CODEWITHSUNDEEP
iphoneobjective-ciosnsset
ticofab
ticofab

Reputation: 7717

NSSet how to extract object randomly?

I am not sure about how NSSet's anyObject work. What does it mean that "The object returned is chosen at the set’s convenience" (from the NSSet class reference) ?

Further, how can I best extract objects randomly from a NSSet? I was thinking about getting allObjects in an array and then myArray[arc4random_uniform(x)] where x is the number of objects in the array.

Upvotes: 8

Views: 4578

Answers (4)

Anne
Anne

Reputation: 27073

Quote from NSSet Class Reference:

The object returned is chosen at the set’s convenience—the selection is not guaranteed to be random.

For "randomness", convert the NSSet to an NSArray using [theSet allObjects].
Next, pick any object randomly using arc4random_uniform().

Upvotes: 14

jay492355
jay492355

Reputation: 594

I use arc4random() and two mutable arrays to get a random and unique set of objects:

NSMutableArray *selectionPool = ...;

int numberOfObjectsToSelect = x;

NSMutableArray *selectedObjects = [[NSMutableArray alloc] initWithCapacity:numberOfObjectsToSelect];

int modulus = selectionPool.count - 1;

for (int i = 0; i < numberOfObjectsToSelect; i++) {

    int j = arc4random() % (modulus--);
    [selectedObjects addObject:[selectionPool objectAtIndex:j]];
    [selectionPool removeObjectAtIndex:j];

}

I'm not sure how efficient it would be for large collections, but it's worked for me with collections that number in the low 100s of objects.

Upvotes: 1

Richard J. Ross III
Richard J. Ross III

Reputation: 55583

Usually, NSSet instances are created with a CFHash backing, so they almost always return the first object in that hash, as it is the fastest to look up. The reason it says

The object returned is chosen at the set’s convenience—the selection is not guaranteed to be random.

Is because you don't always know it will have a backing array. For all you know, the NSSet instance you have has a NSDictionary backing it, or some other similar data structure.

So, in conclusion, if you need a random object from a NSSet, don't use -anyObject, instead use allObjects: and then shuffle that array.

Upvotes: 13

PengOne
PengOne

Reputation: 48398

The documentation reads that anyObject returns

One of the objects in the set, or nil if the set contains no objects. The object returned is chosen at the set’s convenience—the selection is not guaranteed to be random.

Most likely there is some deterministic algorithm at work.

The most reliable thing to do would be, as you suggest, to create an NSArray using the NSSet method allObjects, and then choose a random element from that with arc4random() % N where N is the count of the NSArray.

Upvotes: 4

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