Re: Array value changing for some reason

[choroba]

return doesn't change @_.

@_ is an array of aliases to the actual arguments, so changing @_ changes the actual arguments.

sub { ++$_ for @_ }->(my @arr = 'a' .. 'c'); print "@arr";
# Output: b c d

In your case, it's bit more complex. Changing the function to

sub reverseArray {
    my @arr = @_;
    for my $i (0 .. $#arr) {
        @{ $arr[$i] } = reverse @{ $arr[$i] };
    }
}

wouldn't have helped, as you're working with an array of arrays. The inner arrays are represented as references, so even if you store the references in a different @arr, they still refer to the same inner arrays.

To keep the array unchanged, only assign to the copy, don't change the references from the original array.

sub reverseArray {
    my @arr = @_;
    for my $i (0 .. $#arr) {
        $arr[$i] = [ reverse @{ $arr[$i] } ];
    }
}

[Silt]

Thanks, that helped! Even tough I don't understand what "for my $i (0 .. $#arr)" means, changing

sub reverseArray {
    for(my $i=0;$i<scalar(@_);$i++){
        @{$_[$i]} = reverse @{$_[$i]};
    }
}

to

sub reverseArray {
    my @arr = @_;
    for(my $i=0;$i<scalar(@arr);$i++){
        $arr[$i] = [reverse @{$arr[$i]}];
    }
}

did the trick!
So, if I understand it correctly, a subroutine can change variables from the function it was called from.

[kschwab]

Even though I don't understand what "for my $i (0 .. $#arr)"

$#array is the index of the last element of @array. Which is the same as the scalar(@array)-1;

a subroutine can change variables from the function it was called from

Yes, if you modify @_, it changes for the caller.

[jwkrahn]

$#array is the index of the last element of @array. Which is the same as the scalar(@array)-1;

Depending on which version of perl and the value of $[.     :)

[Silt]

What I don't understand is why I get different results in this code:

use strict;
use warnings;


problem([1,2,3]);

sub problem {
    print2DArray(@_);  #Output: 1 2 3
    reverseArray1(@_);
    print2DArray(@_);  #Output: 1 2 3
    reverseArray2(@_);
    print2DArray(@_);  #Output: 3 2 1
}

sub reverseArray1 {
    my @arr = @_;
    for my $i (0 .. $#arr) {
        $arr[$i] = [reverse @{$arr[$i]}];
    }
}

sub reverseArray2 {
    my @arr = @_;
    for my $i (0 .. $#arr) {
        @{$arr[$i]} = reverse @{$arr[$i]};
    }
}

sub print2DArray {
    for my $i (0 .. $#_) {  # How does that work for the nested for lo
+op?
        for(my $j=0;$j<scalar(@{$_[$i]});$j++){  # $#_[$i] doesn't wor
+k
            print $_[$i][$j]," ";
        }
        print "\n";
    }
}

Whats the difference between

$arr[$i] = [reverse @{$arr[$i]}];

and

@{$arr[$i]} = reverse @{$arr[$i]};

?
I still am quite new to perl, and I appreciate all the comments!

[AnomalousMonk]

... I don't understand what "for my $i (0 .. $#arr)" means ...

The other point to remember about this type of loop (a so-called Perl-style for-loop) is that is "topicalizes" (see "topic" in perlglossary)  $i or whatever variable you specify, or implicitly  $_ if you don't explicitly specify any variable, to each element of the loop list; see Foreach Loops in perlsyn. Also note that foreach and for are exactly synonymous and completely interchangeable keywords in Perl; the differing behavior of Perl- and C-style for/foreach-loops is determined by the syntax of the loop list expression.