http://www.perlmonks.org?node_id=219603
Category: Miscellaneous
Author/Contact Info /msg Aristotle
Description:

Just what you'd expect from the title. :-)

New conversions are very easy to add; add a new row to the table, containing the source and destination unit and the factor to multiply by to get one from the other. If it's not a simple factor, add an array with two subrefs, one to convert from source to destination unit and another to convert backwards.

It will automatically keep applying conversions and reverse conversions to all the results it generates until it has calculated all related units from any given one.

Run it and it will tell you how to use it.

#!/usr/bin/perl -w
use strict;
use Getopt::Std;

sub conv {
    my ($val, $fac) = @_;
    ref $fac ?
        $fac->[0]->($val) :
        $val * $fac;
}

sub reverse_conv {
    my ($val, $fac) = @_;
    ref $fac ?
        $fac->[1]->($val) :
        $val / $fac;
}

use constant SRC => 0;
use constant DST => 1;
use constant FAC => 2;

my @table = (
    #### temperature
    [ c => f => [
        sub { ($_[0] * 9/5) + 32 },
        sub { ($_[0] - 32) * 5/9 },
    ]],
    [ c => k => [
        sub { $_[0] + 273.16 },
        sub { $_[0] - 273.16 },
    ]],

    #### mass/weight
    [ kg   =>  g =>   1000 ],
    [ floz =>  g =>     30 ],
    [ lbs  => kg => 0.4536 ],

    #### distance
    [ ft   => m  =>   0.3048 ],
    [ ft   => in =>       12 ],
    [ in   => cm =>     2.54 ],
    [ m    => cm =>      100 ],
    [ yd   => m  =>   0.9144 ],
    [ km   => m  =>     1000 ],
    [ mile => km => 1.609347 ],
);

my @unit = sort keys %{{
    map +($_->[SRC] => 1, $_->[DST] => 1), @table
}};

die << "END_USAGE" unless getopts('ap:', \my %opt) and @ARGV and (0 ==
+ @ARGV % 2);
usage:  unitconv [-a] [-p precision] <val> <unit> [<val> <unit> [..]]
    -p     the number of decimal places [default: 2]

    -a     force all possible conversions to be shown, even if
           insufficient display precision truncates the value to 0

    <unit> can be one of: @unit

    Results will be printed on one line per input val/unit pair.
END_USAGE

my $prec = $opt{p} || 2;

while(@ARGV) {
    my (%val, $keys_before) = reverse splice @ARGV, 0, 2;

    do {
        $keys_before = keys %val;
        for(@table) {
            $val{$_->[DST]} = conv($val{$_->[SRC]}, $_->[FAC])
                if exists $val{$_->[SRC]}
                and not exists $val{$_->[DST]};

            $val{$_->[SRC]} = reverse_conv($val{$_->[DST]}, $_->[FAC])
                if exists $val{$_->[DST]}
                and not exists $val{$_->[SRC]};
        }
    } until ($keys_before == keys %val);

    unless($opt{a}) {
        sprintf("%.*f", $prec, $val{$_}) == 0 and delete $val{$_}
            for keys %val;
    }

    printf "% 8.${prec}f %s " x keys(%val) . "\n",
        map +($val{$_} => $_), sort keys %val;
}