NAME

DESCRIPTION

Data: Numbers

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

the perlnumber manpage show the gory details of number representations and conversions.

To limit the number of decimal places in your numbers, you can use the printf or sprintf function. See the perlop|``Floating Point Arithmetic'' for more details.

Why isn't my octal data interpreted correctly?

This problem shows up most often when people try using chmod() , mkdir() , umask() , or sysopen() , which by widespread tradition typically take permissions in octal.

Note the mistake in the first line was specifying the decimal literal 644, rather than the intended octal literal 0644. The problem can be seen with:

Surely you had not intended chmod(01204, $file); - did you? If you want to use numeric literals as arguments to chmod() et al. then please try to express them as octal constants, that is with a leading zero and with the following digits restricted to the set 0..7.

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

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.

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

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

How do I convert between numeric representations?

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

How do I convert hexadecimal into decimal

Using perl's built in conversion of 0x notation:

$int = 0xDEADBEEF; $dec = sprintf("%d", $int);

Using the hex function:

$int = hex("DEADBEEF"); $dec = sprintf("%d", $int);

Using pack:

$int = unpack("N", pack("H8", substr("0" x 8 . "DEADBEEF", -8))); $dec = sprintf("%d", $int);

Using the CPAN module Bit::Vector:

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

How do I convert from decimal to hexadecimal

Using sprint:

$hex = sprintf("%X", 3735928559);

Using unpack

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

Using Bit::Vector

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

And Bit::Vector supports odd bit counts:

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

How do I convert from octal to decimal

Using Perl's built in conversion of numbers with leading zeros:

$int = 033653337357; # note the leading 0! $dec = sprintf("%d", $int);

Using the oct function:

$int = oct("33653337357"); $dec = sprintf("%d", $int);

Using Bit::Vector:

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

How do I convert from decimal to octal

Using sprintf:

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

Using Bit::Vector

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

How do I convert from binary to decimal

Perl 5.6 lets you write binary numbers directly with the 0b notation:

$number = 0b10110110;

Using pack and ord

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

Using pack and unpack for larger strings

$int = unpack("N", pack("B32", substr("0" x 32 . "11110101011011011111011101111", -32))); $dec = sprintf("%d", $int); # substr() is used to left pad a 32 character string with zeros.

Using Bit::Vector:

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

How do I convert from decimal to binary

Using unpack;

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

Using Bit::Vector:

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

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

Why doesn't & work the way I want it to?

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

Most problems with & and | arise because the programmer thinks they have a number but really it's a string. The rest arise because the programmer says:

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

How do I multiply matrices?

How do I perform an operation on a series of integers?

For example:

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

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

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:

This situation has been fixed in Perl5.005. Use of .. in a for loop will iterate over the range, without creating the entire range.

will not create a list of 500,000 integers.

How can I output Roman numerals?

Why aren't my random numbers random?

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

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

If you want numbers that are more random than rand with srand provides, you should also check out the Math::TrulyRandom module from CPAN. It uses the imperfections in your system's timer to generate random numbers, but this takes quite a while. If you want a better pseudorandom generator than comes with your operating system, look at ``Numerical Recipes in C'' at http://www.nr.com/ .

How do I get a random number between X and Y?

Data: Dates

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

How do I find the current century or millennium?

You can also use the POSIX strftime() function which may be a bit slower but is easier to read and maintain.

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

How can I compare two dates and find the difference?

How can I take a string and turn it into epoch seconds?

How can I find the Julian Day?

Before you immerse yourself too deeply in this, be sure to verify that it is the Julian Day you really want. Are you interested in a way of getting serial days so that you just can tell how many days they are apart or so that you can do also other date arithmetic? If you are interested in performing date arithmetic, this can be done using modules Date::Manip or Date::Calc.

