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
",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.
Read the Go Programming Language Specification.
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)
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.
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
",slice[1])
// []
fmt.Printf("%d
",array[8])
// panic: runtime error: index out of range
fmt.Printf("%d
",slice[8])
}