perlfaq4 - Data Manipulation ($Revision: 1.19 $,
$Date: 1997/04/24 22:43:57 $)
The section of the FAQ
answers question related to the manipulation of data as numbers, dates, strings, arrays,
hashes, and miscellaneous data issues.
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 $# 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.
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
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.
To turn a string of 1s and 0s like '10110110' into a
scalar containing its binary value, use the pack() function (documented in pack):
$decimal = pack('B8', '10110110');
Here's an example of going the other way:
$binary_string = join('', unpack('B*', "\x29"));
Use the Math::Matrix or Math::MatrixReal modules
(available from CPAN) or the PDL
extension (also available from CPAN).
To call a function on each element in an array, and
collect the results, use:
@results = map { my_func($_) } @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));
}
Get the http://www.perl.com/CPAN/modules/by-module/Roman
module.
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.
The day of the year is in the array returned by localtime()
(see localtime):
$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.
Use the Date::Manip or Date::DateCalc modules from CPAN.
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.
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.
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.
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.
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.
To turn ``abbcccd'' into ``abccd'':
s/(.)\1/$1/g;
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.
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.
Use reverse() in a scalar context, as
documented in reverse.
$reversed = reverse $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);
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).
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/;
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;
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";
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;
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, C<``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);
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 ($string) {
s/^\s+//;
s/\s+$//;
}
Use substr() or unpack(),
both documented in the perlfunc manpage.
Use the standard Text::Soundex module distributed
with perl.
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+)/$1/eeg;
Which is bizarre enough that you'll probably actually
need an EEG afterwards. :-)
See also ``How do I
expand function calls in a string?'' in this section of the FAQ.
The problem is that those double-quotes force
stringification, coercing numbers and references into strings, even when you don't want
them to be.
If you get used to writing odd things like these:
print "$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.
Check for these three things:
- There
must be no space after the << part.
- There (probably) should be a semicolon at the
end.
- You
can't (easily) have any space in front of the tag.
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.
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;
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?).
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 }, $element;
}
You can use this if you care about the index:
for ($i=0; $i < @array; $i++) {
if ($array[$i] eq "Waldo") {
$found_index = $i;
last;
}
}
Now $found_index has what you want.
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.
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
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] = $_;
}
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
}
Use the rand() function (see rand):
srand; # not needed for 5.004 and later
$index = rand @array;
$element = $array[$index];
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]);
}
}
}
Supply a comparison function to sort()
(described in sort):
@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.htm
for more about this approach.
See also the question below on sorting hashes.
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.)
See defined in
the 5.004 release or later of Perl.
Use the each() function (see each) 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.
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.
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.
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;
You can look into using the DB_File module and tie()
using the $DB_BTREE hash bindings as documented in In
Memory Databases.
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!
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.
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.
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;
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.
Use the Tie::IxHash from CPAN.
use Tie::IxHash;
tie(%myhash, Tie::IxHash);
for ($i=0; $i<20; $i++) {
$myhash{$i} = 2*$i;
}
@keys = keys %myhash;
# @keys = (0,1,2,3,...)
If you say something like:
somefunc($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 $_[0],
it has to be ready to write it back into the caller's version.
This has been fixed as of perl5.004.
Normally, merely accessing a key's value for a
nonexistent key does not cause that key to be forever there. This is different
than awk's behavior.
Use references (documented in the perlref manpage).
Examples of complex data structures are given in the perldsc manpage
and the perllol manpage.
Examples of structures and object-oriented classes are in the perltoot manpage.
You can't do this directly, but you could use the
standard Tie::Refhash module distributed with perl.
Perl is binary clean, so this shouldn't be a problem.
For example, this works fine (assuming the files are found):
if (`cat /vmunix` =~ /gzip/) {
print "Your kernel is GNU-zip enabled!\n";
}
On some systems, however, you have to play tedious
games with ``text'' versus ``binary'' files. See binmode.
If you're concerned about 8-bit ASCII data, then see the perllocale
manpage.
If you want to deal with multibyte characters,
however, there are some gotchas. See the section on Regular Expressions.
Assuming that you don't care about IEEE notations like ``NaN'' or ``Infinity'', you probably just want
to use a regular expression.
warn "has nondigits" if /\D/;
warn "not a whole number" unless /^\d+$/;
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.
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.
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.
Use the UNIVERSAL
class (see the UNIVERSAL manpage).
Get the Business::CreditCard module from CPAN.
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