arc4random help

Question

is there any simple tutorial or help available to getting to learn how to use arc4random to select a random set of numbers?

Answer 1

If each number in the set of numbers you need has to be within a specific range, you normally do that using, in this order: arc4random(), modulus, multiplication, addition.

For example, to generate 10 even numbers in the range [6, 90): You will ultimately be adding 6, the lowest value in the range. Your multiplier is 2 because you want every second number (every even number == every second number). The denominator of your modulus will be (the width of the range == 90-6 == 84) / (your multiplier == 2) == 42.

So your code would look like:

  int const kHowManyNumbersToHaveInTheSet = 10;
  int const kLowerBoundInclusive = 6;
  int const kUpperBoundExclusive = 90;
  int const kIntervalBetweenAcceptableValues = 2;

  int rangeWidth = kUpperBoundExclusive - kLowerBoundInclusive;
  int denominatorForModulus = rangeWidth / kIntervalBetweenAcceptableValues;

  NSMutableSet *mySet = [NSMutableSet setWithCapacity: kHowManyNumbersToHaveInTheSet];
  while (mySet.count < kHowManyNumbersToHaveInTheSet)
  {
    int newNumber = arc4random();
    newNumber %= denominatorForModulus;
    newNumber *= kIntervalBetweenAcceptableValues;
    newNumber += kLowerBoundInclusive;
    [mySet addObject: [NSNumber numberWithInt: newNumber]];
  }