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Gurusamy Sarathy

# NAME

perlfaq4 - Data Manipulation (\$Revision: 1.24 \$, \$Date: 1998/07/05 15:07:20 \$)

# 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)?

The infinite set that a mathematician thinks of as the real numbers can only be approximate on a computer, since the computer only has a finite number of bits to store an infinite number of, um, numbers.

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 "\$#" in perlvar 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. See "Floating-point Arithmetic" in perlop.

## 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?

Remember that int() merely truncates toward 0. 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 "pack" in perlfunc):

``    \$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?

John von Neumann said, ``Anyone who attempts to generate random numbers by deterministic means is, of course, living in a state of sin.''

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 "localtime" in perlfunc):

``    \$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? Is Perl Y2K compliant?

Perl is just as Y2K compliant as your pencil--no more, and no less. 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 for 32-bit machines). 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 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.

That doesn't mean that Perl can't be used to create non-Y2K compliant programs. It can. But so can your pencil. It's the fault of the user, not the language. At the risk of inflaming the NRA: ``Perl doesn't break Y2K, people do.'' See http://language.perl.com/news/y2k.html for a longer exposition.

# 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, mail addresses, etc.) for details.

## 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;``

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 perlref. In general, this is fraught with quoting and readability problems, but it is possible. To interpolate a subroutine call (in 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.

One destructive, inside-out approach that you might try is to pull out the smallest nesting parts one at a time:

``````    while (s//BEGIN((?:(?!BEGIN)(?!END).)*)END/gs) {
# do something with \$1
} ``````

## How do I reverse a string?

Use reverse() in scalar context, as documented in "reverse" in perlfunc.

``    \$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;
@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;``````

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

``````    \$WANT = 3;
\$count = 0;
while (/(\w+)\s+fish\b/gi) {
if (++\$count == \$WANT) {
print "The third fish is a \$1 one.\n";
# Warning: don't `last' out of this loop
}
}``````

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:

``````    \$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;``

You can (and probably should) enable locale awareness of those characters by placing a `use locale` pragma in your program. See perllocale for endless details.

## 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?

Although the simplest approach would seem to be:

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

This is unneccesarily slow, destructive, and fails with embedded newlines. It is much better faster to do this in two steps:

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

Or more nicely written as:

``````    for (\$string) {
s/^\s+//;
s/\s+\$//;
}``````

This idiom takes advantage of the for(each) loop's aliasing behavior to factor out common code. You can do this on several strings at once, or arrays, or even the values of a hash if you use a slide:

``````    # trim whitespace in the scalar, the array,
# and all the values in the hash
foreach (\$scalar, @array, @hash{keys %hash}) {
s/^\s+//;
s/\s+\$//;
}``````

## How do I extract selected columns from a string?

Use substr() or unpack(), both documented in perlfunc. If you prefer thinking in terms of columns instead of widths, you can use this kind of thing:

``````    # determine the unpack format needed to split Linux ps output
# arguments are cut columns
my \$fmt = cut2fmt(8, 14, 20, 26, 30, 34, 41, 47, 59, 63, 67, 72);

sub cut2fmt {
my(@positions) = @_;
my \$template  = '';
my \$lastpos   = 1;
for my \$place (@positions) {
\$template .= "A" . (\$place - \$lastpos) . " ";
\$lastpos   = \$place;
}
\$template .= "A*";
return \$template;
}``````

## 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';``

If those were both global variables, then this would suffice:

``    \$text =~ s/\\$(\w+)/\${\$1}/g;``

But since they are probably lexicals, or at least, they could be, you'd have to do this:

``````    \$text =~ s/(\\$\w+)/\$1/eeg;
die if \$@;                  # needed on /ee, not /e``````

It's probably better in the general case to treat those variables as entries in some special hash. For example:

``````    %user_defs = (
foo  => 23,
bar  => 19,
);
\$text =~ s/\\$(\w+)/\$user_defs{\$1}/g;``````

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

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.

Stringification also destroys arrays.

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

## 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.

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

``````    # all in one
(\$VAR = <<HERE_TARGET) =~ s/^\s+//gm;
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.

``````    (\$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 off. Otherwise, it takes the amount of leading white space found on the first line and removes that much off each subsequent line.

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

This works with leading special strings, dynamically determined:

``````    \$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 white space, with remaining indentation correctly preserved:

``````    \$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 \$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: (this assumes all true values in the array)
``````    \$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. It's less nice in that it won't work with false values like undef, 0, or ""; "0 but true" is ok, though.

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 a list or array contains a certain element?

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. 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 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 );
for (@articles) { vec(\$read,\$_,1) = 1 }``````

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. Both pop and shift are both O(1) operations on perl's dynamic arrays. In the absence of shifts and pops, push in general needs to reallocate on the order every log(N) times, and unshift will need to copy pointers each time.

If you really, really wanted, you could use structures as described in perldsc or perltoot 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?

Use this:

``````    # fisher_yates_shuffle( \@array ) :
# generate a random permutation of @array in place
sub fisher_yates_shuffle {
my \$array = shift;
my \$i;
for (\$i = @\$array; --\$i; ) {
my \$j = int rand (\$i+1);
next if \$i == \$j;
@\$array[\$i,\$j] = @\$array[\$j,\$i];
}
}

fisher_yates_shuffle( \@array );    # permutes @array in place``````

You've probably seen shuffling algorithms that works 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
y/XZ/ZX/;       # swap those letters
}``````

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

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

If you want to do the same thing to modify the values of the hash, you may not use the `values` function, oddly enough. You need a slice:

``````    for \$orbit ( @orbits{keys %orbits} ) {
(\$orbit **= 3) *= (4/3) * 3.14159;
}``````

## How do I select a random element from an array?

Use the rand() function (see "rand" in perlfunc):

``````    # at the top of the program:
srand;                      # not needed for 5.004 and later

# then later on
\$index   = rand @array;
\$element = \$array[\$index];``````

Make sure you only call srand once per program, if then. If you are calling it more than once (such as before each call to rand), you're almost certainly doing something wrong.

## 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 permute() function should work on any list:

``````    #!/usr/bin/perl -n
# tsc-permute: permute each word of input
permute([split], []);
sub permute {
my @items = @{ \$_[0] };
my @perms = @{ \$_[1] };
unless (@items) {
print "@perms\n";
} else {
my(@newitems,@newperms,\$i);
foreach \$i (0 .. \$#items) {
@newitems = @items;
@newperms = @perms;
unshift(@newperms, splice(@newitems, \$i, 1));
permute([@newitems], [@newperms]);
}
}
}``````

## 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.

``````    @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 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 "defined" in perlfunc 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 "each" in perlfunc) 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.

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 perllocale).

``````    @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 "In Memory Databases" in DB_File. The Tie::IxHash module from CPAN might also be instructive.

## 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'}``

``````          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'}``

``````          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 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 perlref). Examples of complex data structures are given in perldsc and perllol. Examples of structures and object-oriented classes are in 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 "binmode" in perlfunc.

If you're concerned about 8-bit ASCII 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.

``````   warn "has nondigits"        if     /\D/;
warn "not a natural number" unless /^\d+\$/;             # rejects -3
warn "not an integer"       unless /^-?\d+\$/;           # rejects +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+))?\$/;``````

If you're on a POSIX system, Perl's supports the `POSIX::strtod` function. 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 } ``````

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 AnyDBM_File. 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 UNIVERSAL).

## How do I verify a credit card checksum?

Get the Business::CreditCard module from CPAN.