Go “panic: runtime error: index out of range” when the length of array is not null

Question

I am having a hard time learning how to loop through a string in Go to do some stuff (specifically, to separate words than contain vowels).

I wrote this code snippet: https://play.golang.org/p/zgDtOyq6qf.

Here is the error I’m getting when running it:

panic: runtime error: index out of range

goroutine 1 [running]:
panic(0x1045a0, 0x1040a010)
    /usr/local/go/src/runtime/panic.go:500 +0x720
main.myFunc(0x114130, 0x4, 0x0, 0x0, 0x0, 0x3ba3)
    /tmp/sandbox960520145/main.go:19 +0x1a0
main.main()
    /tmp/sandbox960520145/main.go:10 +0x40

I searched in this forum, and someone said that it’s due to the length of the array, but it’s not the case here. I cannot figure out how to solve this issue. Can someone please suggest something?

Solution 1

The issue is that you are creating a slice with length 0, but with a maximum capacity of 4, but at the same time you are trying to allocate already a value to the zeroth index of the slice created, which is normally empty. This is why you are receiving the index out of range error.

result := make([]string, 0, 4)
fmt.Println(len(result)) //panic: runtime error: index out of range

You can change this code with:

result := make([]string, 4)

which means the capacity will be the same length as the slice length.

fmt.Println(cap(result)) // 4
fmt.Println(len(result)) // 4

You can read about arrays, slices and maps here: https://blog.golang.org/go-slices-usage-and-internals

Solution 2

Index out of range error is appearing because you are not initializing the result array with sufficient length.

In myFunc, you have:

result := make([]string, 0, 4)

This creates a slice of strings which has an underlying array of length 4, but since you have declared slice length to be 0, none of the elements from underlying array are accessible to the slice. So even result[0] is out of range of the available indices.

To fix this, simply supply a sufficiently large length parameter to make:

result := make([]string, 4, 4)

You can read more about how slices operate here.

Comments

[jiji]

Thanks for the aswer!

[jiji]

Thanks for the aswer! Didn't know about slices