将Int转换为格式化的字符串（Swift）

Question

I am trying to format an int to formatted string, like a 23 -> "0023", 100 -> 0100 and so on. I've finished with the function below, but it eats last digit of every number that is multiple of 10, like 900 becomes 090, instead of 0900. Please help me fix that bug, thanks.

func convert(_ score: Int) -> String {
  return String(Float(score) / 1000.0).components(separatedBy: ".").joined()
}

Answer 1

Dividing by 1000 and relying on a certain floating point representation is fragile and a bad idea for this purpose. A simple solution is to use the %ld format with a minimum of 4 digits:

func convert(_ score: Int) -> String {
    return String(format: "%04ld", score)
}

print(convert(23)) // 0023

Answer 2

You can use a NumberFormatter and set minimumIntegerDigits to 4 to achieve your goals.

let nf = NumberFormatter()
nf.minimumIntegerDigits = 4
nf.locale = Locale(identifier: "en_US_POSIX") // Avoid thousands separator
nf.string(for: 23) //"0023"
nf.string(for: 90) //"0900"

For performance reasons, you should avoid recreating the NumberFormatter instance each time you need it and rather define it in a scope (ie by making it an instance/static property of the relevant class) such that it can be reused.