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

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 wil

Answer 1

Both ClosedRange and CountableRange throw this error unless start <= end. One problem with using stride(from:to:by) is that it returns a Strideable and not a Range, which doesn't work with the "is in range" operator ~=. To handle these cases, I use an extension which gives me a bounds-safe range, where invalid ranges become an empty range:

extension Int {
  func until(_ end: Int) -> CountableRange {
    return self <= end ? self.. CountableRange {
    return self <= end ? self..<(end + 1) : self..

Both return CountableRange so that an invalid through range becomes an empty range. (Side note: the name until is chosen because this "safe" range behaves the same as until ranges in Kotlin/Android).

Usage:

for i in 0.until(3) { /* do something */ }

let printRange = { (range: Range) -> Void in
  for i in range {
    print(i, terminator: " ")
  }
  print()
}
printRange(0.until(3))  // prints "0 1 2"
printRange(0.until(0))  // prints ""
printRange(3.until(0))  // prints ""
printRange(0.through(3))  // prints "0 1 2 3"
printRange(0.through(0))  // prints "0"
printRange(3.through(0))  // prints ""
print(0.until(1) ~= -1)  // prints "false"
print(0.until(1) ~= 0)  // prints "true"
print(0.until(1) ~= 1)  // prints "false"
print(0.until(0) ~= 0)  // prints "false"
print(1.until(0) ~= 0)  // prints "false"
print(0.through(1) ~= -1)  // prints "false"
print(0.through(1) ~= 0)  // prints "true"
print(0.through(1) ~= 1)  // prints "true"
print(0.through(0) ~= 0)  // prints "true"
print(1.through(0) ~= 0)  // prints "false"