var numbers = Array(1...15) // Find the indices of all the even numbers let indicesOfEvens = numbers.indices(where: { $0.isMultiple(of: 2) }) // Perform an operation with just the even numbers let sumOfEvens = numbers[indicesOfEvens].reduce(0, +) // sumOfEvens == 56 // You can gather the even numbers at the beginning let rangeOfEvens = numbers.gather(indicesOfEvens, justBefore: numbers.startIndex) // numbers == [2, 4, 6, 8, 10, 12, 14, 1, 3, 5, 7, 9, 11, 13, 15] // numbers[rangeOfEvens] == [2, 4, 6, 8, 10, 12, 14] // Reset `numbers` numbers = Array(1...15) // You can also build range sets by hand using array literals... let notTheMiddle: RangeSet = [0..<5, 10..<15] print(Array(numbers[notTheMiddle])) // Prints [1, 2, 3, 4, 5, 11, 12, 13, 14, 15] // ...or by using set operations let smallEvens = indicesOfEvens.intersection( numbers.indices(where: { $0 < 10 })) print(Array(numbers[smallEvens])) // Prints [2, 4, 6, 8]