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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