延长长度并保持价值

Question

I follow the tour of Go to learning GOLANG.

And I have a question at this step: https://tour.golang.org/moretypes/11

  package main

  import "fmt"

  func main() {
    s := []int{2, 3, 5, 7, 11, 13}
    printSlice(s)

    // Step1 Slice the slice to give it zero length.
    s = s[:0]
    printSlice(s)

    // Step2 Extend its length.
    // Why after extend the length of the slice, the value in this slice is still  [2 3 5 7]
    s = s[:4]
    printSlice(s)

    // Step 3 Drop its first two values.
    s = s[2:]
    printSlice(s)
  }

  func printSlice(s []int) {
    fmt.Printf("len=%d cap=%d %v
", len(s), cap(s), s)
  }

The output:

len=6 cap=6 [2 3 5 7 11 13]
len=0 cap=6 []
len=4 cap=6 [2 3 5 7]
len=2 cap=4 [5 7]

Why after extending the length of the slice at the second step, the value in this slice is still [2 3 5 7]? I think the value in this slice is [0 0 0 0] because I have sliced the origin slice at the first step.

And I have another question is that why the third step can change the capacity of the slice but the first second cannot.

Answer 1

Because the first time extending does not change the slice's pointer address. So s also points to the [2 3 5 7 11 13] address.

package main

import (
    "fmt"
    "unsafe"
)

func main() {
    s := []int{2, 3, 5, 7, 11, 13}
    printSlice(s)

    // Slice the slice to give it zero length.
    s = s[:0]
    printSlice(s)

    // Extend its length.
    s = s[:4]
    printSlice(s)

    // Drop its first two values.
    s = s[2:]
    printSlice(s)
}

func printSlice(s []int) {
    fmt.Printf("len=%d cap=%d %v array ptr: %v 
", len(s), cap(s), s,(*unsafe.Pointer)(unsafe.Pointer(&s)))
}

the terminal shows:

len=6 cap=6 [2 3 5 7 11 13] array ptr: 0xc04200a2a0
len=0 cap=6 [] array ptr: 0xc04200a2a0
len=4 cap=6 [2 3 5 7] array ptr: 0xc04200a2a0
len=2 cap=4 [5 7] array ptr: 0xc04200a2b0

you see, the third step changes the ptr address because of first item is changed. so you know...