2008/8/9 Eliot Miranda <eliot.miranda at gmail.com>:
>
>
> On Fri, Aug 8, 2008 at 3:53 PM, Igor Stasenko <siguctua at gmail.com> wrote:
>>
>> Sets which not allowing contain nil as element is a point of
>> inconvenience.
>>
>> There are two ways how get around that:
>> - initialize an array which contains set elements with unique object ~~
>> nil.
>> and fix methods which testing for an empty slots to compare against
>> this object, not nil.
>> This can cause a slowdown during rehashing, because VM initially
>> creating arrays filled with nils, while we need to fill them with
>> another object.
>> There is also a problem with preserving it 'uniqueness' by not giving
>> this object outside a set.
>>
>> - add an instVar 'containsNil'
>> then when set receiving 'add: nil' , it simply sets this flag to true.
>> modify #collect: , #do: , #remove: to be aware of flag value.
>>
>> I find the second way is more appropriate. While it costs additional
>> memory per Set/IdentitySet instance, it costs almost nothing in speed.
>>
>> What do you think about supporting Sets to contain nils in general,
>> and about methods how to achieve that?
>
> Here's a third approach (a variation on your 2nd approach).  Have an instVar
> 'includesSelf' and fill the array with the Set itself.  So add a new
> primitive new:fillWith: (primitiveNewWithArgAndFillValue?) and use this to
> create the empty array filled with the Set itself.  Check for adding the set
> itself, and itself being the null entry.  The advantage over the flag for
> nil approach is that you kill two birds with one stone.
> 1. You need a unique value anyway, and the Set can nicely be its own unique
> value
> 2. recursive collections are a problem to print and with the explicit flag
> this becomes much easier to deal with.

In math domain, any set includes itself , not as element of course,
but as subset :)
And i don't see how this is better comparing to having 'containsNil' ivar?
You still have to test this flag in each method which deals with
elements , so be it containsFoo or containsBar - no real difference.

>>
>> --
>> Best regards,
>> Igor Stasenko AKA sig.
>>



-- 
Best regards,
Igor Stasenko AKA sig.