Autovivification doesn't work even as lvalue

Question

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?

Answer 1

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!).

Answer 2

my has two effects:

• At compile time, my declares the variable.
• At run time, my allocates a new variable. More or less.

The first effect is what allows you to refer to the variable until the end of the enclosing block.

The second effect means $sum can't possibly hold the sum at the end of the loop since you call my to create a new variable each pass of the loop.

---

[ Warning: This section discusses Perl's guts. Feel free to jump ahead. ]

But why is it undef instead of containing the number from the last pass?

Well, that's cause my doesn't actually allocate a new variable when executed. It places an instruction on the stack to allocate a new one on scope exit!

The for statement modifier creates a lexical scope so that $_ can be properly restored when the statement is complete, so my $sum is replaced with a fresh variable at the end of each loop pass. (It's technically only being cleared rather than deallocated and reallocated thanks to an optimization.)

---

Your code could be written as follows:

#!/usr/bin/perl

use strict;
use warnings;

sub sum { my $acc; $acc += $_ for @_; $acc }

my @inp = map 2**$_, 0..6;
my @cc = grep { ( sum split // ) % 2 } @inp;

or even just

my @cc = grep { ( sum split //, 2**$_ ) % 2 } 0..6;

Always use use strict; use warnings;. Note that use warnings; would have made it more obvious that something was going wrong.

---

By the way, I don't know what you think autovivification means, but it's wrong.

Autovivification is the creation of a variable and a reference to it when deferencing an undefined value.

$ perl -e'
   my $x;
   CORE::say $x // "[undef]";
   $x->[0] = 123;
   CORE::say $x // "[undef]";
'
[undef]
ARRAY(0x35d7f56740)

Less formally, it could also refer to the creation of hash or array elements when using them as lvalues.

$ perl -e'
   my $x;
   CORE::say exists($h{x}) ? 1 : 0;
   my $ref = \( $h{x} );
   CORE::say exists($h{x}) ? 1 : 0;
'
0
1

There's no attempt to autovivify in your code.