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 example if I chose:

 [putNumberUsed objectAtIndex:0]

it would give me anything but "blah1". How would I go about doing this? The following is the code I used thus far:

NSMutableArray *putNumbersUsed = [[NSMutableArray alloc] arrayWithObjects:@"blah1",@"blah2",@"blah3",@"blah4",nil];

possible duplicate - http://stackoverflow.com/questions/5659718/shuffling-an-array-in-objective-c — Peter Kelly, Jun 27 '12 at 10:23
possible duplicate of [What's the Best Way to Shuffle an NSMutableArray?](http://stackoverflow.com/questions/56648/whats-the-best-way-to-shuffle-an-nsmutablearray) — David Rönnqvist, Jun 27 '12 at 10:45

score 9 · Accepted Answer · answered Jun 27 '12 at 10:50

9

I think, You can write a loop for that. Please check the following code,

for (int i = 0; i < putNumberUsed.count; i++) {
    int randomInt1 = arc4random() % [putNumberUsed count];
    int randomInt2 = arc4random() % [putNumberUsed count];
    [putNumberUsed exchangeObjectAtIndex:randomInt1 withObjectAtIndex:randomInt2];
}

I this this may be useful to you.

answered Jun 27 '12 at 10:50

Rajesh

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2
8

6

Not very good algorithm. Use only one random index and exchange object at this index with object at `i`. – onegray Jun 27 '12 at 11:06
Yes, you are right onegray, we can use only one random value. – Rajesh Jun 27 '12 at 11:18
Also, don't use % on a random value, it messes up the distribution. Use arc4random_uniform() instead. – Michael Morris Jun 08 '17 at 08:49

Cœur · Answer 2 · 2017-06-08T08:42:09.783

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

score 3 · Answer 3 · answered Jun 06 '17 at 07:01

3

From iOS 10.x++ new concept of shuffle array is given by Apple,

You need to import the framework :

ObjeC

#import <GameplayKit/GameplayKit.h>

NSArray *shuffledArray = [yourArray shuffledArray];

Swift

import GameplayKit

let shuffledArray = yourArray.shuffled()

answered Jun 06 '17 at 07:01

Mukesh Lokare

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This doesn't answer the question: those methods may return an unmodified array, which is not what is being asked. – Cœur Jul 18 '19 at 16:31

score 2 · Answer 4 · answered Jun 27 '12 at 10:28

2

You can shuffle the object by using the following line of code,

[putNumbersUsed exchangeObjectAtIndex:3 withObjectAtIndex:0];

I think this may useful to you.

answered Jun 27 '12 at 10:28

Rajesh

483
2
8

2

Thank you Rajesh I was looking for something like this but isn't this only exchanging object at 3 with 0? What if I wanted to do this randomly with all. Thanks! – Alex G Jun 27 '12 at 10:32
Yes, you're right. But I think Rajesh wanted to guide you to the right direction. I posted a very similiar code in my answer, so have a look. – Fabio Poloni Jun 27 '12 at 10:52

score 1 · Answer 5 · answered Jun 27 '12 at 10:22

1

generate a random number for index

int randomInt = arc4random() % [putNumberUsed count];
[putNumberUsed objectAtIndex:randomInt];

answered Jun 27 '12 at 10:22

janusfidel

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what if the same index was generated several times ? 0_o – Ahmad Al-Attal Jul 22 '17 at 22:24

Fabio Poloni · Answer 6 · 2014-02-21T18:49:04.060

0

Use this:

for (int i = 0; i < [putNumberUsed count]; i++) {
    int random = arc4random() % [putNumberUsed count]; 
    [putNumbersUsed exchangeObjectAtIndex:random withObjectAtIndex:i]; 
}

edited Feb 21 '14 at 18:49

answered Jun 27 '12 at 10:51

Fabio Poloni

8,219
5
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74

1

you must recalculate random in each iteration!! – ragnarius Feb 21 '14 at 18:48

Force Shuffle NSMutableArray

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