FAQ 4.57 What happens if I add or remove keys from a hash while iterating over it?
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4.57: What happens if I add or remove keys from a hash while iterating over it?
(contributed by brian d foy)
The easy answer is "Don't do that!"
If you iterate through the hash with each(), you can delete the key most
recently returned without worrying about it. If you delete or add other
keys, the iterator may skip or double up on them since perl may
rearrange the hash table. See the entry for "each()" in perlfunc.
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Ben Morrow
Quoth PerlFAQ Server <br...@stonehenge.com>:
>
> 4.57: What happens if I add or remove keys from a hash while iterating over it?
>
> (contributed by brian d foy)
>
> The easy answer is "Don't do that!"
I thought mjd'd fixed this entry?
> If you iterate through the hash with each(), you can delete the key most
> recently returned without worrying about it. If you delete or add other
> keys, the iterator may skip or double up on them since perl may
> rearrange the hash table. See the entry for "each()" in perlfunc.
Deleting *any* key is safe, and has the obvious result. If you've seen
it already, you won't see it again; if you haven't, you won't see it at
all. Deleting an entry never causes Perl hashes to rehash, so it won't
affect any other keys.
Adding a key is not safe. It may cause the hash to rehash, which will
change the order of all the keys.
Ben
Uri Guttman
BM> Quoth PerlFAQ Server <br...@stonehenge.com>:
>>
>> 4.57: What happens if I add or remove keys from a hash while iterating over it?
>>
>> (contributed by brian d foy)
>>
>> The easy answer is "Don't do that!"
BM> I thought mjd'd fixed this entry?
>> If you iterate through the hash with each(), you can delete the key most
>> recently returned without worrying about it. If you delete or add other
>> keys, the iterator may skip or double up on them since perl may
>> rearrange the hash table. See the entry for "each()" in perlfunc.
BM> Deleting *any* key is safe, and has the obvious result. If you've seen
BM> it already, you won't see it again; if you haven't, you won't see it at
BM> all. Deleting an entry never causes Perl hashes to rehash, so it won't
BM> affect any other keys.
BM> Adding a key is not safe. It may cause the hash to rehash, which will
BM> change the order of all the keys.
minor nit: it MAY change the order of all the keys. it could rehash the
keys to the same order but you can't expect that. your point of deleting
any key is valid. it would make no sense to rehash on that anyway. you
could be deleting a key from an overflow list in a bucket so why rehash
then?
so i agree this FAQ needs to be edited to clarify the safety of deleting
keys in a loop while emphasizing the error of adding keys in a loop.
uri
--
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brian d foy
<b...@morrow.me.uk> wrote:
> Quoth PerlFAQ Server <br...@stonehenge.com>:
> >
> > 4.57: What happens if I add or remove keys from a hash while iterating over
> > it?
> Deleting *any* key is safe, and has the obvious result. If you've seen
> it already, you won't see it again; if you haven't, you won't see it at
> all. Deleting an entry never causes Perl hashes to rehash, so it won't
> affect any other keys.
This is contrary to the documentation for each() in perlfunc. I'm
taking perlfunc as the more authoratative source. If there's a problem,
someone needs to fix it there first.
Eric Pozharski
I think there *is* problem with F<perlfunc>
perl -wle '
$h{$_} = $_ foreach 0 .. 1E4;
print scalar %h;
while( $x = each %h ) {
delete $h{$x + 1};
push @z, $x + 1;
print "already deleted: $x"
if grep $_ == $x, @z;
};
print scalar %h;
'
7391/16384
3526/16384
--
Torvalds' goal for Linux is very simple: World Domination
Stallman's goal for GNU is even simpler: Freedom
Uri Guttman
EP> $h{$_} = $_ foreach 0 .. 1E4;
since i like slices:
%h{ 0 .. 1e4 } = 0 .. 1E4 ;
EP> while( $x = each %h ) {
EP>
EP> delete $h{$x + 1};
EP> push @z, $x + 1;
and very few seem to know that delete returns its value and since your
keys and values are the same:
push @z, delete $h{$x + 1} ;
:)
John W. Krahn
>>>>>> "EP" == Eric Pozharski <why...@pozharski.name> writes:
>
> EP> $h{$_} = $_ foreach 0 .. 1E4;
>
> since i like slices:
>
> %h{ 0 .. 1e4 } = 0 .. 1E4 ;
ITYM:
@h{ 0 .. 1e4 } = 0 .. 1E4 ;
> EP> while( $x = each %h ) {
> EP>
> EP> delete $h{$x + 1};
> EP> push @z, $x + 1;
>
> and very few seem to know that delete returns its value and since your
> keys and values are the same:
>
> push @z, delete $h{$x + 1} ;
>
> :)
John
--
Those people who think they know everything are a great
annoyance to those of us who do. -- Isaac Asimov
Uri Guttman
JWK> Uri Guttman wrote:
>>>>>>> "EP" == Eric Pozharski <why...@pozharski.name> writes:
>>
EP> $h{$_} = $_ foreach 0 .. 1E4;
>>
>> since i like slices:
>>
>> %h{ 0 .. 1e4 } = 0 .. 1E4 ;
JWK> ITYM:
JWK> @h{ 0 .. 1e4 } = 0 .. 1E4 ;
yep. i cut/pasted your code and forgot to change it.
Eric Pozharski
>>>>>> "EP" == Eric Pozharski <why...@pozharski.name> writes:
>
> EP> $h{$_} = $_ foreach 0 .. 1E4;
>
> since i like slices:
>
> %h{ 0 .. 1e4 } = 0 .. 1E4 ;
Those who trash hashes know. I don't.
> EP> while( $x = each %h ) {
> EP>
> EP> delete $h{$x + 1};
> EP> push @z, $x + 1;
>
> and very few seem to know that delete returns its value and since your
> keys and values are the same:
>
> push @z, delete $h{$x + 1} ;
Those who B<delete> know. I don't.