looping through a list in Perl

Question

I am starting to learn Perl and this is just a basic little for loop for which I get a strange output and was hoping for some clarity on this.

@numbers = {1,4,5,6,7,8,9};
for(my$i = 0; $i<=$#numbers; $i++)
{
    print ("$numbers[$i}\n");
}

the output is HASH(0x23a09c).

What does this actually mean and why do I get this result.

regards Arian

Answer 1

You want this:

@numbers = (1,4,5,6,7,8,9);
foreach my $number (@numbers)
{
    print ("$number\n");
}

With {1,4,5,6,7,8,9} you're actually creating a reference to an anonymous hash containing the key value pairs (1 => 4, 5 => 6, 7 => 8, 9 => undef). When you write @numbers = {1,4,5,6,7,8,9}; that reference becomes the sole scalar stored in the @numbers array.

Furthermore, if you just want to iterate over the elements, no need to use the "classic" style with a counter.

You can do:

for my $number (1 .. 9) {
    print "$number\n";
}

Make sure you have use strict; and use warnings; at the beginning of every Perl script you write. Those directives enable perl to catch errors and warn about certain possibly erroneous code. As a beginner, you might want to couple those with warnings with diagnostics to get more detailed information.

These are very handy, especially when starting out with Perl as they help you prevent shooting yourself in the foot.

Answer 2

What you're doing there is creating an array with a hash ref at index 0, i.e.:

$numbers[0] = {
    1 => 4,
    5 => 6,
    7 => 8,
    9 => undef,
};

If you had used strict you would have seen:

Global symbol "@numbers" requires explicit package name

And warnings would have told you:

Odd number of elements in anonymous hash

Start all your perl scripts with something like:

#!/usr/bin/perl -w
use strict;

(alternatively use warnings;) and you'll always be able to catch these kinds of errors right away. Even seasoned Perl programmers make them from time to time. There's really never any good reason to leave them out (unless you're golfing that is).

Answer 3

try:

@numbers = (1,4,5,6,7,8,9);
foreach(@numbers) {
    print $_;
}