Calling arc4random several times and getting the same array set

Question

I need a method to generate 4 numbers positioned randonly in an array. This method must be able to be called several times. The code that I tried below seems to be working.. except that everytime I call it, it generates the very same numbers sequence.

At my header file:

NSMutableSet * numberSet;
NSArray * numbers;

Code file:

    numberSet = [NSMutableSet setWithCapacity:4];
    [self placeRandomLine];
    numbers = [numberSet allObjects];

... using the generated array

    [self placeRandomLine];
    numbers = [numberSet allObjects];

... using the generated array

    [self placeRandomLine];
    numbers = [numberSet allObjects];

... using the generated array

Random Method:

-(void)placeRandomLine
{
    [numberSet removeAllObjects];
    while ([numberSet count] < 4 ) {
        NSNumber * randomNumber = [NSNumber numberWithInt:(arc4random() % 4)];
        [numberSet addObject:randomNumber];
    }
}

I am sure I am missing something here..

Thanks for your help!

Answer 1

Use an ordered set:

NSMutableOrderedSet *numberSet = [NSMutableOrderedSet new];
int setSize = 4;
while ([numberSet count] < setSize ) {
    NSNumber * randomNumber = [NSNumber numberWithInt:arc4random_uniform(setSize)];
    [numberSet addObject:randomNumber];
}

NSLog output:

numberSet: {(
    2,
    0,
    1,
    3
)}

Alternatively using an array or arbitrary numbers
Create an NSMutableArray with the four integers.
Create an empty NSMutableArray.
Use arc4random_uniform() to pick one of the numbers in the first array, remove it and place it in the second array.
Repeat for all four numbers.
The second array will have the four numbers in a random order.

Example:

NSMutableArray *a0 = [@[@3, @5, @4, @8] mutableCopy];
NSMutableArray *a1 = [NSMutableArray new];
while (a0.count) {
    int randomIndex = arc4random_uniform(a0.count);
    NSNumber *randomValue = a0[randomIndex];
    [a1 addObject:randomValue];
    [a0 removeObject:randomValue];
}   
NSLog(@"a1: %@", a1);

NSLog output:

a1: (
    8,
    5,
    3,
    4
)

Alternatively using an ordered set

Answer 2

while ([numberSet count] < 4 ) will cause the loop to run until its elements are 0,1,2,3, because the set doesn't contain repeated elements.