Slicing: Out of bounds error in Go

Question

package main

import "fmt"

func main() {
    a := make([]int, 5)
    printSlice("a", a)
    b := make([]int, 0, 5)
    printSlice("b", b)
    c := b[1:]
    printSlice("c", c)
}


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

The above gives me an out of bounds error:

a len=5 cap=5 [0 0 0 0 0]
b len=0 cap=5 []
panic: runtime error: slice bounds out of range

goroutine 1 [running]:
main.main()
   /private/var/folders/q_/53gv6r4s0y5f50v9p26qhs3h00911v/T/compile117.go:10 +0x150

Why is the slicing expression to create the c slice results in an error?

Answer 1

In short: The problem is not with the lower bound which can be equal to or greater than len() (the upper limit is dictated by cap() in case of slices). The problem is with the higher bound: it must be greater than or equal to the lower bound. And since you didn't specify the higher bound, it defaults to len() (and not to cap()!) which is 0. And 1 is not less than or equal to 0.

Spec: Slice expressions:

For arrays or strings, the indices are in range if 0 <= low <= high <= len(a), otherwise they are out of range. For slices, the upper index bound is the slice capacity cap(a) rather than the length.

Since you are slicing a slice, indices are in range if:

0 <= low <= high <= cap(a)

So this line:

c := b[1:]

Is invalid, because:

A missing low index defaults to zero; a missing high index defaults to the length of the sliced operand.

So in your case low = 1 and high = 0 (implicit), which does not satisfy:

0 <= low <= high <= cap(a)

So for example the following expressions are valid:

c := b[1:1]        // c len=0 cap=4 []
c := b[1:2]        // c len=1 cap=4 [0]
c := b[1:cap(b)]   // c len=4 cap=4 [0 0 0 0]