does Swift array.count get evaluated each time in a loop - swift

Question

I suppose this is a question that must get asked for every language, but when you write for example:
while i < array.count {
...
}
does array.count get evaluated each time the loop runs? Is it better to store it in a let constant before running the loop like this?
let length = array.count
while i < length {
...
}

Answer 1

Array gets the count property because it conforms to Collection. The documentation for count in Collection states
Complexity: O(1) if the collection conforms to RandomAccessCollection; otherwise, O(n), where n is the length of the collection.
Source
Since Array also conforms to RandomAccessCollection, it is a constant time operation to get the count of the array. There shouldn't be any major performance difference between getting it once at the start vs every loop iteration.

Answer 2

while loops (and do while loops) have their predicates evaluated on each iteration.
for loops evaluate the sequences once.
Here's is a demonstration:
var array: [Int]
print("Test Case 1 - while i < array.count")
array = [1, 2, 3, 4, 5, 6]
var i = 0
while i < array.count {
print(array[i])
if i < 3 { array.append(123) }
i += 1
}
print("\r\nTest Case 2 - for i in array.indices")
array = [1, 2, 3, 4, 5, 6]
for i in array.indices {
print(array[i])
if i < 3 { array.append(123) }
}
print("\r\nTest Case 3 - for i in 0 ..< array.count")
array = [1, 2, 3, 4, 5, 6]
for i in 0 ..< array.count {
print(array[i])
if i < 3 { array.append(123) }
}
Test Case 1 - while i < array.count
1
2
3
4
5
6
123
123
123
Test Case 2 - for i in array.indices
1
2
3
4
5
6
Test Case 3 - for i in 0 ..< array.count
1
2
3
4
5
6

Answer 3

Yes it's evaluated on each iteration.
Assigning to a constant will be slightly more performant. However with all of the optimisations in a modern compiler I wouldn't bother. Unless the loop count is going to be humongous.

Answer 4

Careful with for loops like the following. Since elems.count is part of a range that gets constructed once at the top of the loop, it is evaluated exactly once. The following code will die when i = 4:
var elems = [1, 2, 3, 4, 5, 6]
for i in 0 ..< elems.count {
if i % 2 == 0 { // remove even elements
elems.remove(at: i)
}
}
The array.count in your while does indeed get evaluated each time the condition is evaluated.

Answer 5

Yes, it gets called every time
Let's run a simple test, first of all we need the following Array Extension
extension Array {
var myCustomCount: Int {
print("myCustomCount")
return self.count
}
}
And then we can try this code
let nums = [1, 2, 3, 4, 5]
var i = 0
while i < nums.myCustomCount {
i += 1
}
The output is
myCustomCount
myCustomCount
myCustomCount
myCustomCount
myCustomCount
myCustomCount