Indexes as Ints in Swift - ios

Question

I have the following code which I've commented:
func hash (s:String) -> Int {
var z = 19
let key = "abcdefghijk"
for var i = 0; i < s.characters.count; i++ {
let index = s.startIndex.advancedBy(i)
let char = s[index]
if let f = key.characters.indexOf(char) {
print(f) //This outputs the number I want to use in the next line, but as String.CharacterView.Index, not an Int
z = (z * f) //obviously this won't work
}
// So instead we try this:
z = z * s.startIndex.distanceTo(key.characters.indexOf(char)!))
// It works for the first few characters then I get "fatal error: can not increment endIndex" and a EXC_BAD_INSTRUCTION
}
return z
}
I'm struggling to use Swift String to find the index I want to use as an Int in some kind of a hash function. In case it isn't clear:
User inputs a string, the function iterates through each character, finding that character in the key and then takes the index of that character in key and multiplies it to the existing z counter.
I get the results I want but only in the wrong type which doesn't let me convert to Int. Anyone know how?
Thanks

Answer 1

f = key.characters.indexOf(char) is an index into the characters
of key, therefore you have to compute the distance to the start index of key, not s:
z = z * key.startIndex.distanceTo(key.characters.indexOf(char)!))
which you can move back to your if-let block:
if let f = key.characters.indexOf(char) {
z = z * key.startIndex.distanceTo(f)
}
You also might want to use the overflow operator &*
if let f = key.characters.indexOf(char) {
z = z &* key.startIndex.distanceTo(f)
}
otherwise the application will crash if the result of the
multiplication does not fit into an Int.
Generally, the indexes of a Swift String can only be used
with the same string (regardless of string length), as the following
example demonstrates:
let s1 = "abc"
let i1 = s1.characters.indexOf("b")!
print(i1) // 1
let s2 = "🇩🇪a🇩🇪b🇩🇪c"
print(s2.characters.count) // 6
let d2 = s2.startIndex.distanceTo(i1) // fatal error: can not increment endIndex

Answer 2

If you make your key into an array of Characters, then indexOf will return the Int you need:
func hash (s:String) -> Int {
var z = 19
let key = Array("abcdefghijk".characters)
for char in s.characters {
if let f = key.indexOf(char) {
print(f)
z = (z &* (f + 1))
}
}
return z
}
Also, if your character is the first index in the key you will get a value of 0 which will make your hash value 0, so I've added 1 to f. I also incorporated &* as suggested by #MartinR to prevent Int overflow.

Answer 3

First, don't use forced unwrap as you can end up easily crashing your app (as it seems it already happened).
Second, you already test for the validity of key.characters.indexOf(char), you can place the z computation within the if-let:
z = z * s.startIndex.distanceTo(f)
What happens with your code is that the hash() function crashes as soon as it encounters a character beyond k, so you should also add all possible characters to key.