全局的dropLast()函数作用于序列，因此也作用于字符串:

var expression  = "45+22"
expression = dropLast(expression)  // "45+2"

// in Swift 2.0 (according to cromanelli's comment below)
expression = String(expression.characters.dropLast())

Swift 4.0(也叫Swift 5.0)

var str = "Hello, World"                           // "Hello, World"
str.dropLast()                                     // "Hello, Worl" (non-modifying)
str                                                // "Hello, World"
String(str.dropLast())                             // "Hello, Worl"

str.remove(at: str.index(before: str.endIndex))    // "d"
str                                                // "Hello, Worl" (modifying)

斯威夫特3.0

api变得更加敏捷了，因此Foundation扩展也做了一些改变:

var name: String = "Dolphin"
var truncated = name.substring(to: name.index(before: name.endIndex))
print(name)      // "Dolphin"
print(truncated) // "Dolphi"

或者就地版本:

var name: String = "Dolphin"
name.remove(at: name.index(before: name.endIndex))
print(name)      // "Dolphi"

谢谢Zmey, Rob Allen!

Swift 2.0+方式

有几种方法可以做到这一点:

通过Foundation扩展，尽管不是Swift库的一部分:

var name: String = "Dolphin"
var truncated = name.substringToIndex(name.endIndex.predecessor())
print(name)      // "Dolphin"
print(truncated) // "Dolphi"

使用removeRange()方法(改变名称):

var name: String = "Dolphin"    
name.removeAtIndex(name.endIndex.predecessor())
print(name) // "Dolphi"

使用dropLast()函数:

var name: String = "Dolphin"
var truncated = String(name.characters.dropLast())
print(name)      // "Dolphin"
print(truncated) // "Dolphi"

旧的字符串。索引(Xcode 6 Beta 4 +)方式

由于Swift中的字符串类型旨在提供出色的UTF-8支持，您不能再使用Int类型访问字符索引/范围/子字符串。相反，您使用String。指数:

let name: String = "Dolphin"
let stringLength = count(name) // Since swift1.2 `countElements` became `count`
let substringIndex = stringLength - 1
name.substringToIndex(advance(name.startIndex, substringIndex)) // "Dolphi"

或者(对于一个更实用，但教育意义更小的例子)，你可以使用endIndex:

let name: String = "Dolphin"
name.substringToIndex(name.endIndex.predecessor()) // "Dolphi"

注意:我发现这是理解String的一个很好的起点。指数

旧(pre-Beta 4)方式

你可以简单地使用substringToIndex()函数，提供它比String的长度小1:

let name: String = "Dolphin"
name.substringToIndex(countElements(name) - 1) // "Dolphi"

let str = "abc"
let substr = str.substringToIndex(str.endIndex.predecessor())  // "ab"

使用新的Substring类型用法:

斯威夫特4:

var before: String = "Hello world!"
var lastCharIndex: Int = before.endIndex
var after:String = String(before[..<lastCharIndex])
print(after) // Hello world

短:

var before: String = "Hello world!"
after = String(before[..<before.endIndex])
print(after) // Hello world

Swift 3:当你想删除尾随字符串:

func replaceSuffix(_ suffix: String, replacement: String) -> String {
    if hasSuffix(suffix) {
        let sufsize = suffix.count < count ? -suffix.count : 0
        let toIndex = index(endIndex, offsetBy: sufsize)
        return substring(to: toIndex) + replacement
    }
    else
    {
        return self
    }
}

函数的作用是:删除字符串的最后一个元素。

var expression = "45+22"
expression = expression.dropLast()