swift - 在 Swift 中向数组中插入一个新元素

Question

代码:

let oldNums: [Int] = [1, 2, 3, 4, 5 ,6 , 7, 8, 9, 10]
var newArray = oldNums[1..<4]
newArray.insert(99, atIndex: 0) // <-- crash here
newArray.insert(99, atIndex: 1) // <-- work very well

感谢 newArray 是一个新的可变变量。所以我很困惑。 为什么？我无法将新元素插入“newArray”

Answer 1

oldNums[1..<4]不是一个数组，而是一个 ArraySlice :

An Array-like type that represents a sub-sequence of any Array, ContiguousArray, or other ArraySlice.

数组切片的索引不是从零开始的，而是对应的 到原始数组的索引。这是一个改变 Swift 2 附带并记录在 Xcode 7.0 Release notes 中:

For consistency and better composition of generic code, ArraySlice indices are no longer always zero-based but map directly onto the indices of the collection they are slicing and maintain that mapping even after mutations.

在你的情况下:

let oldNums: [Int] = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
var newArray = oldNums[1..<4]
print(newArray.indices)
// 1..<4

所以 0是 insert() 的无效索引, 这就是为什么

newArray.insert(99, atIndex: 0)

崩溃。要在切片的开头插入一个元素，您 可以用

newArray.insert(99, atIndex: newArray.startIndex)

要创建“真正的”数组而不是切片，请使用 Array() 构造函数:

let oldNums: [Int] = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
var newArray = Array(oldNums[1..<4])
print(newArray.indices)
// 0..<3

newArray.insert(99, atIndex:0)
print(newArray)
// [99, 2, 3, 4]