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Re: Is it safe to append to the array you are iterating over

by bart (Canon)
on Mar 14, 2013 at 17:53 UTC ( #1023515=note: print w/ replies, xml ) Need Help??


in reply to Is it safe to append to the array you are iterating over

The question then is is this violation of the general prohibition safe or was I lucky
Now I am confused. What do you expect,
  1. Perl loops over the original array, whatever you do to the array in the meantime
  2. Perl loops over the modified array
Frankly, I have no idea what to expect, you can make a case for both behaviors. In such a case, I'd say: don't do it. What will happen in this case might change over time, possibly because someone decided to fix perl's "erratic behavior".

What I think is safe, is this:

  1. Loop over the original array: make a temporary copy.
    foreach(my @temp = @arr) { push @arr, 'd' if $_ eq 'a'; push @arr, 'e' if $_ eq 'b'; push @arr, 'f' if $_ eq 'c'; print $_; }
  2. if you want to loop over the modified array, use the loop index:
    for(my $i = 0; $i < @arr; $i++) { local $_ = $arr[$i]; push @arr, 'd' if $_ eq 'a'; push @arr, 'e' if $_ eq 'b'; push @arr, 'f' if $_ eq 'c'; print $_; }
p.s. I was expecting this to work:
foreach(() = @arr) { print; }
but it doesn't loop even once. I think that's a bit weird, since
$x = () = @arr;
actually sets $x to the size of the array.


Comment on Re: Is it safe to append to the array you are iterating over
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Re^2: Is it safe to append to the array you are iterating over
by DrWhy (Chaplain) on Mar 14, 2013 at 19:11 UTC
    My expectation was your number 2. Loop over the modified array. And that's what it does in my experiments. It looks like Perl is being efficient and rather than constructing a new list to loop over that is just a verbatim copy of the array it already has access to, it's using that array as it's loop list and the implementation of arrays in Perl is such that adding elements on the end of the array doesn't mess up the iterator used by the for loop. At least for the small size arrays I've been working with. Thinking about it further I can imagine Perl doing this until the array gets to a large enough size that it can't be held in the current location in memory so it moves it around in a way that could invalidate or reset the iterator.

    --DrWhy

    "If God had meant for us to think for ourselves he would have given us brains. Oh, wait..."

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