Slice append() 给出旧切片版本的值

Question

这是我的代码：

package main

import "fmt"

func main() {
    numList := make([]int, 4)
    numList[0] = 1
    numList[1] = 2
    numList[2] = 3
    numList[3] = 4
    fmt.Println(numList)

    numList = append(numList[1:2])
    fmt.Println("numList: ", numList)
    numList = append(numList[0:])
    fmt.Println(numList)
    numList2 := append(numList[0:2])
    fmt.Println(numList2)
    numList3 := append(numList[0:3])
    fmt.Println(numList3)
    numList4 := append(numList[0:4])
    fmt.Println(numList4)
}

输出为：

[1 2 3 4] // EXPECTED
numList:  [2] //EXPECTED
[2]           // EXPECTED
[2 3]         
[2 3 4]
panic: runtime error: slice bounds out of range [:4] with capacity 3

输出的最后三行非常令人困惑。

numList = append(numList[1:2])

和

numList = append(numList[0:])

给出

[2]

。

numList2 := append(numList[0:2])

如何将输出提供为

[2 3]

和

numList3 := append(numList[0:3])

如何将输出提供为

[2 3 4]

？ 3和4是从哪里来的？

有人可以解释一下这里到底发生了什么吗？

Answer 1

我将逐步解释发生了什么：

// 1. Initial creation and assignments
numList := make([]int, 4)     // [0 0 0 0]    len=4, cap=4
numList[0] = 1                // [1 0 0 0]
numList[1] = 2                // [1 2 0 0]
numList[2] = 3                // [1 2 3 0]
numList[3] = 4                // [1 2 3 4]
fmt.Println(numList)          // Prints: [1 2 3 4]

// 2. First slice operation
numList[1:2]                  // Creates view: [2]
numList = append(numList[1:2])// New slice: [2]    len=1, cap=3
                             // Underlying array: [2 3 4 ?]
fmt.Println("numList: ", numList) // Prints: [2]

// 3. Second slice operation
numList[0:]                   // Creates view: [2]
numList = append(numList[0:]) // Still: [2]    len=1, cap=3
                             // Underlying array: [2 3 4 ?]
fmt.Println(numList)          // Prints: [2]

// 4. Creating numList2
numList[0:2]                  // Even though len=1, can access up to cap=3
                             // Creates view: [2 3]
numList2 := append(numList[0:2]) // [2 3]    len=2, cap=3
fmt.Println(numList2)         // Prints: [2 3]

// 5. Creating numList3
numList[0:3]                  // Can access up to cap=3
                             // Creates view: [2 3 4]
numList3 := append(numList[0:3]) // [2 3 4]    len=3, cap=3
fmt.Println(numList3)         // Prints: [2 3 4]

// 6. Creating numList4 - PANIC!
numList[0:4]                  // Trying to access beyond cap=3
                             // PANIC: slice bounds out of range [:4] with capacity 3
numList4 := append(numList[0:4])
fmt.Println(numList4)

注意事项：

• 切片的容量决定了切片的最大有效索引。
• 长度仅决定正常操作中可见的内容。
• 切片时，您可以访问最大容量（因此您会恐慌）