How to transform iterating over characters in String to Swift from Objective C?

user_2725435 0
Viewed 29

This is my function in objc:

+ (NSString *)hexForString:(NSString *)string{
    NSUInteger charIndex = 0;
    for (NSUInteger i = 0; i < string.length; i++){
        charIndex += (NSUInteger )[string characterAtIndex:i];
    }
    NSArray *colors = @[
        @"1abc9c", @"2ecc71", @"3498db",
        @"9b59b6", @"34495e", @"16a085",
        @"27ae60", @"2980b9", @"8e44ad",
        @"2c3e50", @"f1c40f", @"e67e22",
        @"e74c3c", @"95a5a6", @"f39c12",
        @"d35400", @"c0392b", @"bdc3c7",
        @"7f8c8d"
    ];
    charIndex %= colors.count;
    NSLog(@">>po modulo %lld", charIndex); // output is 13 for "Johny ivan"
    return colors[charIndex];
}

and now Swift:

func hex(for string: String) -> String {
    var index = 0
    string.forEach { character in
        index += character.wholeNumberValue ?? 0
    }
    let colors = [
        "1abc9c", "2ecc71", "3498db",
        "9b59b6", "34495e", "16a085",
        "27ae60", "2980b9", "8e44ad",
        "2c3e50", "f1c40f", "e67e22",
        "e74c3c", "95a5a6", "f39c12",
        "d35400", "c0392b", "bdc3c7",
        "7f8c8d",
    ]
    index %= colors.count
    print("+++ \(index)") // output is 0 for "Johny ivan"
    return colors[index]
}
swift objective-c string
2 Answers

NSStrings are a sequence of UTF-16 code units:

An NSString object encodes a Unicode-compliant text string, represented as a sequence of UTF–16 code units. All lengths, character indexes, and ranges are expressed in terms of 16-bit platform-endian values, with index values starting at 0.

Therefore, your Objective-C for loop was looping through the UTF-16 code units, and adding up the numerical value of each of them.

You can do the same thing in Swift:

for codeUnit in string.utf16 {
    index += Int(codeUnit)
}

You can also rewrite this into a reduce:

func hex(for string: String) -> String {
    let colors = [
        "1abc9c", "2ecc71", "3498db",
        "9b59b6", "34495e", "16a085",
        "27ae60", "2980b9", "8e44ad",
        "2c3e50", "f1c40f", "e67e22",
        "e74c3c", "95a5a6", "f39c12",
        "d35400", "c0392b", "bdc3c7",
        "7f8c8d",
    ]
    let index = string.utf16.reduce(0, { result, acc in result + Int(acc) })
                % colors.count
    print("+++ \(index)") // output is 0 for "Johny ivan"
    return colors[index]
}

Your swift code is looping through the Characters of a String (which are a different concept from UTF-16 code units), and adding up their wholeNumberValues, which is:

The numeric value this character represents, if it represents a whole number.

Clearly, none of the characters in "Johny ivan" represents a whole number.

user_51335850

The Objective-C version basically adds up the ASCII value of each character (actually, UTF16-scalar value), while your Swift version checks whether a character is a number and returns that value. A "more correct" approach would be to iterate over character.unicodeScalars and adding their .value. But there's a shortcut on String already for that:

for scalar in string.unicodeScalars {
    index += scalar.value
}
user_4000560
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