canonical way to randomize an NSArray in Objective C

Question

Is there a canonical way to randomize an array in Objective C?

Answer 1

There is none built into the SDK if that's what you are asking.

You can use just about any randomization or shuffling algorithm you want however. Different algorithms have different tradeoffs in terms of randomness, efficiency, etc.

http://en.wikipedia.org/wiki/Shuffling#Shuffling_algorithms

For algorithms that shuffle "in-place" start with a mutable array use

insertObject:atIndex:
removeObjectAtIndex:

For algorithms that reconstruct the array, feed it the original and build a new array.

Answer 2

if ([array count] > 1) {
    for (NSUInteger shuffleIndex = [array count] - 1; shuffleIndex > 0; shuffleIndex--)
        [array exchangeObjectAtIndex:shuffleIndex withObjectAtIndex:random() % (shuffleIndex + 1)];
}

Make sure to seed the random() function with either srandomdev() or srandom().

Answer 3

HowToGenerateARandomOrder

Answer 4

My utility library defines this category on NSMutableArray to do it:

@interface NSMutableArray (ArchUtils_Shuffle)
- (void)shuffle;
@end

// Unbiased random rounding thingy.
static NSUInteger random_below(NSUInteger n) {
    NSUInteger m = 1;

    do {
     m <<= 1;
    } while(m < n);

    NSUInteger ret;

    do {
     ret = random() % m;
    } while(ret >= n);

    return ret;
}

@implementation NSMutableArray (ArchUtils_Shuffle)

- (void)shuffle {
    // http://en.wikipedia.org/wiki/Knuth_shuffle

    for(NSUInteger i = [self count]; i > 1; i--) {
     NSUInteger j = random_below(i);
     [self exchangeObjectAtIndex:i-1 withObjectAtIndex:j];
    }
}

@end