Using arc4random function to produce unique values

Question

I am creating a quiz app for the iPhone. Currently my questions are being selected randomly with the arc4random function.

The problem is that I want each question to be displayed only once. Is there a way that I make the arc4random function generate unique numbers and then stop once it has generated all possible numbers?

This is what i am currently using to generate my random number:

QuestionSelected = arc4random() %4;

Any help would be great.

One solution: Create an array of k elements (with the id of your questions). Then: int index = arc4Randcom()%[theArray count]; You'll get the question with [theArray objectAtIndex:index]. Then, remove the question: [theArray removeObjectAtIndex:index]; — Larme, Apr 08 '14 at 13:50
What you are looking for is called a "random permutation" or "random shuffle". — Martin R, Apr 08 '14 at 13:50
Use the method mentioned by @Larme and `arc4random_uniform([theArray count])`. The reasons for `arc4random_uniform` can be found here:http://stackoverflow.com/questions/160890/generating-random-numbers-in-objective-c — Robotic Cat, Apr 08 '14 at 14:20

nicael · Accepted Answer · 2014-04-09T19:23:17.070

2

NSMutableArray *questions=[NSMutableArray new];
//creating an array to save questions
// Place in viewDidLoad
for(;;) {
    //randomly select question
    QuestionSelected = arc4random() % 4;
    //check if question contains this number
    //if it does - continue looping
    if(![questions containsObject:@(QuestionSelected)]){
        //so it doesn't - we add this number to array
        [questions addObject:@(QuestionSelected)];
        break;
        //and exit loop
    }

 }

Thats all

edited Apr 09 '14 at 19:23

answered Apr 08 '14 at 13:50

nicael

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You fancy explaining your code???? – Popeye Apr 08 '14 at 13:53
2

Depending on how many question there are this could slow down randomly trying to find a number thats not used – Flexicoder Apr 08 '14 at 13:56
I tried this but I was getting error "implicit conversion of 'int' to 'Id' is disallowed with ARC – Sztucki Apr 09 '14 at 18:54

score 1 · Answer 2 · answered Apr 08 '14 at 13:51

1

You could use an NSMutableArray with the questions, and when a question is selected, remove it from the array. In this case you would generate the random number between 0 and array.count - 1

answered Apr 08 '14 at 13:51

Levi

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Using arc4random function to produce unique values

2 Answers2