<div dir="ltr"><br><br><div class="gmail_quote">On Fri, Aug 8, 2008 at 5:46 PM, Igor Stasenko <span dir="ltr"><<a href="mailto:siguctua@gmail.com">siguctua@gmail.com</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex;">
2008/8/9 Eliot Miranda <<a href="mailto:eliot.miranda@gmail.com">eliot.miranda@gmail.com</a>>:<br>
<div><div></div><div class="Wj3C7c">><br>
><br>
> On Fri, Aug 8, 2008 at 3:53 PM, Igor Stasenko <<a href="mailto:siguctua@gmail.com">siguctua@gmail.com</a>> wrote:<br>
>><br>
>> Sets which not allowing contain nil as element is a point of<br>
>> inconvenience.<br>
>><br>
>> There are two ways how get around that:<br>
>> - initialize an array which contains set elements with unique object ~~<br>
>> nil.<br>
>> and fix methods which testing for an empty slots to compare against<br>
>> this object, not nil.<br>
>> This can cause a slowdown during rehashing, because VM initially<br>
>> creating arrays filled with nils, while we need to fill them with<br>
>> another object.<br>
>> There is also a problem with preserving it 'uniqueness' by not giving<br>
>> this object outside a set.<br>
>><br>
>> - add an instVar 'containsNil'<br>
>> then when set receiving 'add: nil' , it simply sets this flag to true.<br>
>> modify #collect: , #do: , #remove: to be aware of flag value.<br>
>><br>
>> I find the second way is more appropriate. While it costs additional<br>
>> memory per Set/IdentitySet instance, it costs almost nothing in speed.<br>
>><br>
>> What do you think about supporting Sets to contain nils in general,<br>
>> and about methods how to achieve that?<br>
><br>
> Here's a third approach (a variation on your 2nd approach). Have an instVar<br>
> 'includesSelf' and fill the array with the Set itself. So add a new<br>
> primitive new:fillWith: (primitiveNewWithArgAndFillValue?) and use this to<br>
> create the empty array filled with the Set itself. Check for adding the set<br>
> itself, and itself being the null entry. The advantage over the flag for<br>
> nil approach is that you kill two birds with one stone.<br>
> 1. You need a unique value anyway, and the Set can nicely be its own unique<br>
> value<br>
> 2. recursive collections are a problem to print and with the explicit flag<br>
> this becomes much easier to deal with.<br>
<br>
</div></div>In math domain, any set includes itself , not as element of course,<br>
but as subset :)<br>
And i don't see how this is better comparing to having 'containsNil' ivar?<br>
You still have to test this flag in each method which deals with<br>
elements , so be it containsFoo or containsBar - no real difference.</blockquote><div><br></div><div>One difference is the use of self for the unique object instead of another.</div><div>Another difference is that recursive sets no longer fail to print, inspect, etc.</div>
<div><br></div><div>I think those are real differences.</div><div> </div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex;"><br>
<div class="Ih2E3d"><br>
>><br>
>> --<br>
>> Best regards,<br>
>> Igor Stasenko AKA sig.<br>
>><br>
<br>
<br>
<br>
</div>--<br>
<div><div></div><div class="Wj3C7c">Best regards,<br>
Igor Stasenko AKA sig.<br>
<br>
</div></div></blockquote></div><br></div>