this section.

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

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

     How do I strip blank space	from the beginning/end of a string?

     The simplest approach, albeit not the fastest, is probably	like this:

	 $string =~ s/^\s*(.*?)\s*$/$1/;

     It	would be faster	to do this in two steps:

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

     Or	more nicely written as:

	 for ($string) {

     How do I extract selected columns from a string?

     Use substr() or unpack(), both documented in the perlfunc manpage.

									Page 8

PERLFAQ4(1)							   PERLFAQ4(1)

     How do I find the soundex value of	a string?

     Use the standard Text::Soundex module distributed with perl.

     How can I expand variables	in text	strings?

     Let's assume that you have	a string like:

	 $text = 'this has a $foo in it	and a $bar';
	 $text =~ s/\$(\w+)/${$1}/g;

     Before version 5 of perl, this had	to be done with	a double-eval

	 $text =~ s/(\$\w+)/$1/eeg;

     Which is bizarre enough that you'll probably actually need	an EEG
     afterwards. :-)

     See also "How do I	expand function	calls in a string?" in this section of
     the FAQ.

     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.

     If	you get	used to	writing	odd things like	these:

	 print "$var";	     # BAD
	 $new =	"$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;
	 $new =	$old;

     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

	 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,

									Page 9

PERLFAQ4(1)							   PERLFAQ4(1)

     such as the magical ++ autoincrement operator or the syscall() function.

     Why don't my <<HERE documents work?

     Check for these three things:

     1.	There must be no space after the << part.

     2.	There (probably) should	be a semicolon at the end.

     3.	You can't (easily) have	any space in front of the tag.

Data: Arrays
     What is the difference between $array[1] and @array[1]?

     The former	is a scalar value, the latter an array slice, which makes it a
     list with one (scalar) value.  You	should use $ when you want a scalar
     value (most of the	time) and @ when you want a list with one scalar value
     in	it (very, very rarely; nearly never, in	fact).

     Sometimes it doesn't make a difference, but sometimes it does.  For
     example, compare:

	 $good[0] = `some program that outputs several lines`;


	 @bad[0]  = `same program that outputs several lines`;

     The -w flag will warn you about these matters.

     How can I extract just the	unique elements	of an array?

     There are several possible	ways, depending	on whether the array is
     ordered and whether you wish to preserve the ordering.

     a)	If @in is sorted, and you want @out to be sorted:

	     $prev = 'nonesuch';
	     @out = grep($_ ne $prev &&	($prev = $_), @in);

	 This is nice in that it doesn't use much extra	memory,	simulating
	 uniq(1)'s behavior of removing	only adjacent duplicates.

     b)	If you don't know whether @in is sorted:

	     undef %saw;
	     @out = grep(!$saw{$_}++, @in);

								       Page 10

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     c)	Like (b), but @in contains only	small integers:

	     @out = grep(!$saw[$_]++, @in);

     d)	A way to do (b)	without	any loops or greps:

	     undef %saw;
	     @saw{@in} = ();
	     @out = sort keys %saw;  # remove sort if undesired

     e)	Like (d), but @in contains only	small positive integers:

	     undef @ary;
	     @ary[@in] = @in;
	     @out = @ary;

     How can I tell whether an array contains a	certain	element?

     There are several ways to approach	this.  If you are going	to make	this
     query many	times and the values are arbitrary strings, the	fastest	way is
     probably to invert	the original array and keep an associative array lying
     about whose keys are the first array's values.

	 @blues	= qw/azure cerulean teal turquoise lapis-lazuli/;
	 undef %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:

	 @primes = (2, 3, 5, 7,	11, 13,	17, 19,	23, 29,	31);
	 undef @is_tiny_prime;
	 for (@primes) { $is_tiny_prime[$_] = 1; }

     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:

	 @articles = ( 1..10, 150..2000, 2017 );
	 undef $read;
	 grep (vec($read,$_,1) = 1, @articles);

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

								       Page 11

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     Please do not use

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

     or	worse yet

	 $is_there = grep /$whatever/, @array;

     These are slow (checks every element even if the first matches),
     inefficient (same reason),	and potentially	buggy (what if there are
     regexp characters in $whatever?).

