Counting character frequencies in a Swift string - swift

Question

I'm trying to convert Java code to Swift and facing the issue:
single-quoted string literal found, use '"' charArray[(s[i].asciiValue)! - ('a'.asciiValue)!]++ ^~~ "a"
Java Code:
for(String s: str){
char arr[] = new char[26]
for(int i =0;i< s.length(); i++){
arr[s.charAt(i) -'a']++;
}
}
Swift Code:
extension String {
var asciiArray: [UInt32] {
return unicodeScalars.filter{$0.isASCII}.map{$0.value}
}
}
extension Character {
var asciiValue: UInt32? {
return String(self).unicodeScalars.filter{$0.isASCII}.first?.value
}
}
class GroupXXX {
func groupXXX(strList: [String]) {
for str in strList {
var charArray = [Character?](repeating: nil, count: 26)
var s = str.characters.map { $0 }
for i in 0..<s.count {
charArray[(s[i].asciiValue)! - ('a'.asciiValue)!]++
}
}
}
}

Answer 1

There are several problems in your Swift code:
There are no single-quoted character literals in Swift (as already explained
by JeremyP).
The ++ operator has been removed in Swift 3.
s[i] does not compile because Swift strings are not indexed by
integers.
Defining the array as [Character?] makes no sense, and you cannot
increment a Character?. The Swift equivalent of the Java char
would be UInt16.
You don't check if the character is in the range "a"..."z".
Apparently you want to count the number of occurrences of
each character "a" to "z" in a string.
This is how I would do it in Swift:
Define the "frequency" array as an array of integers.
Enumerate the unicodeScalars property of the string.
Use a switch statement to check for the valid range of characters.
Then the custom extension are not needed anymore and the code becomes
var frequencies = [Int](repeating: 0, count: 26)
for c in str.unicodeScalars {
switch c {
case "a"..."z":
frequencies[Int(c.value - UnicodeScalar("a").value)] += 1
default:
break // ignore all other characters
}
}

Answer 2

charArray[(s[i].asciiValue)! - ('a'.asciiValue)!]++
As the error says, use double quotes, Swift doesn't have a syntax that differentiates between characters and strings (characters themselves may be sequences of bytes in swift).
You may need to explicitly force the character to be a Character if the compiler can't differentiate.
charArray[(s[i].asciiValue)! - (Character("a").asciiValue)!]++