Converting a method from Objective C to Swift - objective-c

Question

I am starting to learn Swift and hope to find it an excellent replacement for Objective C.
I am attempting to convert my Objective C classes into Swift and I cannot find the best way to translate the following method into Swift.
#implementation VersionReader
- (NSString *)readVersionFromString:(NSString *)string {
if (string.length == 0) {
return nil;
}
unichar firstChar = [string characterAtIndex:0];
if (firstChar < '0' || firstChar > '9') {
return nil;
}
NSUInteger length = string.length;
for (NSUInteger i = 0; i < length; ++i) {
if ([string characterAtIndex:i] == ' ') {
return [string substringToIndex:i];
}
}
return string;
}
#end
So far my Swift code looks like this:
import Cocoa
class VersionReader {
func readVersionFromString(string: String) -> String? {
if (string.isEmpty) {
return nil
}
var firstChar = string.characterAtIndex[0]
if (firstChar < 48 || firstChar > 57) {
return nil
}
var length = string.utf16Count
for (var i = 0; i < length; ++i) {
if (string.characterAtIndex(i) == 32) {
return string.substringToIndex(i)
}
}
return string
}
}
Tom this, I get the same error on two lines:
'String' does not have a member named 'characterAtIndex'
What would be an alternative to make this work in Swift? Thanks in advance.

Answer 1

A possible Swift solution:
func readVersionFromString(string: String) -> String? {
if string.isEmpty {
return nil
}
let firstChar = string[string.startIndex]
if !find("0123456789", firstChar) {
return nil
} else if let pos = find(string, " ") {
return string.substringToIndex(pos)
} else {
return string
}
}

Answer 2

Swift's String type doesn't have a characterAtIndex method. You can cast it to an NSString and use it as below:
var firstChar = (string as NSString).characterAtIndex(0)
Or
var firstChar = string.bridgeToObjectiveC().characterAtIndex(0)
Note that the characterAtIndex method returns an unichar, which seems to be what you want. But the correct approach to get a Swift Character would be that suggested by FreeAsInBeer in his answer: Array(string)[0]

Answer 3

Swift doesn't have a characterAtIndex selector. Instead, you need to use Array(string)[0].