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```#!/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;
}
[download]```

In reply to The Definitive Unit Conversion Script by Aristotle

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