Why is my Swift loop failing with error "Can't form range with end

Question

I have a for loop that checks if a number is a factor of a number, then checks if that factor is prime, and then it adds it to an array. Depending on the original number, I will get an error saying fatal error: Can't form range with end < start This happens almost every time, but for some numbers it works fine. The only numbers I have found to work with it are 9, 15, and 25. Here is the code: ``` let num = 16 // or any Int var primes = [Int]() for i in 2...(num/2) { if ((num % i) == 0) { var isPrimeFactor = true for l in 2...i-1 { if ((i%l) == 0) { isPrimeFactor = false; }//end if }//end for if (isPrimeFactor == true) { primes.append(i) }//end if }//end if }//end for ```

Accepted Answer

Swift 5

If you need a loop with dynamic value-range, I suggest that using `stride(to:by:)` instead of `..<` or `...`

Basically `..<` or `...` will be crashed if `start_index > end_index`.

This will be crash:

```
let k = 5
for i in 10...k { print("i=\(i)") }
for i in 10..<k { print("i=\(i)") }
```

How to fix:

```
let k = 5
for i in stride(from: 10, through: k, by: 1) { print("i=\(i)") }
for i in stride(from: 10, to: k, by: 1) { print("i=\(i)") }
```

NOTE:

The code above won't print out anything, but it won't be crash when execution.

Also, if you want to stride from a higher number to a lower number then the `by` parameter needs to be changed to a negative number.

Reference:

- http://michael-brown.net/2016/using-stride-to-convert-c-style-for-loops-to-swift-2.2/

- http://swiftdoc.org/v2.1/protocol/Strideable/

Why is my Swift loop failing with error "Can't form range with end < start"?

Solution

Problem