A correct and idiomatic way to iterate through a collection with index in Swift?

Question

I want to iterate over an arbitrary Swift collection and get both the element and its index.

Basically an alternative to:

for (idx, el) in collection.enumerate() {
    print("element at \(idx) is \(el)")
}

but that gives me real generic indexes, not just sequential integers starting at 0.

Of course, the solution is going to be a part of a generic function that accepts any kind of collection, otherwise the difference wouldn't be very important.

Is there a better way than a naïve loop like below?

var idx = collection.startIndex, endIdx = collection.endIndex
while idx < endIdx {
    let el = collection[idx]
    print("element at \(idx) is \(el)")
    idx = idx.successor()
}

Writing that seems fairly error-prone. I know I can turn that code into a snippet, but I'd like to find a more concise and more idiomatic solution if possible.

from my tests idx.dynamicType is Int but you say generic index am i missing something? — Ali Kıran, Jul 09 '16 at 12:31
@AliKıran: It depends on the collection. Arrays have integer indices, but strings for example have a special String.CharacterView.Index type. Also the indices of a collection need not start at zero (e.g. array slices). — Martin R, Jul 09 '16 at 12:43
I hope that the linked-to "duplicate" solves your problem. Otherwise let me know and I'll reopen the question. — Martin R, Jul 09 '16 at 13:16
@MartinR thank you i get it i just misunderstood the question — Ali Kıran, Jul 09 '16 at 13:30
@MartinR Thanks, the other question is more narrowly formulated but the answer is exactly correct. I think the answer to this one was better and more to the point, though. I'm not sure what the SO policy is for such cases; if it were left to my discretion, I'd leave both. Thank you! — Andrey Tarantsov, Jul 09 '16 at 16:01
@AndreyTarantsov: I have re-activated the answer and reopened the question. — Martin R, Jul 10 '16 at 21:54

score 1 · Accepted Answer · edited May 23 '17 at 12:24

For any collection the indices property returns a range of the valid indices. To iterate over the indices and the corresponding elements in parallel you can use zip():

for (idx, el) in zip(collection.indices, collection) {
    print(idx, el)
}

Example for an array slice:

let a = ["a", "b", "c", "d", "e", "f"]
let slice = a[2 ..< 5]

for (idx, el) in zip(slice.indices, slice) {
    print("element at \(idx) is \(el)")
}

Output:

element at 2 is c
element at 3 is d
element at 4 is e

You can define a custom extension method for that purpose (taken from How to enumerate a slice using the original indices?):

// Swift 2:
extension CollectionType {
    func indexEnumerate() -> AnySequence<(index: Index, element: Generator.Element)> {
        return AnySequence(zip(indices, self))
    }
}

// Swift 3:
extension Collection {
    func indexEnumerate() -> AnySequence<(Indices.Iterator.Element, Iterator.Element)> {
        return AnySequence(zip(indices, self))
    }
}

Example for a character view:

let chars = "az".characters
for (idx, el) in chars.indexEnumerate() {
    print("element at \(idx) is \(el)")
}

Output:

element at 0 is a
element at 1 is 
element at 3 is 
element at 7 is z

Why are the `chars` example's indices 0, 1, 3, 7 instead of 0, 1, 2, 3? — dantiston, Jul 09 '16 at 16:23
@dantiston: Because the string indices internally refer to UTF-16 code points. But you should not care about these numbers. If you use the proper methods (like advancedBy, distanceTo) to operate on indices, everything works as expected. — Martin R, Jul 09 '16 at 16:26

A correct and idiomatic way to iterate through a collection with index in Swift?

1 Answers1