How to generate non repeating random number

Question

I am trying to randomize numbers in an array. I am able to do that using arc4random() % [indexes count]

My problem is - If an array consists of 20 item

Answer 1

Declare array that contains already generated number in extension or header file

@property (strong, nonatomic)NSMutableArray *existingNums;
@property (assign, nonatomic)NSInteger maxLimit;
@property (assign, nonatomic)NSInteger minLimit;

Then implement given code in implementation file

@synthesize existingNums;
@synthesize maxLimit;
@synthesize minLimit;

- (NSInteger)randomNumber {

    if(!existingNums)
        existingNums = [NSMutableArray array];

    while (YES) {

        NSNumber *randonNum = [NSNumber numberWithInteger:minLimit+arc4random()%maxLimit];

        if([existingNums containsObject:randonNum]) {

            if([existingNums count] == (maxLimit - minLimit))
                return -1; // All possible numbers generated in the given range

            continue;
        }

        [existingNums addObject:randonNum];

        return [randonNum integerValue];
    }

    return -1;   // return error
}

Hope this will help you :)