Codewars / reduce

codewars.com is a “coding practice” website. You chose a language and a skill level, then it offers up a task (or kata) for you to write a suitable function. The first one it gave me was seemed too hard, so I changed my level to beginner and skipped to the next one. This was my task:

 Given an array of integers, find the one that appears an odd number of times.
 
 There will always be only one integer that appears an odd number of times.
 
 Examples
 [7] should return 7, because it occurs 1 time (which is odd).
 [0] should return 0, because it occurs 1 time (which is odd).
 [1,1,2] should return 2, because it occurs 1 time (which is odd).
 [0,1,0,1,0] should return 0, because it occurs 3 times (which is odd).
 [1,2,2,3,3,3,4,3,3,3,2,2,1] should return 4, because it appears 1 time (which is odd).

I know there’s a cool “Set” container type in Swift, so my plan was to iterate through the array, then for each number if there’s no entry in the set, then add one, but if there is, remove it. That way whatever is left in the set at the end must be in the original array an odd number of times. I was pretty pleased with myself. Here’s the code I Playground’d up: