Force Shuffle NSMutableArray

Question

I have a NSMutableArray called putNumberUsed. It contains the following objects @\"blah1,@\"blah2\",@\"blah3\",@\"blah4\". I want to shuffle these objects randomly so for ex

Answer 1

Here is a shuffling solution with all positions forced to change when count > 1.

Add a category like NSMutableArray+Shuffle.m:

@implementation NSMutableArray (Shuffle)
// Fisher-Yates shuffle variation with all positions forced to change
- (void)unstableShuffle
{
    for (NSInteger i = self.count - 1; i > 0; i--)
        // note: we use u_int32_t because `arc4random_uniform` doesn't support int64
        [self exchangeObjectAtIndex:i withObjectAtIndex:arc4random_uniform((u_int32_t)i)];
}
@end

Then you can shuffle like:

[putNumbersUsed unstableShuffle];

This solution:

• has no modulo bias
• has no naive bias
• has no sorting bias

A Swift 3.2 and Swift 4 equivalent is:

extension Array {
    mutating func unstableShuffle() {
        for i in stride(from: count - 1, to: 0, by: -1) {
            swapAt(i, Int(arc4random_uniform(UInt32(i))))
        }
    }
}

A Swift 3.0 and 3.1 equivalent is:

extension Array {
    mutating func unstableShuffle() {
        for i in stride(from: count - 1, to: 0, by: -1) {
            swap(&self[i], &self[Int(arc4random_uniform(UInt32(i)))])
        }
    }
}

Note: An algorithm for regular shuffling (where a result with same positions being possible) is also available