Swift Array 可选类型和下标(Beta 3) [英] Swift Array optional Type and subscripting (Beta 3)

问题描述

我正在关注 2014 年 WWDC 教程 408:使用 XCode Beta 3 的 Swift Playgrounds(30 分钟).Swift 语法自 Beta 2 以来发生了变化.

I'm following the 2014 WWDC tutorial 408: Swift Playgrounds using XCode Beta 3 (30 minutes in). The Swift syntax has changed since Beta 2.

var data = [27, 46, 96, 79, 56, 85, 45, 34, 2, 57, 29, 66, 99, 65, 66, 40, 40, 58, 87, 64]

func exchange<T>(data: [T], i: Int, j: Int) {
    let temp = data[i]
    data[i] = data[j]  // Fails with error '@lvalue $T8' is not identical to 'T'
    data[j] = temp     // Fails with error '@lvalue $T5' is not identical to 'T'
}

exchange(data, 0 , 2)
data

为什么我不能以这种方式修改可变整数数组?

Why I can't modify a mutable integer array in this way?

推荐答案

因为子程序参数是用 let 隐式定义的，因此不可变.尝试将声明更改为:

Because subroutine parameters are implicitly defined with let hence, non mutable. Try changing the declaration to:

func exchange<T>(inout data: [T], i: Int, j: Int) {

和调用:

exchange(&date, 0, 2)

您也可以使用 var 但这只会允许在子例程中修改数组.Beta 3 的重大变化是让数组真正按值传递，而不是在某些时候只是按值传递，但其他时候则不然.

You can also use var but that would only allow the array to be modified within the subroutine. The big change for beta 3 was to make arrays really pass by value instead of just kind of sorta pass by value some of the time, but not the rest.

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