1) Try to write a program that does the following. Create a file with these lines in it:

one hello
two world
three goodbye
two apple
one banana
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Now, collect all the words in the second column that have the same word in the first column. You want your program to display this(in any order):

one: hello, banana
two: world, apple
three: goodbye
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Spend an hour trying to write that program.

2) Next, read perlreftut. There is no point in discussing references unless you know the basics.

3) A line such as this:

push @{$hash_name{$key}}, $string;
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is magical. This part:

$hash_name{$key}
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tells perl to retrieve the value from the hash that corresponds to the key. The @{ } around $hash_name{$key} tells perl to convert the value to an array--so you know the retrieved value has to be an array reference. Then push() adds $string to the array.

Now, here is the magic part: if the key does not exist in the hash, normally $hash_name{$key} would return undef, and then you would be using @{ } to convert undef to an array, which doesn't work. However, perl magically creates a reference to an empty array for you, and subsequently @{ } converts the reference to an empty array. Then push adds $string to the empty array.

In effect, the line:

push @{$hash_name{$key}}, $string;
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tells perl to add $string to the array corresponding to $hash_name{$key}, but if the $key does not exist in the hash, create the key with a corresponding empty array, and add $string to the empty array.

Here's the program:

data1.txt:

one hello
two world
three goodbye
two apple
one banana
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========

use strict;
use warnings;
use 5.010;

open my $INFILE, '<', 'data1.txt'
    or die "Couldn't open file: $!";


my %hash;

while (<$INFILE>) {
    chomp;

    my ($first_col, $second_col) = split;
    push @{$hash{$first_col}}, $second_col;
}

while ( my($first_col, $aref) = each %hash ) {
    say "$first_col: ", join(', ', @$aref);
}

close $INFILE;

--output:--
three: goodbye
one: hello, banana
two: world, apple
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