恐慌：分配给单个简单地图上的nil地图中的条目

Question

I was under the impression that the assignment to entry in nil map error would only happen if we would want to assign to a double map, that is, when a map on a deeper level is trying to be assigned while the higher one doesn't exist, e.g.:

var mm map[int]map[int]int
mm[1][2] = 3

But it also happens for a simple map (though with struct as a key):

package main

import "fmt"

type COO struct {
    x int
    y int
}

var neighbours map[COO][]COO

func main() {
    for i := 0; i < 30; i++ {
        for j := 0; j < 20; j++ {
            var buds []COO
            if i < 29 {
                buds = append(buds, COO{x: i + 1, y: j})
            }
            if i > 0 {
                buds = append(buds, COO{x: i - 1, y: j})
            }
            if j < 19 {
                buds = append(buds, COO{x: i, y: j + 1})
            }
            if j > 0 {
                buds = append(buds, COO{x: i, y: j - 1})
            }
            neighbours[COO{x: i, y: j}] = buds // <--- yields error
        }
    }


    fmt.Println(neighbours)

}

What could be wrong?

Answer 1

You need to initialize neighbours: var neighbours = make(map[COO][]COO)

See the second section in: https://blog.golang.org/go-maps-in-action

You'll get a panic whenever you try to insert a value into a map that hasn't been initialized.