Pairs as hash keys - perl

Question

Does anyone know how to make a hash with pairs of strings serving as keys in perl?
Something like...
{
($key1, $key2) => $value1;
($key1, $key3) => $value2;
($key2, $key3) => $value3;
etc....

Answer 1

You can't have a pair of scalars as a hash key, but you can make a multilevel hash:
my %hash;
$hash{$key1}{$key2} = $value1;
$hash{$key1}{$key3} = $value2;
$hash{$key2}{$key3} = $value3;
If you want to define it all at once:
my %hash = ( $key1 => { $key2 => $value1, $key3 => $value2 },
$key2 => { $key3 => $value3 } );
Alternatively, if it works for your situation, you could just concatenate your keys together
$hash{$key1 . $key2} = $value1; # etc
Or add a delimiter to separate the keys:
$hash{"$key1:$key2"} = $value1; # etc

Answer 2

You could use an invisible separator to join the coordinates:
Primarily for mathematics, the Invisible Separator (U+2063) provides a separator between characters where punctuation or space may be omitted such as in a two-dimensional index like i⁣j.
#!/usr/bin/env perl
use utf8;
use v5.12;
use strict;
use warnings;
use warnings qw(FATAL utf8);
use open qw(:std :utf8);
use charnames qw(:full :short);
use YAML;
my %sparse_matrix = (
mk_key(34,56) => -1,
mk_key(1200,11) => 1,
);
print Dump \%sparse_matrix;
sub mk_key { join("\N{INVISIBLE SEPARATOR}", #_) }
sub mk_vec { map [split "\N{INVISIBLE SEPARATOR}"], #_ }
~/tmp> perl mm.pl |xxd
0000000: 2d2d 2d0a 3132 3030 e281 a331 313a 2031 ---.1200...11: 1
0000010: 0a33 34e2 81a3 3536 3a20 2d31 0a .34...56: -1.

Answer 3

Usage: Multiple keys of a single value in a hash can be used for implementing a 2D matrix or N-dimensional matrix!
#!/usr/bin/perl -w
use warnings;
use strict;
use Data::Dumper;
my %hash = ();
my ($a, $b, $c) = (2,3,4);
$hash{"$a, $b ,$c"} = 1;
$hash{"$b, $c ,$a"} = 1;
foreach(keys(%hash) )
{
my #a = split(/,/, $_);
print Dumper(#a);
}

Answer 4

I do this:
{ "$key1\x1F$key2" => $value, ... }
Usually with a helper method:
sub getKey() {
return join( "\x1F", #_ );
}
{ getKey( $key1, $key2 ) => $value, ... }
----- EDIT -----
Updated the code above to use the ASCII Unit Separator per the recommendation from #chepner above

Answer 5

Use $; implicitly (or explicitly) in your hash keys, used for multidimensional emulation, like so:
my %hash;
$hash{$key1, $key2} = $value; # or %hash = ( $key1.$;.$key2 => $value );
print $hash{$key1, $key2} # returns $value
You can even set $; to \x1F if needed (the default is \034, from SUBSEP in awk):
local $; = "\x1F";