var str = "This is my string"

print(str.replacingOccurrences(of: " ", with: "+"))

输出是

This+is+my+string

Swift 3, Swift 4, Swift 5解决方案

let exampleString = "Example string"

//Solution suggested above in Swift 3.0
let stringToArray = exampleString.components(separatedBy: " ")
let stringFromArray = stringToArray.joined(separator: "+")

//Swiftiest solution
let swiftyString = exampleString.replacingOccurrences(of: " ", with: "+")

从Swift 2开始，String不再符合SequenceType。换句话说，你不能用for…在循环。

简单而简单的方法是将String转换为Array以获得索引的好处:

let input = Array(str)

我记得当我试图索引到字符串不使用任何转换。我真的很沮丧，因为我不能想出或达到一个理想的结果，我准备放弃了。 但我最终创建了我自己的解决方案，这里是扩展的完整代码:

extension String {
    subscript (_ index: Int) -> String {
    
        get {
             String(self[self.index(startIndex, offsetBy: index)])
        }
    
        set {
            remove(at: self.index(self.startIndex, offsetBy: index))
            insert(Character(newValue), at: self.index(self.startIndex, offsetBy: index))
        }
    }
}

现在，你可以像你最初想要的那样，使用索引从字符串中读取和替换单个字符:

var str = "cat"
for i in 0..<str.count {
 if str[i] == "c" {
   str[i] = "h"
 }
}

print(str)

这是一种简单而有用的方式来使用它，并通过Swift的字符串访问模型。 现在，下次你会觉得它很顺利，因为你可以循环字符串，而不是将它强制转换到数组中。

尝试一下，看看它是否有帮助!

Xcode 11•Swift 5.1

StringProtocol replacingOccurrences的突变方法可以实现如下:

extension RangeReplaceableCollection where Self: StringProtocol {
    mutating func replaceOccurrences<Target: StringProtocol, Replacement: StringProtocol>(of target: Target, with replacement: Replacement, options: String.CompareOptions = [], range searchRange: Range<String.Index>? = nil) {
        self = .init(replacingOccurrences(of: target, with: replacement, options: options, range: searchRange))
    }
}

var name = "This is my string"
name.replaceOccurrences(of: " ", with: "+")
print(name) // "This+is+my+string\n"

基于Ramis回答的Swift 3解决方案:

extension String {
    func withReplacedCharacters(_ characters: String, by separator: String) -> String {
        let characterSet = CharacterSet(charactersIn: characters)
        return components(separatedBy: characterSet).joined(separator: separator)
    }
}

尝试根据Swift 3的命名约定，提出一个合适的函数名。

这在swift 4.2中很容易做到。只需使用replacingOccurrences(of: " "， with: "_")进行替换

var myStr = "This is my string"
let replaced = myStr.replacingOccurrences(of: " ", with: "_")
print(replaced)