perlfaq4

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NAME

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


DESCRIPTION

       The section of the FAQ answers question related to the
       manipulation of data as 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)?

       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.

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


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

       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 functions.

       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, use:

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

       For example:

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

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

           foreach $iterator (@array) {
               &my_func($iterator);
           }

       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:

           $week_of_year = int($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
       http://www.perl.com/CPAN/authors/David_Muir_Sharnoff/modules/Time/JulianDay.pm.gz,
       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:

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

       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":

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


       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 distribution).

           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.

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


       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:

           $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 <comdog@computerdog.com>):

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

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

       If you want to represent quotation marks inside a
       quotation-mark-delimited field, escape them with
       backslashes (eg, "like \"this\"".  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 ($string) {
               s/^\s+//;
               s/\s+$//;
           }


       How do I extract selected columns from a string?

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

       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 substitution:

           $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 = $old;
           somefunc($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.

       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`;

       with

           @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);


       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.

       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;
                   last;
               }
           }

       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.

       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:

           srand;
           @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:

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

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

       Use for/foreach:

           for (@lines) {
               s/foo/bar/;
               tr[a-z][A-Z];
           }

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

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


       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 = @{ $_[1] };
               unless (@head) {
                   # stop recursing when there are no elements in the head
                   print "@tail\n";
               } else {
                   # for all elements in @head, move one from @head to @tail
                   # and call permut() on the new @head and @tail
                   my(@newhead,@newtail,$i);
                   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 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.

           @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 above.

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

       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:

           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.

       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 use:

           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 hashes.

       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).

           @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:

                 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.

       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)) {
               $seen{$element}++;
           }
           @uniq = keys %seen;

       Or more succinctly:

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

       Or if you really want to save space:

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


       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 behavior.

       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
       manpage.

       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 manpage.

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

       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.

          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.


AUTHOR AND COPYRIGHT

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