There is too many details and much confusion on this issue to cover in this FAQ, but the term is applied (correctly) to a calendar now supplanted by the Gregorian Calendar, with the Julian Calendar failing to adjust properly for leap years on centennial years (among other annoyances). The term is also used (incorrectly) to mean: [1] days in the Gregorian Calendar; and [2] days since a particular starting time or `epoch', usually 1970 in the Unix world and 1980 in the MS-DOS/Windows world. If you find that it is not the first meaning that you really want, then check out the Date::Manip and Date::Calc modules. (Thanks to David Cassell for most of this text.)

How do I find yesterday's date?

Most people try to use the time rather than the calendar to figure out dates, but that assumes that your days are twenty-four hours each. For most people, there are two days a year when they aren't: the switch to and from summer time throws this off. Russ Allbery offers this solution.

All of this is because during those days when one switches off or onto DST, a ``day'' isn't 24 hours long; it's either 23 or 25.

The explicit settings of $ndst and $tdst are necessary because localtime only says it returns the system tm struct, and the system tm struct at least on Solaris doesn't guarantee any particular positive value (like, say, 1) for isdst, just a positive value. And that value can potentially be negative, if DST information isn't available (this sub just treats those cases like no DST).

Note that between 2am and 3am on the day after the time zone switches off daylight savings time, the exact hour of ``yesterday'' corresponding to the current hour is not clearly defined. Note also that if used between 2am and 3am the day after the change to daylight savings time, the result will be between 3am and 4am of the previous day; it's arguable whether this is correct.

This sub does not attempt to deal with leap seconds (most things don't).

Does Perl have a Year 2000 problem? Is Perl Y2K compliant?

Long answer: The question belies a true understanding of the issue. Perl is just as Y2K compliant as your pencil--no more, and no less. Can you use your pencil to write a non-Y2K-compliant memo? Of course you can. Is that the pencil's fault? Of course it isn't.

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 a list 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?

How do I unescape a string?

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

How do I remove consecutive pairs of characters?

Here's a solution that turns ``abbcccd'' to ``abcd'':

How do I expand function calls in a string?

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

How do I find matching/nesting anything?

If you are serious about writing a parser, there are a number of modules or oddities that will make your life a lot easier. There are the CPAN modules Parse::RecDescent, Parse::Yapp, and Text::Balanced; and the byacc program. Starting from perl 5.8 the Text::Balanced is part of the standard distribution.

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

A more complicated and sneaky approach is to make Perl's regular expression engine do it for you. This is courtesy Dean Inada, and rather has the nature of an Obfuscated Perl Contest entry, but it really does work:

How do I reverse a string?

How do I expand tabs in a string?

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

How do I reformat a paragraph?

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

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

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

How can I access or change N characters of a string?

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

How do I change the Nth occurrence of something?

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

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

How can I count the number of occurrences of a substring within a 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:

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

How do I capitalize all the words on one line?

This has the strange effect of turning ``don't do it'' into "C

To make the whole line upper case:

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

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

This is sometimes referred to as putting something into "title case", but that's not quite accurate. Consider the proper capitalization of the movie I

How can I split a [character] delimited string except when inside [character]?

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

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

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

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

There's also a Text::CSV (Comma-Separated Values) module on CPAN.

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

not only is this unnecessarily slow and destructive, it also fails with embedded newlines. It is much faster to do this operation in two steps:

Or more nicely written as:

This idiom takes advantage of the foreach 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 slice:

How do I pad a string with blanks or pad a number with zeroes?

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

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

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

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

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

How do I extract selected columns from a string?

How do I find the soundex value of a string?

How can I expand variables in text strings?

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

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

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

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

What's wrong with always quoting ``$vars''?

If you get used to writing odd things like these:

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

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:

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.

Why don't my <<HERE documents work?

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

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

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

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

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 whitespace, 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 a list and an array?

As a side note, there's no such thing as a list in scalar context. When you say

you're using the comma operator in scalar context, so it uses the scalar comma operator. There never was a list there at all! This causes the last value to be returned: 9.

