How can I efficiently select several unique random numbers from 1 to 50, excluding x?

Question

I have 2 numbers which are between 0 and 49. Let\'s call them x and y. Now I want to get a couple of other numbers which are not x or y, but are al

Answer 1

First, make a set of valid numbers:

// Create a set of all the possible numbers
NSRange range = { 0, 50 };// Assuming you meant [0, 49], not [0, 49)
NSMutableSet *numbers = [NSMutableSet set];
for (NSUInteger i = range.location; i < range.length; i++) {
    NSNumber *number = [NSNumber numberWithInt:i];
    [numbers addObject:number];
}

// Remove the numbers you already have
NSNumber *x = [NSNumber numberWithInt:(arc4random() % range.length)];
NSNumber *y = [NSNumber numberWithInt:(arc4random() % range.length)];
NSSet *invalidNumbers = [NSSet setWithObjects:x, y, nil];
[numbers minusSet:invalidNumbers];

Then, if you don't need the numbers to be guaranteed to be random, you could use -anyObject and -removeObject to pull out a couple of other numbers. If you do need them to be random, then follow LBushkin's answer, but be careful not to accidentally implement Sattolo's algorithm:

// Shuffle the valid numbers
NSArray *shuffledNumbers = [numbers allObjects];
NSUInteger n = [shuffledNumbers count];
while (n > 1) {
    NSUInteger j = arc4random() % n;
    n--;
    [shuffledNumbers exchangeObjectAtIndex:j withObjectAtIndex:n];
}