     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:

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

     How do I find the first array element for which a condition is true?

     You can use this if you care about	the index:

	 for ($i=0; $i < @array; $i++) {
	     if	($array[$i] eq "Waldo")	{
		 $found_index =	$i;

     Now $found_index has what you want.

     How do I handle linked lists?

     In	general, you usually don't need	a linked list in Perl, since with
     regular arrays, you can push and pop or shift and unshift at either end,
     or	you can	use splice to add and/or remove	arbitrary number of elements
     at	arbitrary points.

     If	you really, really wanted, you could use structures as described in
     the perldsc manpage or the	perltoot manpage and do	just what the
     algorithm book tells you to do.

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     How do I handle circular lists?

     Circular lists could be handled in	the traditional	fashion	with linked
     lists, or you could just do something like	this with an array:

	 unshift(@array, pop(@array));	# the last shall be first
	 push(@array, shift(@array));	# and vice versa

     How do I shuffle an array randomly?

     Here's a shuffling	algorithm which	works its way through the list,
     randomly picking another element to swap the current element with:

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

     For large arrays, this avoids a lot of the	reshuffling:

	 @new =	();
	 @old =	1 .. 10000;  # just a demo
	 for( @old ){
	     my	$r = rand @new+1;
	     $new[$r] =	$_;

     How do I process/modify each element of an	array?

     Use for/foreach:

	 for (@lines) {

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

	 for (@radii) {
	     $_	**= 3;
	     $_	*= (4/3) * 3.14159;  # this will be constant folded

								       Page 13

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     How do I select a random element from an array?

     Use the rand() function (see the rand entry in the	perlfunc manpage):

	 srand;			     # not needed for 5.004 and	later
	 $index	  = rand @array;
	 $element = $array[$index];

     How do I permute N	elements of a list?

     Here's a little program that generates all	permutations of	all the	words
     on	each line of input.  The algorithm embodied in the permut() function
     should work on any	list:

	 #!/usr/bin/perl -n
	 # permute - tchrist@perl.com
	 permut([split], []);
	 sub permut {
	     my	@head =	@{ $_[0] };
	     my	@tail =	@{ $_[1] };
	     unless (@head) {
		 # stop	recursing when there are no elements in	the head
		 print "@tail\n";
	     } else {
		 # for all elements in @head, move one from @head to @tail
		 # and call permut() on	the new	@head and @tail
		 foreach $i (0 .. $#head) {
		     @newhead =	@head;
		     @newtail =	@tail;
		     unshift(@newtail, splice(@newhead,	$i, 1));
		     permut([@newhead],	[@newtail]);

     How do I sort an array by (anything)?

     Supply a comparison function to sort() (described in the sort entry in
     the perlfunc manpage):

	 @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

     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

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     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.

	 @idx =	();
	 for (@data) {
	     ($item) = /\d+\s*(\S+)/;
	     push @idx,	uc($item);
	 @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:

	 @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.

	 @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

     See http://www.perl.com/CPAN/doc/FMTEYEWTK/sort.html for more about this

     See also the question below on sorting hashes.

     How do I manipulate arrays	of bits?

     Use pack()	and unpack(), or else vec() and	the bitwise operations.

     For example, this sets $vec to have bit N set if $ints[N] was set:

	 $vec =	'';
	 foreach(@ints)	{ vec($vec,$_,1) = 1 }

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

								       Page 15

PERLFAQ4(1)							   PERLFAQ4(1)

	 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.)

     Why does defined()	return true on empty arrays and	hashes?

     See the defined entry in the perlfunc manpage in the 5.004	release	or
     later of Perl.

Data: Hashes (Associative Arrays)
     How do I process an entire	hash?

     Use the each() function (see the each entry in the	perlfunc manpage) if
     you don't care whether it's sorted:

	 while (($key,$value) =	each %hash) {
	     print "$key = $value\n";

     If	you want it sorted, you'll have	to use foreach() on the	result of
     sorting the keys as shown in an earlier question.

