Re: Strange behavior of iteration while creating Perl/Tk widgets dynamically

Your problem can be demonstrated by following simple code:

my @subs;
for(my $i=1;$i<=5;$i++){
        push @subs, sub {
                print "i=$i\n";
        };
}
$subs[3]->();
[download]

Inside the sub, $i is evaluated at the time the sub is called in the context/scope of the for-loop which has now terminated, so $i=5+1=6, which is the expected behaviour.

A workaround would be to create a sub via eval which will force it to consider $i's value at the time of the eval. But I am not sure if this is an elegant solution:

my @subs;
for(my $i=1;$i<=5;$i++){
        push @subs, eval <<EOE;
sub {
    print "i=$i\n";
};
EOE
}
$subs[3]->();
[download]

btw, once you sort this out, you should start using arrays instead of ${"xx$i"}!

Update: another solution would be to introduce another variable local to the loop scope and use that:

my @subs;
for(my $i=1;$i<=5;$i++){
        my $j = $i;
        push @subs, sub {
          print "i=$i, j=$j\n";
        };
}
$subs[3]->();
[download]

Question: what's the difference between $i and $j, don't they both have the same scope?

bw, bliako

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Re^2: Strange behavior of iteration while creating Perl/Tk widgets dynamically by BillKSmith (Monsignor) on Oct 29, 2021 at 14:34 UTC
The perl `foreach` (or its alias `for`) loop does not have this problem. `my @subs; #for(my $i=1;$i<=5;$i++){ for my $i (1..5) { push @subs, sub { print "i=$i\n"; }; } $subs[3]->();` [download] OUTPUT: `i=4` [download] Bill	[reply] [d/l] [select]
Re^3: Strange behavior of iteration while creating Perl/Tk widgets dynamically by bliako (Abbot) on Oct 29, 2021 at 17:01 UTC
And this code confirms your finding: `my @subs; for(my $i=1;$i<=2;$i++){ print \$i."\n" } print "----\n"; for my $i (1..2) { print \$i."\n" }` [download] OUTPUT: `SCALAR(0x55a429b66a28) SCALAR(0x55a429b66a28) ---- SCALAR(0x56399466d4d8) SCALAR(0x5639946924a0)` [download] Which made me try this: `my @subs; my $i; for($i=1;$i<=5;$i++){ push @subs, sub { print "i=$i\n"; }; } $i = 100; $subs[3]->();` [download] It prints `i=100` bw, bliako	[reply] [d/l] [select]