Modifying a Swift array while iterating over its range - 0 to count vs. startIndex to endIndex

Question

I'm new to Swift and I'm exploring what happens if a collection is modified while iterating over it using a Range. I can't figure out why these two loops have different behavior:

var things = ["a", "b", "c"]
for i in 0..<things.count {
    print("i: \(i) count: \(things.count) value: \(things[i])")
    if i == 1 {
        things.append("x")
    }
}
// i: 0 count: 3 value: a
// i: 1 count: 3 value: b
// i: 2 count: 4 value: c

vs

var things = ["a", "b", "c"]
for i in things.startIndex...things.endIndex {
    print("i: \(i) count: \(things.count) value: \(things[i])")
    if i == 1 {
        things.append("x")
    }
}
// i: 0 count: 3 value: a
// i: 1 count: 3 value: b
// i: 2 count: 4 value: c
// i: 3 count: 4 value: x

I created this Array class extension so I could see when the Range literal accesses the array's properties:

extension Array {
    var myCount: Int {
        print("myCount (\(self.count))")
        return self.count
    }
    var myStartIndex: Int {
        print("myStartIndex (\(self.startIndex))")
        return self.startIndex
    }
    var myEndIndex: Int {
        print("myEndIndex (\(self.endIndex))")
        return self.endIndex
    }
}

If I use these properties instead of the normal ones, I get:

myCount (3)
i: 0 count: 3 value: a
i: 1 count: 3 value: b
i: 2 count: 4 value: c

and

myStartIndex (0)
myEndIndex (3)
i: 0 count: 3 value: a
i: 1 count: 3 value: b
i: 2 count: 4 value: c
i: 3 count: 4 value: x

I must be missing something, because this feels like magic! It seems like endIndex is being re-evaluated, but count is not. What gives?

for i in things.indices { or if you don't need the index for thing in things { — Leo Dabus
– Leo Dabus, Commented Dec 24, 2017 at 7:10

Papershine · Accepted Answer · 2017-12-24 06:28:10Z

3

This is because in the first piece of code, you have used the half-open range operator (..<) while in the second piece of code you used the normal range operator (...). They are different. From the apple docs:

The half-open range operator (a..<b) defines a range that runs from a to b, but doesn’t include b.

So, the for loop in the first piece of code does not continue if things.count is 4 as the range (0 - 4) when used with the half-open range operator only loops when i is 0, 1, 2, or 3.

If you use ... instead of ..<, the result of the first piece of code will be the same as the second one.

answered Dec 24, 2017 at 6:28

Papershine

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1 Comment

jibberia Over a year ago

Thanks for making me smack my head -- what a simple mistake! It seems that the only way to iterate over a changing collection is to use a while loop, because it evaluates its condition each time through the loop. Is that right -- for loops in Swift only evaluate once? (I'm not saying this is a good idea ;) -- just trying to learn the language)

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