canonical way to randomize an NSArray in Objective-C
Here it is!
- (NSArray*)shuffleArray:(NSArray*)array {
NSMutableArray *temp = [[NSMutableArray alloc] initWithArray:array];
for(NSUInteger i = [array count]; i > 1; i--) {
NSUInteger j = arc4random_uniform(i);
[temp exchangeObjectAtIndex:i-1 withObjectAtIndex:j];
}
return [NSArray arrayWithArray:temp];
}
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().
My utility library defines this category on NSMutableArray to do it:
@interface NSMutableArray (ArchUtils_Shuffle)
- (void)shuffle;
@end
// Chooses a random integer below n without bias.
// Computes m, a power of two slightly above n, and takes random() modulo m,
// then throws away the random number if it's between n and m.
// (More naive techniques, like taking random() modulo n, introduce a bias
// towards smaller numbers in the range.)
static NSUInteger random_below(NSUInteger n) {
NSUInteger m = 1;
// Compute smallest power of two greater than n.
// There's probably a faster solution than this loop, but bit-twiddling
// isn't my specialty.
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
Make sure you seed the random number generator (with e.g. srandom(time(NULL))
) sometime before you call it; otherwise the output won't be very random.