去切片-[：n]和[n：]之间的区别

Question

Go Slice question, please check below and comment if I am missing something.

   import "fmt"
   func main() {
       s := []int{2, 3, 5, 7, 11, 13}
       s = s[1:]
       fmt.Println(s)
       s = s[2:]
       fmt.Println(s)
       s = s[5:]
       fmt.Println(s)
  }

Output:

[3 5 7 11 13] [7 11 13] panic: runtime error: slice bounds out of range

The above makes sense.

func main() {
       s := []int{2, 3, 5, 7, 11, 13}
       s = s[:1]
       fmt.Println(s)
       s = s[:2]
       fmt.Println(s)
       s = s[:5]
       fmt.Println(s)
   }

Output:

[2] [2 3] [2 3 5 7 11]

Should this also get array out of bounds panic from s=s[:2]?

Answer 1

Subslicing in Go allows you to slice beyond the end of the slice, as long as it's still within range of the underlaying array's capacity. You cannot slice before the start of that slice, but you can slice after it so long as you don't go past that last allocated index.

As an example, s[3:] then s[:3] works, but s[4:] then s[:4] will panic, as you're requesting indexes 4 through 7 of the underlying array, which only has allocated indexes 0-5.

It's a bit of an oddity, but it does allow you to max out any slice simply by doing slice = slice[:cap(slice)].

https://play.golang.org/p/Gq5xoXc3Vd

The language specification annotes this, btw. I've paraphrased it below for the simple slice notation you're using (there's an alternative that also specifies the maximum index for the new slice).

For a string, array, pointer to array, or slice a, the primary expression a[low : high] constructs a substring or slice. The indices are in range if 0 <= low <= high <= cap(a), otherwise they are out of range.