[Newbies] Deleting elements from a Collection while looping
leves at elte.hu
Sat Dec 10 20:30:05 UTC 2011
On Sat, 10 Dec 2011, Jan Teske wrote:
> I have a question concerning the behavior of Collections in Squeak. Recently
> I face a problem when trying to delete some elements from a set during a
> 'do:' message.
> Have a look at this sample method:
> doFallForAllIn: aSet
> aSet do:
> [:block | block fall ifFalse: [aSet remove: block]]
> It loops over a set of blocks and performs the fall message on each of them.
> If a block is not falling I don't want it to be in aSet after the function
> returned so I remove it from the set. Squeak doesn't seem to like that since
> some rather strange error now occures: If a block is removed from the set,
> the next block in the set will be left out from looping... sometimes. Nearly
> all the time it works as exspected, only sometimes a block is ignored. I
> couldn't find any pattern when this happens.
> So my question is: Has anyone an explanation for this? Does the error occure
This happens, because #remove: will rehash elements in the same chain
after the removed element. The rehashed elements may be moved to a lower
index, so the loop may not iterate over it at all. You should use
#removeAllSuchThat: instead. E.g.:
aSet removeAllSuchThat: [ :block | block fall not ]
> because the way I'm doing it is fundamentally wrong? Or am I just overlooking
> something and it is possible to remove blocks from a collection while looping
> over it?
As others pointed out, modifying collections while iterating over them is
not supported in general.
> Thanks in advance!
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