What's the Best Way to Shuffle an NSMutableArray?

Question

If you have an NSMutableArray, how do you shuffle the elements randomly?

(I have my own answer for this, which is posted below, but I\'m new to Cocoa an

Answer 1

I solved this by adding a category to NSMutableArray.

Edit: Removed unnecessary method thanks to answer by Ladd.

Edit: Changed (arc4random() % nElements) to arc4random_uniform(nElements) thanks to answer by Gregory Goltsov and comments by miho and blahdiblah

Edit: Loop improvement, thanks to comment by Ron

Edit: Added check that array is not empty, thanks to comment by Mahesh Agrawal

//  NSMutableArray_Shuffling.h

#if TARGET_OS_IPHONE
#import <UIKit/UIKit.h>
#else
#include <Cocoa/Cocoa.h>
#endif

// This category enhances NSMutableArray by providing
// methods to randomly shuffle the elements.
@interface NSMutableArray (Shuffling)
- (void)shuffle;
@end


//  NSMutableArray_Shuffling.m

#import "NSMutableArray_Shuffling.h"

@implementation NSMutableArray (Shuffling)

- (void)shuffle
{
    NSUInteger count = [self count];
    if (count <= 1) return;
    for (NSUInteger i = 0; i < count - 1; ++i) {
        NSInteger remainingCount = count - i;
        NSInteger exchangeIndex = i + arc4random_uniform((u_int32_t )remainingCount);
        [self exchangeObjectAtIndex:i withObjectAtIndex:exchangeIndex];
    }
}

@end

Answer 2

If you import GameplayKit, there is a shuffled API:

https://developer.apple.com/reference/foundation/nsarray/1640855-shuffled

let shuffledArray = array.shuffled()

Answer 3

Edit: This is not correct. For reference purposes, I did not delete this post. See comments on the reason why this approach is not correct.

Simple code here:

- (NSArray *)shuffledArray:(NSArray *)array
{
    return [array sortedArrayUsingComparator:^NSComparisonResult(id obj1, id obj2) {
        if (arc4random() % 2) {
            return NSOrderedAscending;
        } else {
            return NSOrderedDescending;
        }
    }];
}

Answer 4

You don't need the swapObjectAtIndex method. exchangeObjectAtIndex:withObjectAtIndex: already exists.

Answer 5

A slightly improved and concise solution (compared to the top answers).

The algorithm is the same and is described in literature as "Fisher-Yates shuffle".

In Objective-C:

@implementation NSMutableArray (Shuffle)
// Fisher-Yates shuffle
- (void)shuffle
{
    for (NSUInteger i = self.count; i > 1; i--)
        [self exchangeObjectAtIndex:i - 1 withObjectAtIndex:arc4random_uniform((u_int32_t)i)];
}
@end

In Swift 3.2 and 4.x:

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

In Swift 3.0 and 3.1:

extension Array {
    /// Fisher-Yates shuffle
    mutating func shuffle() {
        for i in stride(from: count - 1, to: 0, by: -1) {
            let j = Int(arc4random_uniform(UInt32(i + 1)))
            (self[i], self[j]) = (self[j], self[i])
        }
    }
}

Note: A more concise solution in Swift is possible from iOS10 using GameplayKit.

Note: An algorithm for unstable shuffling (with all positions forced to change if count > 1) is also available

Answer 6

NSUInteger randomIndex = arc4random() % [theArray count];