perlautovivification

Autovivification doesn't work even as lvalue

I have this simple code:

#!/usr/bin/perl
@inp = map 2**$_, 0..6;
@cc = grep {
    my $num = $inp[$_];
    my $sum;    #---- HERE, I have to have the var declared first, before init.
    $sum += $_ for (split //, $num);
    print "$sum\n";
    $sum % 2;
    } 0..$#inp;

Here, the $sum will be used in for loop, However in this case:

#!/usr/bin/perl
@inp = map 2**$_, 0..6;
@cc = grep {
    my $num = $inp[$_];
    my $sum += $_ for (split //, $num); # HERE, Trying to autovificate - wont work
    print "$sum\n";
    $sum % 2;
    } 0..$#inp;

But when I used var $sum at the same line with for loop - that means I am trying to declare and initiate at once - where should work the autovivifaction - As i would expect to autovivificate the $sum to zero (because used with math operator +=), but will not work, but why so? What are the rules for autovivification?

Solution

  • This is not autovivification. You have a syntax mistake. If you had use strict and use warnings turned on, it would be more obvious.

    The post-fix for construct treats the left-hand side like a block. So there is a scope for the body of the loop. Therefore you are declaring your my $sum inside that loop body scope, and it's not visible outside.

    If you turn on use warnings, you'll get Use of uninitialized value $sum in concatenation (.) or string at ... line 6, which is the print after.

    You need to declare the variable first (and use strict and warnings!).