in reply to Avoiding vivification with array slice

I was thinking that the defined test would exclude any bits of the slice I didn't want - with no side effects. But clearly reading the value to test it's defined-ness is enough to vivify.

You're testing too late. Besides betterworld's trick, or moritz's optimization, you could specifically test for defined-ness per element, rather than taking the slice first:

@b = map { defined($a[$_]) ? ($a[$_]) : () } (0..5);

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Re^2: Avoiding vivification with array slice
by jbert (Priest) on Sep 09, 2008 at 14:45 UTC

    Cool. Accessing $a[$i] isn't enough to vivify (otherwise your defined test would do it).

    So we don't actually need the defined test in your map and can get away with:

    # This *doesn't* vivify $a[3,4,5]
@b = map { $a[$_] } (0..5);

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    Thanks.

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      The point of the defined wasn't to prevent autovivification, but to implement the grep you had in your original code. The following two snippets are equivalent. 
      @b = map { defined($a[$_]) ? ($a[$_]) : () } 0..5;

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      @b = grep { defined } map { $a[$_] } 0..5;

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      What you suggested (@b = map { $a[$_] } 0..5;) is *not* equivalent.

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