Iterating over hash while deleting elements

Eily asked in the CB, here the answer: *

Basically:

each has no problem if you delete the current element
if you delete other elements, you need to reset the iterator, e.g. with keys
then the iterator starts anew with the remaining elements
NB if you don't empty the whole hash, you risk an endless loop.

The following demo is always resetting, but doesn't need to.

 
  DB<41> @h{a..c,A..C} = (1..3,11..13)

  DB<42> x \%h
0  HASH(0x33bf940)
   'A' => 11
   'B' => 12
   'C' => 13
   'a' => 1
   'b' => 2
   'c' => 3
  DB<43> while (my ($k,$v) = each %h ) { delete $h{$k}; print "$k,$v\n
+"; delete $h{uc($k)} } continue {keys %h }
c,3
a,1
B,12
b,2
[download]

Question to follow ;-P

Cheers Rolf
_{(addicted to the Perl Programming Language :)

Wikisyntax for the Monastery
FootballPerl is like chess, only without the dice}

UPDATE

*) this was a root post which was re-parented to the later question. Partly because it toook eily more than 15 min to ask, but mainly because I liked the idea to defy causality! ;-)

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Re: Iterating over hash while deleting elements by Eily (Monsignor) on Feb 06, 2020 at 17:02 UTC
each has no problem if you delete the current element Nice to know but I wouldn't rely on it. I think "don't use each if the hash changes" is an easier rule to follow/understand. And it's safer against possible future changes as well (you might start by only deleting the current key, and then find out you can delete a few more at the same time). Question to follow ;-P Spooky...	[reply]
Re^2: Iterating over hash while deleting elements by LanX (Saint) on Feb 06, 2020 at 17:05 UTC
> Nice to know but I wouldn't rely on it It's documented. > Exception: It is always safe to delete the item most recently returned by each, so the following code works properly: > `while (my ($key, $value) = each %hash) { print $key, "\n"; delete $hash{$key}; # This is safe }` [download] Cheers Rolf _{(addicted to the Perl Programming Language :) Wikisyntax for the Monastery FootballPerl is like chess, only without the dice}	[reply] [d/l]
Re^3: Iterating over hash while deleting elements by Eily (Monsignor) on Feb 06, 2020 at 17:07 UTC
Oh I didn't mean I don't trust perl with that. It's whoever is using perl I have trust issues against ;-)	[reply]
Re^4: Iterating over hash while deleting elements by LanX (Saint) on Feb 06, 2020 at 17:21 UTC
Re^5: Iterating over hash while deleting elements by Eily (Monsignor) on Feb 06, 2020 at 17:29 UTC
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