Perl deleting array elements after a few iterations of code, with a call to a sub function

Question

My code works for the first few iterations, but after a few times through the while loop, it seems that my array elements are being deleted.

I'm taking numbers off the array constructed from the input parameters and all I can tell is that when I get to a number which was passed in twice, I get an error.

I am calling my script like this

./branchandboundNoComments.pl 1 2 3 4 5 5 7 7 9 9 10 10 12 14 19

I should get this as output

0, 7, 9, 10, 14, 19

This is my script

#!/usr/bin/perl -w 

use strict;

my @input  = @ARGV;
my $maxAll = $input[-1];
$#input = $#input - 1;
my @multiset = ( 0, $maxAll );
my @stack;

my $rotation = 0;    # this is 0,1, or 2.

while ( @input != 0 ) {

    my $max = $input[-1];

    my @deltamultiset;
    for ( my $i = 1; $i <= $#multiset; $i++ ) {
        push @deltamultiset, $multiset[$i] - $max;
    }
    push @deltamultiset, $max;

    my @deltamultiset2;
    for ( my $i = 1; $i <= $#multiset; $i++ ) {
        push @deltamultiset2, $multiset[$i] - ( $maxAll - $max );
    }
    push @deltamultiset2, $max;

    if ( subset( \@deltamultiset, \@input ) and $rotation == 0 ) {

        for ( my $i = 0; $i < $#deltamultiset; $i++ ) {
            pop @input;
        }

        push @multiset, $max;
        push @stack,    $max;
        push @stack,    0;
    }
    elsif ( subset( \@deltamultiset2, \@input ) and $rotation <= 1 ) {

        for ( my $j = 0; $j < $#deltamultiset; $j++ ) {
            pop @input;
        }

        push @multiset, ( $maxAll - $max );
        push @stack,    ( $maxAll - $max );
        push @stack, 1;
        $rotation = 0;
    }
    elsif ( @stack != 0 ) {

        $rotation = $stack[-1];
        $#stack--;
        $max = $stack[-1];
        $#stack--;
        $rotation++;

        for ( my $i = 0; $i < $#multiset; $i++ ) {
            if ( $multiset[$i] == $max ) {
                delete $multiset[$i];
                last;
            }
        }

        for ( my $i = 0; $i < $#deltamultiset; $i++ ) {
            push @input, $deltamultiset[$i];
        }
    }
    else {
        print "no solutions \n";
        exit;
    }
}

print "@multiset is a solution \n";

sub subset {
    my ( $deltamultisetSubref, $multisetSubref ) = @_;
    my @deltamultisetSub = @{$deltamultisetSubref};
    my @multisetSub      = @{$multisetSubref};

    while ( @deltamultisetSub != 0 ) {

        for ( my $i = $#multisetSub; $i >= -1; $i-- ) {

            if ( $multisetSub[$i] == $deltamultisetSub[-1] ) {
                pop @deltamultisetSub;
                $#multisetSub--;
                last;
            }

            if ( $i == -1 ) {
                return 0;
            }
        }
    }

    return 1;
}

This is what is output

Use of uninitialized value in subtraction (-) at ./branchandboundNoComments.pl line 20.
Use of uninitialized value in subtraction (-) at ./branchandboundNoComments.pl line 26.
no solutions

You can use -1 to index the last array element, no need for `$#`. I'm not sure if you're realized that assigning to `$#` modifies the array. Your loops seem to sometimes skip the first element, and sometimes skip the last. You're unnecessarily copying array references to examine their contents. Anyway, I'm too confused by now to figure out what you're actually trying to achieve. I'm pretty sure the problem is the if statement that assigns instead of comparing, and the subsequent `delete` on an array (I think you meant to use `splice`). — nothingmuch, Dec 13 '16 at 01:25
Re "*I'm pretty sure the problem is the if statement that assigns instead of comparing*", meaning that `if ($multiset[$i] = $max)` should be `if ($multiset[$i] == $max)` — ikegami, Dec 13 '16 at 02:56
And please!!! Always use `use strict; use warnings qw( all );`! (I see that you used `-w`, which is an acceptable replacement for the latter.) — ikegami, Dec 13 '16 at 02:56
@ikegami Oh right, sorry about that, I fixed a few things and updated my code, I took some of the print tests out because I decided it was more clear — Sam, Dec 13 '16 at 04:13
`$max = $stack[-1]; $#stack--;` is better written as `$max = pop @stack;` — Borodin, Dec 13 '16 at 12:55
Is this algorithm described on line anywhere? I'd like to understand what your code is meant to do, as there's no way of distinguishing what's correct from what's not. — Borodin, Dec 13 '16 at 13:19

score 1 · Answer 1 · answered Dec 13 '16 at 13:32

I can't understand the algorithm you're trying to implement, so there are probably more errors, but the immediate problem is that the statement

delete $multiset[$i]

won't remove that element from the array unless it is the last element; otherwise the array stays the same length, exists on that element will return false, and it will evaluate to undef

If you want to remove the element, which seems most likely, then you want

splice @multiset, $i, 1;

But I have tested your code with that fix in place, and while it no longer produces Use of uninitialized value in subtraction errors, the result is still no solutions

Unfortunately I can't understand what you're trying to implement, and can't make any useful guesses about what may be wrong unless you can offer me a description of the underlying algorithm

Perl deleting array elements after a few iterations of code, with a call to a sub function

1 Answers1