Hey, not bad. Does the -allObjects method always return the same sequence?
Did you just cobble this up or is this running in a system somewhere? It
doesn't do everything I want but it is definitely the better part of the 80/20
rule.
-Michael
On May 25, 2010, at 12:52 PM, Dave DeLong wrote:
> What about something like:
>
> @implementation NSSet (Random)
>
> - (id) randomObject {
> NSArray * allObjects = [self allObjects];
> if ([allObjects count] == 0) { @throw ...; }
> return [allObjects objectAtIndex:(arc4random() % [allObjects count])];
> }
>
> @end
>
> On May 25, 2010, at 10:49 AM, Michael A. Crawford wrote:
>
>> I'd like to extend Apple implementation of NSMutableSet to include a
>> randomObject method. The purpose of this method is self-explanatory and its
>> potential use is I'm sure obvious to most of you.
>>
>> For my first idea I thought about simply adding a category to NSMutableSet
>> but any obvious implementation of -randomObject would require accessing
>> existing instance variables or adding my own in order to implement the
>> random access I would need to for the set. I also don't know how objects
>> are stored internally. Is it an array? Is it a key/value pair? I don't
>> know. How am I going to correlate the random numbers I generate with the
>> objects in the set?
>>
>> Next, I thought about using inheritance with NSMutableArray. I even wrote
>> some code:
>>
>> + (void)initialize
>> {
>> if ( self == [MCRandomlyAccessedArray class] )
>> {
>> srandom(time(NULL));
>> }
>> }
>>
>> - (id)randomObject
>> {
>> // If the array is empty, throw an exception. The caller should know
>> better.
>> // If there is only one, return object zero every time.
>> // If there are only two, alternate with every access.
>> // If there are three or more, randomize but never provide the same object
>> // twice in a row.
>> if ( [self count] )
>> {
>> NSUInteger index = 0;
>>
>> if ( [self count] > 1 )
>> {
>> if ( 2 == [self count] )
>> {
>> index = (++lastObjectIndex % 2);
>> }
>> else
>> {
>> do
>> {
>> index = (random() % [self count]);
>> } while (index == lastObjectIndex);
>> }
>> lastObjectIndex = index;
>> }
>> return [self objectAtIndex:index];
>> }
>> else
>> {
>> [[NSException exceptionWithName:NSRangeException
>> reason:@"Array is empty"
>> userInfo:nil] raise];
>> }
>> return nil; // unreachable
>> }
>>
>> Come to find out at runtime that this won't work either because
>> NSMutableArray is abstract!
>>
>> *** Terminating app due to uncaught exception 'NSInvalidArgumentException',
>> reason: '*** -[NSMutableArray addObject:]: method only defined for abstract
>> class. Define -[MCRandomlyAccessedArray addObject:]!'
>>
>> This would seem to indicate that I need to write a lot more code in order to
>> have a complete implementation. I'm not in the mood to re-invent the wheel.
>> I'm not lazy, I just have a lot of other code that needs to be written,
>> which pertains to the big picture.
>>
>> Next, I thought maybe I could simply wrap-up an NSMutableArray in my own
>> class and let it do most of the heavy lifting when it comes to all of those
>> existing methods. The only problem is that I'm no expert on how to
>> duplicate all of the class and instance init methods and memory management
>> code.
>>
>> So, that brings me to you lovely people. What do you recommend? Do I bite
>> the bullet and re-implement NSMutableArray? If so, where can I find
>> guidance on this? I love the containers provided by Apple. They simply
>> work and I don't have to think about them. So much so, that I don't really
>> know how they work. My expertise is/was in C/C++ and STL.
>>
>> Is there fourth alternative or something I missed in my brief examination as
>> described above?
>>
>> Thanks.
>>
>> -Michael
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