How to create a for loop with a dynamic increment amount on each iteration in swift now that the C-style loop has been removed

Question

I am trying to do something that with a C-style loop was easy but I am struggling with in Swift3.

I have a loop that I need to increment by different amounts depending on a value stored in an array. The code below describes it better.

let scaleIntervals: [Int] = [2,2,1,2,2,2,1] 
let notes: [String] = ["A", "A#", "B", "C", "C#" "D", "D#" "E", "F" ,"G", "G#"]
var scale: [String] = []

for(var i = 0; i < notes.length; i++){

/* *Sketchy - untested* If the note in the first index of the output array 
matches the current note we have completed the scale. Exit loop */

    if scale[0] == notes[i]{ 
       break
    }

//Add the note to the output array and increment by the next interval

    scale.append(notes[i])
    i += scaleIntervals![i]

//If interval makes i larger than the notes array, loop back round

    if i >= notes.length{
        i -= notes.length
    }


}

If you have read this far and are thinking 'that code doesn't look much like swift', that is because I am currently transitioning from JavaScript to Swift and some habits die hard.

I am after an alternative looping arrangement as for in creates i as a let making it immutable throwing an error at the increment line i += scaleIntervals![i]. I thought stride might work but I can't get my head around how to set it up.

Also using a for in where loop with the modulo operator only works to a point because I make have large increments that could cause a false positive. That being said if I am wrong and you can make it work I would love to learn how.

I'd even accept a completely different structure (i.e. non for-loop).

Answer 1

Here's a corrected version using a regular for-in loop over the scaleIntervals. You also forgot about F# :-)

let scaleIntervals: [Int] = [2,2,1,2,2,2,1]
let notes: [String] = ["A", "A#", "B", "C", "C#", "D", "D#", "E", "F", "F#", "G", "G#"]

func scale(fromNote tonicIndex: Int) -> [String] {
    var result: [String] = []

    var currentIndex = tonicIndex
    for interval in scaleIntervals {
        result.append(notes[currentIndex])
        currentIndex = (currentIndex + interval) % notes.count
    }

    return result
}

print(scale(fromNote: 0))  // ["A", "B", "C#", "D", "E", "F#", "G#"]
print(scale(fromNote: 3))  // ["C", "D", "E", "F", "G", "A", "B"]

---

Just for fun, here's a single-expression version using reduce. This is really slow to typecheck and probably less efficient, but it's cute:

func scale(fromNote tonicIndex: Int) -> [String] {
    return scaleIntervals.reduce((notes: [], index: 0), {
        ($0.notes + [notes[$0.index]], ($0.index + $1) % notes.count)
    }).notes
}