对于导致恐慌的循环：由于nil映射或切片导致运行时错误

Question

Hey I am trying to create a simple scalar vector for my program.

I was starting with a simple variable and incrementing it to make it a 32 by 1 size vector matrix.

var x []int
for i := 0; i < 32 ; i++{
    x[i] = i + 1
}

Simple enough but when trying to compile this I get this error.

panic: runtime error: index out of range

goroutine 1 [running]:
main.main()
    /Users/jeanmac/go/src/matrices/main.go:69 +0x7d

Process finished with exit code 2

Not sure why. FYI Line 69 refers to x[i] = i + 1. When trying to assign x[i] I receive the following warning.

Reports indexing of nil map or slice that may lead to runtime panic.

Not sure why this seems to be happening.

Answer 1

Allocate the slice. For example,

package main

import "fmt"

func main() {
    x := make([]int, 32)
    for i := range x {
        x[i] = i + 1
    }
    fmt.Println(x == nil, len(x), cap(x), x)
}

Playground: https://play.golang.org/p/UVrUAZHtTw-

Output:

false 32 32 [1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32]

---

In your example, you have a zero-value (nil) slice which has length 0. Therefore, x[i] or x[0] is panic: runtime error: index out of range.

package main

import "fmt"

func main() {
    var x []int
    fmt.Println(x == nil, len(x), cap(x), x)
    for i := 0; i < 32; i++ {
        x[i] = i + 1
    }
}

Playground: https://play.golang.org/p/3hG5FpV3_dC

Output:

true 0 0 []
panic: runtime error: index out of range
main.go:9

---

References:

A Tour of Go

The Go Programming Language Specification

Slice types

Index expressions

Making slices, maps and channels