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

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

with

The use warnings pragma and the -w flag will warn you about these matters.

How can I remove duplicate elements from a list or array?

a)

If @in is sorted, and you want @out to be sorted: (this assumes all true values in the array)

$prev = "not equal to $in[0]"; @out = grep($_ ne $prev && ($prev = $_, 1), @in);

This is nice in that it doesn't use much extra memory, simulating uniq(1)'s behavior of removing only adjacent duplicates. The ``, 1'' guarantees that the expression is true (so that grep picks it up) even if the $_ is 0, ``'', or undef.

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 = grep {defined} @ary;

How can I tell whether a certain element is contained in a list or array?

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 maintain a hash whose keys are the first array's values.

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:

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:

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

Please do not use

or worse yet

These are slow (checks every element even if the first matches), inefficient (same reason), and potentially buggy (what if there are regex characters in $whatever?). If you're only testing once, then use:

How do I compute the difference of two arrays? How do I compute the intersection of two arrays?

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

How do I test whether two arrays or hashes are equal?

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

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

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

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

If you want the array index, you can iterate through the indices and check the array element at each index until you find one that satisfies the condition.

How do I handle linked lists?

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. For example, imagine a list node like this:

You could walk the list this way:

You could add to the list this way:

But again, Perl's built-in are virtually always good enough.

How do I handle circular lists?

How do I shuffle an array randomly?

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

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

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

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?

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

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

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

How do I select a random element from an array?

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 is discussed in Volume 4 (still unpublished) of Knuth's The Art of Computer Programming and will work on any list:

How do I sort an array by (anything)?

The default sort function is cmp, string comparison, which would sort (1, 2, 10) into (1, 10, 2). C<< <=> >>, 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.

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

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

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

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

See also the question below on sorting hashes.

How do I manipulate arrays of bits?

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

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

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

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

Or use the CPAN module Bit::Vector:

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

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

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

Data: Hashes (Associative Arrays)

How do I process an entire hash?

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?

[lwall] In Perl 4, you were not allowed to modify a hash at all while iterating over it. In Perl 5 you can delete from it, but you still can't add to it, because that might cause a doubling of the hash table, in which half the entries get copied up to the new top half of the table, at which point you've totally bamboozled the iterator code. Even if the table doesn't double, there's no telling whether your new entry will be inserted before or after the current iterator position.

Either treasure up your changes and make them after the iterator finishes or use keys to fetch all the old keys at once, and iterate over the list of keys.

How do I look up a hash element by value?

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

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

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

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

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

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

How can I always keep my hash sorted?

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

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

And these conditions hold

If you now say

your table now reads:

and these conditions now hold; changes in caps:

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

Now, consider this:

your table now reads:

and these conditions now hold; changes in caps:

See, the whole entry is gone!

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

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

How can I get the unique keys from two hashes?

Or more succinctly:

Or if you really want to save space:

How can I store a multidimensional array in a DBM file?

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

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

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?

References are documented in the perlref manpage and the upcoming the perlreftut 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?

Data: Misc

How do I handle binary data correctly?

On less elegant (read: Byzantine) systems, however, you have to play tedious games with ``text'' versus ``binary'' files. See ``binmode'' or the perlopentut 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?

You can also use the Data::Types|Data::Types module on the CPAN, which exports functions that validate data types using these and other regular expressions, or you can use the Regexp::Common module from CPAN which has regular expressions to match various types of numbers.

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?''

Or you could check out the String::Scanf|String::Scanf module on the CPAN instead. The POSIX module (part of the standard Perl distribution) provides the strtod and strtol for converting strings to double and longs, respectively.

How do I keep persistent data across program calls?

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

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

How do I define methods for every class/object?

How do I verify a credit card checksum?

How do I pack arrays of doubles or floats for XS code?

AUTHOR AND COPYRIGHT

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

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