Swiftier Swift用于“添加阵列,或者如果不存在则创建……”

var x:[String:[Thing]] = [:]

x[which].append(t)

if x.index(forKey: which) == nil {
    x[which] = []
    }
x[which]!.append(s!)

x[index?!?!].append??(s?!)

Swift 4更新：

从Swift 4开始,字典有一个subscript(_:default:)方法,所以

Dict[key,default: []].append(newElement)

追加到已存在的数组或空数组.例：

var Dict: [String: [Int]] = [:]
print(Dict["foo"]) // nil

Dict["foo",default: []].append(1)
print(Dict["foo"]) // Optional([1])

Dict["foo",default: []].append(2)
print(Dict["foo"]) // Optional([1,2])

从Swift 4.1(目前处于测试版)开始,这也很快,
比较Hamish的评论here.

以前的答案为Swift< = 3：据我所知 - 没有办法“创建或更新”字典
使用单个下标调用的值.

除了你写的内容,你可以使用Nil-coalescing运算符

Dict[key] = (Dict[key] ?? []) + [elem]

或可选链接(如果追加操作,则返回nil
无法执行)：

if Dict[key]?.append(elem) == nil {
     Dict[key] = [elem]
}

正如SE-0154 Provide Custom Collections for Dictionary Keys and Values中提到的那样,也是@Hamish在评论中提到的两种方法
制作一个数组的副本.

随着SE-0154的实施,您将能够变异
没有复制的字典值：

if let i = Dict.index(forKey: key) {
    Dict.values[i].append(elem)
} else {
    Dict[key] = [key]
}

目前,Rob Napier提供了最有效的解决方案
在Dictionary in Swift with Mutable Array as value is performing very slow? How to optimize or construct properly?：

var array = Dict.removeValue(forKey: key) ?? []
array.append(elem)
Dict[key] = array

一个简单的基准确认“Rob的方法”是最快的：

let numKeys = 1000
let numElements = 1000

do {
    var Dict: [Int: [Int]] = [:]

    let start = Date()
    for key in 1...numKeys {
        for elem in 1...numElements {
            if Dict.index(forKey: key) == nil {
                Dict[key] = []
            }
            Dict[key]!.append(elem)

        }
    }
    let end = Date()
    print("Your method:",end.timeIntervalSince(start))
}

do {
    var Dict: [Int: [Int]] = [:]

    let start = Date()
    for key in 1...numKeys {
        for elem in 1...numElements {
            Dict[key] = (Dict[key] ?? []) + [elem]
        }
    }
    let end = Date()
    print("Nil coalescing:",end.timeIntervalSince(start))
}


do {
    var Dict: [Int: [Int]] = [:]

    let start = Date()
    for key in 1...numKeys {
        for elem in 1...numElements {
            if Dict[key]?.append(elem) == nil {
                Dict[key] = [elem]
            }
        }
    }
    let end = Date()
    print("Optional chaining",end.timeIntervalSince(start))
}

do {
    var Dict: [Int: [Int]] = [:]

    let start = Date()
    for key in 1...numKeys {
        for elem in 1...numElements {
            var array = Dict.removeValue(forKey: key) ?? []
            array.append(elem)
            Dict[key] = array
        }
    }
    let end = Date()
    print("Remove and add:",end.timeIntervalSince(start))
}

1000键/ 1000个元素的结果(在1.2 GHz Intel Core m5 MACBook上)：

Your method:      0.470084965229034
Nil coalescing:   0.460215032100677
Optional chaining 0.397282958030701
Remove and add:   0.160293996334076

对于1000个键/ 10,000个元素：

Your method:      14.6810429692268
Nil coalescing:   15.1537700295448
Optional chaining 14.4717089533806
Remove and add:   1.54668599367142