								       Page 16

PERLFAQ4(1)							   PERLFAQ4(1)

     What happens if I add or remove keys from a hash while iterating over it?

     Don't do that.

     How do I look up a	hash element by	value?

     Create a reverse hash:

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

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

	 while (($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.

     How can I know how	many entries are in a hash?

     If	you mean how many keys,	then all you have to do	is take	the scalar
     sense of the keys() function:

	 $num_keys = scalar keys %hash;

     In	void context it	just resets the	iterator, which	is faster for tied

     How do I sort a hash (optionally by value instead of key)?

     Internally, hashes	are stored in a	way that prevents you from imposing an
     order on key-value	pairs.	Instead, you have to sort a list of the	keys
     or	values:

	 @keys = sort keys %hash;    # sorted by key
	 @keys = sort {
			 $hash{$a} cmp $hash{$b}
		 } keys	%hash;	     # and by value

     Here we'll	do a reverse numeric sort by value, and	if two keys are
     identical,	sort by	length of key, and if that fails, by straight ASCII
     comparison	of the keys (well, possibly modified by	your locale -- see the
     perllocale	manpage).

								       Page 17

PERLFAQ4(1)							   PERLFAQ4(1)

	 @keys = sort {
		     $hash{$b} <=> $hash{$a}
		     length($b)	<=> length($a)
			   $a cmp $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 the	section	on In Memory Databases in the
     DB_File manpage.

     What's the	difference between "delete" and	"undef"	with hashes?

     Hashes are	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 $key is
     present in	the array, exists($key)	will return true.  The value for a
     given key can be undef, in	which case $array{$key}	will be	undef while
     $exists{$key} will	return true.  This corresponds to ($key, undef)	being
     in	the hash.

     Pictures help...  here's the %ary table:

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

     And these conditions hold

	     $ary{'a'}			     is	true
	     $ary{'d'}			     is	false
	     defined $ary{'d'}		     is	true
	     defined $ary{'a'}		     is	true
	     exists $ary{'a'}		     is	true (perl5 only)
	     grep ($_ eq 'a', keys %ary)     is	true

     If	you now	say

	     undef $ary{'a'}

     your table	now reads:

								       Page 18

PERLFAQ4(1)							   PERLFAQ4(1)

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

     and these conditions now hold; changes in caps:

	     $ary{'a'}			     is	FALSE
	     $ary{'d'}			     is	false
	     defined $ary{'d'}		     is	true
	     defined $ary{'a'}		     is	FALSE
	     exists $ary{'a'}		     is	true (perl5 only)
	     grep ($_ eq 'a', keys %ary)     is	true

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

     Now, consider this:

	     delete $ary{'a'}

     your table	now reads:

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

     and these conditions now hold; changes in caps:

	     $ary{'a'}			     is	false
	     $ary{'d'}			     is	false
	     defined $ary{'d'}		     is	true
	     defined $ary{'a'}		     is	false
	     exists $ary{'a'}		     is	FALSE (perl5 only)
	     grep ($_ eq 'a', keys %ary)     is	FALSE

     See, the whole entry is gone!

     Why don't my tied hashes make the defined/exists distinction?

     They may or may not implement the EXISTS()	and DEFINED() methods
     differently.  For example,	there isn't the	concept	of undef with hashes
     that are tied to DBM* files. This means the true/false tables above will
     give different results when used on such a	hash.  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.

								       Page 19

PERLFAQ4(1)							   PERLFAQ4(1)

     How do I reset an each() operation	part-way through?

     Using keys	%hash in a scalar context returns the number of	keys in	the
     hash and resets the iterator associated with the hash.  You may need to
     do	this if	you use	last to	exit a loop early so that when you re-enter
     it, the hash iterator has been reset.

     How can I get the unique keys from	two hashes?

     First you extract the keys	from the hashes	into arrays, and then solve
     the uniquifying the array problem described above.	 For example:

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

     Or	more succinctly:

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

     Or	if you really want to save space:

	 %seen = ();
	 while (defined	($key =	each %foo)) {
	 while (defined	($key =	each %bar)) {
	 @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.

