Can't form Range with end < start Check range before doing for loop?

Question

I\'m encountering a change in the swift code which i do not quite understand.

var arr = []
for var i = 1; i <= arr.count; i += 1
{
    print(\"i want to          


        

Answer 1

If you just want to iterate over a collection, then use the for <element> in <collection> syntax.

for element in arr {
    // do something with element
}

If you also need access to the index of the element at each iteration, you can use enumerate(). Because indices are zero based, the index will have the range 0..<arr.count.

for (index, element) in arr.enumerate() {

    // do something with index & element

    // if you need the position of the element (1st, 2nd 3rd etc), then do index+1
    let position = index+1
}

You can always add one to the index at each iteration in order to access the position (to get a range of 1..<arr.count+1).

If none of these solve your problem, then you can use the range 0..<arr.count to iterate over the indices of your array, or as @vacawama says, you could use the range 1..<arr.count+1 to iterate over the positions.

for index in 0..<arr.count {

    // do something with index
}

for position in 1..<arr.count+1 {

    // do something with position
}

0..<0 cannot crash for an empty array as 0..<0 is just an empty range, and 1..<arr.count+1 cannot crash for an empty array as 1..<1 is also an empty range.

Also see @vacawama's comment below about using stride for safely doing more custom ranges. For example (Swift 2 syntax):

let startIndex = 4
for i in startIndex.stride(to: arr.count, by: 1) {
    // i = 4, 5, 6, 7 .. arr.count-1
}

Swift 3 syntax:

for i in stride(from: 4, to: arr.count, by: 1) {
    // i = 4, 5, 6, 7 .. arr.count-1
}

This is where startIndex is the number to start the range at, arr.count is the number that the range will stay below, and 1 is the stride length. If your array has less elements than the given starting index, then the loop will never be entered.

Answer 2

This should produce the same results as your first example, without error...

var arr = []
var i=1
for _ in arr
{
    print("i want to see the i \(i)")
    i += 1
}

...although that just seems like a complicated way to count the elements in an array (arr.count) so I suspect that there is more to this question than meets the eye.

Answer 3

The obvious solution in this case would be:

var arr = []
for i in arr.indices {
    print("I want to see the i \(i)") // 0 ... count - 1
    print("I want to see the i \(i + 1)") // 1 ... count
}

but carefully read through originaluser2's answer