在Xcode 8.3.2, iOS 10.3 这对现在来说很好

Sample1:

let dayMoveRaw = 5 
let dayMove = String(format: "%02d", arguments: [dayMoveRaw])
print(dayMove) // 05

Sample2:

let dayMoveRaw = 55 
let dayMove = String(format: "%02d", arguments: [dayMoveRaw])
print(dayMove) // 55

细节

Xcode 9.0.1, swift 4.0

解决方案

Data

import Foundation

let array = [0,1,2,3,4,5,6,7,8]

解决方案1

extension Int {

    func getString(prefix: Int) -> String {
        return "\(prefix)\(self)"
    }

    func getString(prefix: String) -> String {
        return "\(prefix)\(self)"
    }
}

for item in array {
    print(item.getString(prefix: 0))
}

for item in array {
    print(item.getString(prefix: "0x"))
}

解决方案2

for item in array {
    print(String(repeatElement("0", count: 2)) + "\(item)")
}

解决方案3

extension String {

    func repeate(count: Int, string: String? = nil) -> String {

        if count > 1 {
            let repeatedString = string ?? self
            return repeatedString + repeate(count: count-1, string: repeatedString)
        }
        return self
    }
}

for item in array {
    print("0".repeate(count: 3) + "\(item)")
}

与其他使用格式化器的答案不同，你也可以在循环内的每个数字前面添加一个“0”文本，就像这样:

for myInt in 1...3 {
    println("0" + "\(myInt)")
}

但是，当您必须为每个单独的数字添加指定数量的0时，formatter通常更好。如果你只需要加一个0，那么它就是你的选择。

使用Swift 5新奇的可扩展插值:

extension DefaultStringInterpolation {
    mutating func appendInterpolation(pad value: Int, toWidth width: Int, using paddingCharacter: Character = "0") {
        appendInterpolation(String(format: "%\(paddingCharacter)\(width)d", value))
    }
}

let pieCount = 3
print("I ate \(pad: pieCount, toWidth: 3, using: "0") pies")  // => `I ate 003 pies`
print("I ate \(pad: 1205, toWidth: 3, using: "0") pies")  // => `I ate 1205 pies`

Swift 4*及以上，你也可以试试这个:

func leftPadding(valueString: String, toLength: Int, withPad: String = " ") -> String {
        guard toLength > valueString.count else { return valueString }
        
        let padding = String(repeating: withPad, count: toLength - valueString.count)
        return padding + valueString
    }

调用函数:

leftPadding(valueString: "12", toLength: 5, withPad: "0")

输出: “00012”

对于左填充，添加一个字符串扩展，如下所示:

Swift 5.0 +

extension String {

    func padLeft(totalWidth: Int, with byString: String) -> String {
        let toPad = totalWidth - self.count
        if toPad < 1 {
            return self
        }
    
        return "".padding(toLength: toPad, withPad: byString, startingAt: 0) + self
    }
}

使用这种方法:

for myInt in 1...3 {
    print("\(myInt)".padLeft(totalWidth: 2, with: "0"))
}