     How can I make my hash remember the order I put elements into it?

     Use the Tie::IxHash from CPAN.

	 use Tie::IxHash;
	 tie(%myhash, Tie::IxHash);
	 for ($i=0; $i<20; $i++) {
	     $myhash{$i} = 2*$i;
	 @keys = keys %myhash;
	 # @keys = (0,1,2,3,...)

								       Page 20

PERLFAQ4(1)							   PERLFAQ4(1)

     Why does passing a	subroutine an undefined	element	in a hash create it?

     If	you say	something like:

	 somefunc($hash{"nonesuch key here"});

     Then that element "autovivifies"; that is,	it springs into	existence
     whether you store something there or not.	That's because functions get
     scalars passed in by reference.  If somefunc() modifies $_[0], it has to
     be	ready to write it back into the	caller's version.

     This has been fixed as of perl5.004.

     Normally, merely accessing	a key's	value for a nonexistent	key does not
     cause that	key to be forever there.  This is different than awk's

     How can I make the	Perl equivalent	of a C structure/C++ class/hash	or
     array of hashes or	arrays?

     Use references (documented	in the perlref manpage).  Examples of complex
     data structures are given in the perldsc manpage and the perllol manpage.
     Examples of structures and	object-oriented	classes	are in the perltoot

     How can I use a reference as a hash key?

     You can't do this directly, but you could use the standard	Tie::Refhash
     module distributed	with perl.

Data: Misc
     How do I handle binary data correctly?

     Perl is binary clean, so this shouldn't be	a problem.  For	example, this
     works fine	(assuming the files are	found):

	 if (`cat /vmunix` =~ /gzip/) {
	     print "Your kernel	is GNU-zip enabled!\n";

     On	some systems, however, you have	to play	tedious	games with "text"
     versus "binary" files.  See the section on	binmode	in the perlfunc

     If	you're concerned about 8-bit ASCII data, then see the perllocale

     If	you want to deal with multibyte	characters, however, there are some
     gotchas.  See the section on Regular Expressions.

								       Page 21

PERLFAQ4(1)							   PERLFAQ4(1)

     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.

	warn "has nondigits"	    if	   /\D/;
	warn "not a whole number"   unless /^\d+$/;
	warn "not an integer"	    unless /^-?\d+$/;  # reject	+3
	warn "not an integer"	    unless /^[+-]?\d+$/;
	warn "not a decimal number" unless /^-?\d+\.?\d*$/;  # rejects .2
	warn "not a decimal number" unless /^-?(?:\d+(?:\.\d*)?|\.\d+)$/;
	warn "not a C float"
	    unless /^([+-]?)(?=\d|\.\d)\d*(\.\d*)?([Ee]([+-]?\d+))?$/;

     Or	you could check	out http://www.perl.com/CPAN/modules/by-
     module/String/String-Scanf-1.1.tar.gz instead.  The POSIX module (part of
     the standard Perl distribution) provides the strtol and strtod for
     converting	strings	to double and longs, respectively.

     How do I keep persistent data across program calls?

     For some specific applications, you can use one of	the DBM	modules.  See
     the AnyDBM_File manpage.  More generically, you should consult the
     FreezeThaw, Storable, or Class::Eroot modules from	CPAN.

     How do I print out	or copy	a recursive data structure?

     The Data::Dumper module on	CPAN is	nice for printing out data structures,
     and FreezeThaw for	copying	them.  For example:

	 use FreezeThaw	qw(freeze thaw);
	 $new =	thaw freeze $old;

     Where $old	can be (a reference to)	any kind of data structure you'd like.
     It	will be	deeply copied.

     How do I define methods for every class/object?

     Use the UNIVERSAL class (see the UNIVERSAL	manpage).

     How do I verify a credit card checksum?

     Get the Business::CreditCard module from CPAN.


     Copyright (c) 1997	Tom Christiansen and Nathan Torkington.	 All rights
     reserved.	See the	perlfaq	manpage	for distribution information.

								       Page 22

PERLFAQ4(1)							   PERLFAQ4(1)

								       PPPPaaaaggggeeee 22223333
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