Peter Gaffney Peter Gaffney - 3 years ago 254
iOS Question

Randomly choosing an item from a Swift array without repeating

This code picks a random color from a array of pre-set colors. How do I make it so the same color doesn't get picked more than once?

var colorArray = [(UIColor.redColor(), "red"), (UIColor.greenColor(), "green"), (UIColor.blueColor(), "blue"), (UIColor.yellowColor(), "yellow"), (UIColor.orangeColor(), "orange"), (UIColor.lightGrayColor(), "grey")]

var random = { () -> Int in
return Int(arc4random_uniform(UInt32(colorArray.count)))
} // makes random number, you can make it more reusable

var (sourceColor, sourceName) = (colorArray[random()])

Answer Source

Create an array of indexes. Remove one of the indexes from the array and then use that to fetch a color.

Something like this:

var colorArray = [
  (UIColor.redColor(), "red"), 
  (UIColor.greenColor(), "green"), 
  (UIColor.blueColor(), "blue"), 
  (UIColor.yellowColor(), "yellow"), 
  (UIColor.orangeColor(), "orange"), 
  (UIColor.lightGrayColor(), "grey")]

var indexes = [Int]();

func randomItem() -> UIColor
  if indexes.count == 0
    print("Filling indexes array")
    indexes = (0... colorArray.count-1).map{$0}
  let randomIndex = Int(arc4random_uniform(UInt32(indexes.count)))
  let anIndex = indexes.removeAtIndex(randomIndex)
  return colorArray[anIndex].0;

The code above creates an array indexes. The function randomItem looks to see if indexes is empty. if it is, it populates it with index values ranging from 0 to colorArray.count - 1.

It then picks a random index in the indexes array, removes the value at that index in the indexes array, and uses it to fetch and return an object from your colorArray. (It doesn't remove objects from the colorArray. It uses indirection, and removes objects from the indexesArray, which initially contains an index value for each entry in your colorArray.

The one flaw in the above is that after you fetch the last item from indexArray, you populate it with a full set of indexes, and it's possible that the next color you get from the newly repopulated array will be the same as the last one you got.

It's possible to add extra logic to prevent this.

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