为什么这样做有效，我在最大索引后切片了1个，却没有收到错误？

Question

I was playing around with slices, and I found something that I can't explain. I create a string of total length 10 (index ranges from 0 to 9) and then I create a slice from it specifying one past the max index and it does not panic. It will panic if I go more than one past the last index. What am I not understanding please?

I have investigated with integer slices as well thinking there might be something odd about strings, but it showed the same behavior.

Here is an example: I expected a failure at x:= str[10:].

package main

import "fmt"

func main() {

    var str = "attribute="

    x := str[10:]
    fmt.Printf("type of x is %T
", x)
    //fmt.Printf("x is a %T and length of x is %d
", x, len(x))

    for k, v := range []byte(str) {
        fmt.Printf("%d, %x
", k, v)
    }

}

Playground: https://play.golang.org/p/Z-3YvTx3-s6

Output:

type of x is string
0, 61
1, 74
2, 74
3, 72
4, 69
5, 62
6, 75
7, 74
8, 65
9, 3d

Answer 1

It's in the spec. Having low or high equal to the length is allowed.

For arrays or strings, the indices are in range if 0 <= low <= high <= len(a), otherwise they are out of range.

As to "why is this not an error": you can visualize the slicing operation as cutting between the elements. Like this:

│   a   t   t   r   i   b   u   t   e   =    │
│ ↑   ↑   ↑   ↑   ↑   ↑   ↑   ↑   ↑   ↑   ↑  │
│ 0   1   2   3   4   5   6   7   8   9   10 │

Technically slice point 10 is still within bounds, but it has no elements after it. That's why [10:] results in an empty string (that, or because there are no elements between low and high). This works the same way in ruby, for example. Maybe other languages too.