How to print specific key in an array (Perl) [duplicate] - perl

Question

This question already has answers here:
Simple hash search by value
(5 answers)
Closed 5 years ago.
I recently started learning Perl, so I'm not too familiar with the functions and syntax.
If I have a Perl array and some variables,
#!/usr/bin/perl
use strict;
use warnings;
my #numbers = (a =>1, b=> 2, c => 3, d =>4, e => 5);
my $x;
my $range = 5;
$x = int(rand($range));
print "$x";
to generate a random number between 1-5, how can I get the program to print the actual key (a, b, c, etc.) instead of just the number (1, 2, 3, 4, 5)?

Answer 1

It seems that you want to do a reverse lookup, key-by-value, opposite to what we get from a hash. Since a hash is a list you can reverse it and use the resulting hash to look up by number.
A couple of corrections: you need a hash variable (not an array), and you need to add 1 to your rand integer generator so to have the desired 1..5 range
use warnings;
use strict;
use feature 'say';
my %numbers = (a => 1, b => 2, c => 3, d => 4, e => 5);
my %lookup_by_number = reverse %numbers; # values need be unique
my $range = 5;
my $x = int(rand $range) + 1;
say $lookup_by_number{$x};
Without reversing the hash you'd need to iterate the hash %numbers over values, testing each against $x so to find its key.
If there are same values for various keys in your original hash then you have to do it by hand since reverse-ing would attempt to create a hash with duplicate keys, in which case only the last one assigned remains. So you'd lose some values. One way
my #at_num = grep { $x == $numbers{$_} } keys %numbers;
as in the post that this was marked as duplicate of.
But then you should build a data structure for reverse lookup so to not search through the list every time information is needed. This can be a hash where keys are the list of unique numbers while their values are then array references (arrayrefs) with corresponding keys from the original hash
use warnings;
use strict;
my %num = (a => 1, b => 2, c => 1, d => 3, e => 2); # with duplicate values
my %lookup_by_num;
foreach my $key (keys %num) {
push #{ $lookup_by_num{$num{$key}} }, $key;
}
say "$_ => [ #{$lookup_by_num{$_}} ]" for keys %lookup_by_num;
This prints
1 => [ c a ]
3 => [ d ]
2 => [ e b ]
A nice way to display complex data structures is via Data::Dumper, or Data::Dump (or others).
The expression #{ $lookup_by_num{ $num{$key} } } extracts the value of %lookup_by_num for the key $num{$key}and dereferences it #{ ... }, so that it can then push the $key to it. The critical part of this is that the first time it encounters $num{$key} it autovivifies the arrayref and its corresponding key. See this post with its references for details.

Answer 2

There's many ways to do it. For example, declare "numbers" as a hash rather than an array. Note that the keys come first in each key-value pair, and here you want to use your random int as the key:
my %numbers = ( 0 => 'a', 1 => 'b', 2 => 'c', 3 => 'd', 4 => 'e' );
Then you can look up the "key" as you call it using:
my $key = $numbers{$x};
Note that rand( $x ); returns a number greater than or equal to zero and less than $x. So if you want integers in the range 1-5, you must add 1 in your code: at the moment you'll get 0-4, not 1-5.

Answer 3

Firstly, arrays don't have keys (well, they kind of do, but they're integers and not the values you want). So I think you want a hash, not an array.
my %numbers = (a =>1, b=> 2, c => 3, d =>4, e => 5);
And if you want to get the letter, given the integer then you need the reverse of this hash:
my %rev_numbers = %numbers;
Note that reversing a hash like this only works if the values in your original hash are unique (because reversing a hash makes the values into keys and hash keys are always unique).
Then, you can just look up an integer in your %rev_hash to get its associated letter.
my $integer = 3;
say $rev_numbers{$integer}; # prints 'c'