Slice index greater than length and less than capacity gives error

Following code gives a error at runtime.

package main

import fmt "fmt"

func main(){

    type b []int
    var k = make([]b, 5, 10)
    fmt.Printf("%d\n",k[8])
    fmt.Printf("%d", len(k))
}

Error is as follows.

panic: runtime error: index out of range

runtime.panic+0x9e /go/src/pkg/runtime/proc.c:1060
        runtime.panic(0x453b00, 0x300203f0)
runtime.panicstring+0x94 /go/src/pkg/runtime/runtime.c:116
        runtime.panicstring(0x4af6c6, 0xc)
runtime.panicindex+0x26 /go/src/pkg/runtime/runtime.c:73
        runtime.panicindex()
main.main+0x8d C:/GOEXCE~1/basics/DATATY~1/slice.go:9
        main.main()
runtime.mainstart+0xf 386/asm.s:93
        runtime.mainstart()
runtime.goexit /go/src/pkg/runtime/proc.c:178
        runtime.goexit()
----- goroutine created by -----
_rt0_386+0xbf 386/asm.s:80

While if k[0] or k[1] is printed, it runs fine. Can you please explain what exactly capacity means for slices.

You are simply indexing, so the index must be less than the length. The relevant section of the Go specification says that

> A primary expression of the form
>
> a[x]
>
> ...
>
> For a of type A or *A where A is an array type, or for a of type S
> where S is a slice type:
>
> x must be an integer value and 0 <= x < len(a)

However, if you were "slicing" (e.g. a[6:9]), then it would work with indexes that are greater than the length but within the capacity.

Read the Go Programming Language Specification.

> Length and capacity
>
> The capacity of a slice is the number of elements for which there is
> space allocated in the underlying array. At any time the following
> relationship holds:
>
> 0 <= len(s) <= cap(s)

var slice = make([]b, 5, 10)

is equal to

var array [10]b
slice := array[:5]

The difference is that when you use var slice = make([]b, 5, 10), you can't access the array under slice. The slice := array[:5] means the first element of slice is array[0] and the length of slice is 5, which means slice[0] == array[0], slice[1] == array[1], ... slice[4] == array[4]. Because you can only access the index that is less than the length(which means 0 <= index < length). The length of slice is 5 and the length of array is 10, so you can access array[8](0<=8<10) but can't access slice[8](8>5).

Full sample:

package main

import fmt "fmt"

func main(){

	type b []int
	var array [10]b
	slice := array[:5]
	// []
	fmt.Printf("%d\n",slice[1])
	// []
	fmt.Printf("%d\n",array[8])
	// panic: runtime error: index out of range
	fmt.Printf("%d\n",slice[8])
}

Reference

1. https://blog.golang.org/go-slices-usage-and-internals