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Re^2: references--hard vs anonymous operational weirdness

by Fletch (Chancellor)
on Mar 23, 2008 at 03:23 UTC ( #675722=note: print w/replies, xml ) Need Help??


in reply to Re: references--hard vs anonymous operational weirdness
in thread references--hard vs anonymous operational weirdness

Only if @array was declared outside the loop.</nit>

my @array; for my $tab ( @things ) { @array = frobnicate( $tab ); ## As you've seen, this leaves everything pointing at the same array $data{ $tab } = \@array; }

Were @array declared inside the loop you'd get a fresh instance each time through.

for my $tab ( @things ) { my @array = frobnicate( $tab ); $data{ $tab } = \@array; }

The cake is a lie.
The cake is a lie.
The cake is a lie.

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Re^3: references--hard vs anonymous operational weirdness
by hipowls (Curate) on Mar 23, 2008 at 03:43 UTC

    Quite right, I'd got caught by an optimization. I ran (adjusting code to your example)

    for my $tab ( 1 .. 3 ) { my @array = frobnicate($tab); printf "Array ref: %s\n", \@array; printf "Annoymous: %s\n", [@array]; } sub frobnicate { return ( 0 .. shift ); } __END__ Array ref: ARRAY(0x826a924) Annoymous: ARRAY(0x8183a54) Array ref: ARRAY(0x826a924) Annoymous: ARRAY(0x826d818) Array ref: ARRAY(0x826a924) Annoymous: ARRAY(0x82115f8)
    The same address even though @array is declared in the loop. The variable is being reused. Assigning \@array to something prevents the optimization.
    my %data; for my $tab ( 1 .. 3 ) { my @array = frobnicate($tab); printf "Array ref: %s\n", \@array; printf "Annoymous: %s\n", [@array]; $data{$tab} = \@array; } sub frobnicate { return ( 0 .. shift ); } __END__ Array ref: ARRAY(0x826a924) Annoymous: ARRAY(0x8183a54) Array ref: ARRAY(0x819f318) Annoymous: ARRAY(0x8183798) Array ref: ARRAY(0x826ade8) Annoymous: ARRAY(0x819f304)
    Something to bear in mind when benchmarking.

      Assigning \@array to something prevents the optimization.

      Which optimization is that? Can you show the (incorrect) code which triggers this optimization?

        The first of the two examples in the grandparent of this post is the "incorrect" code. To cut it down to a minimum

        for my $tab ( 1 .. 3 ) { my @array = ( 0 .. $tab); printf "Array ref: %s\n", \@array; } __END__ Array ref: ARRAY(0x826acf4) Array ref: ARRAY(0x826acf4) Array ref: ARRAY(0x826acf4)
        As you can see the same address is being reused. The latest perl review has an article about closures (PDF) that mentions it in passing.

        I don't see how the code is incorrect. In the first example the value of @array is discarded but I could quite legitimately do something like

        for my $value ( @values ) { my @array = frobinate($value); foreach my $bit (@array) { drill($bit); } }
        and the memory location of @array is reused.

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