in reply to Using exists to check element in array

Perl source and tests have had all use of 'exists' on array elements / array refs removed. So running exists on an array element can lead to surprising behavior - because no one is testing what the behavior should be. So essentially the maintainers are reserving the right to modify this behavior by accident.

This situation seems to have come about because originally "exists" on array elements returned true if Perl had allocated a CV pointer for that index, where as now it returns True only if a CV pointer is both allocated and is not set to null. But this change over was not clean and and I think there are situations where it would not always be true. Imagine an array that is extremely large and fragmented in its use of indexes, where memory allocation has trimmed, extended, realloacted various CV linked lists etc. Does perl always only have a CV for each actually 'used' index position? The answer is apparently no.

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Re^2: Using exists to check element in array
by LanX (Saint) on Jan 29, 2024 at 03:55 UTC
    I have problems to follow...

    > The answer is apparently no

    Is it possible to demonstrate this problem with Perl code?

    Cheers Rolf
    (addicted to the Perl Programming Language :)
    see Wikisyntax for the Monastery

[reply]
Re^2: Using exists to check element in array
by david (Novice) on Feb 04, 2024 at 03:20 UTC

    Thanks. This is really interesting. I thought under "normal" circumstances memory allocation for variables was fairly fixed and persistent for PERL and was the reason that 'referencing' works. Perhaps a little knowledge is truely dangerous but I have to push through to turn a little into a little-more, so thanks for the input. Does CV stand for "Code Value"? I found that in 'perlguts' which was pretty heavy reading!

    Musings... It seems to me that it is fairly reasonable for a user to want to know if an element of an array or hash 'exists'. Exists could return more than a binary choice and the user could be expected to test the returned value to make sure it is what they wanted. I am guessing that a fairly large portion of PERL code includes arrays and hashes, so predictable results in this area now and in the future seems important. I wonder what code may break in the future and indeed where I might have used it previously and erroneously! The warning certainly made me stop but I needed the why and the guidance on what to do instead.

    Thanks again to you and the other contributors.

[reply]

      Its because the 'high level' concept of an Array is that it is a string of 0 to n items. So all items in the array always exist conceptually.

      So if you want to know if index x 'exists' in the array, simply do either: (x <= $#array) or (x < scalar @array).

      If you haven't set an array index to a value, then its value will be undef and you can check this simply with (defined $array[x]). If x is beyond the length of array, this will not change the length of the array. However if you assign an index to a value - including undef, the array will expand in size to at least that index.

      .
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