Slice out of bounds using [][]int but works with map[int][]int

Question

Why does this code work

graph := make(map[int][]int, 0)

graph[0] = append(graph[0], 1)

But if you replace first line with graph := make([][]int, 0) I get panic: runtime error: index out of range? It's very weird.

Answer 1

When you do make in graph := make(map[int][]int, 0), you are allocating memory to your map, not to array. So you might do this only graph := make(map[int][]int).

Decomping you code:

type a []int
type m map[int]a

func main() {
    fmt.Println("Hello, playground")

    //decomping graph := make(map[int][]int, 0)
    graph := make(m)

    //map is empty
    fmt.Println(len(graph))

    //decomping graph[0] := append(graph[0], 1)
    itm_a := 1
    arr_a := []int{}

    //appeding item to "a" type
    arr_a = append(arr_a, itm_a)

    //appending array of a to graph
    graph[0] = arr_a

    //print graph
    fmt.Println(graph)
}

See in playground.

The error that you are getting is caused for conceptual error. When you does graph := make([][]int, 0), you are allocation memory to a slice of slice, not an array. See in https://blog.golang.org/go-slices-usage-and-internals.

So you can does this (decomping solution):

type a []int
type m []a

func main() {
    fmt.Println("Hello, playground")

    //decomping graph := make([][]int, 0)
    //see that you must be set the length
    graph := make(m, 0)

    //map is empty
    fmt.Println(len(graph))

    //this is incorrect: graph[0] := append(graph[0], 1)
    //this is correct:   graph[0] := append(graph[0], []int{1})
    //see:
    itm_a := 1
    arr_a := []int{}

    //appeding item to "a" type
    arr_a = append(arr_a, itm_a)

    //appending slice of a to graph (slice)
    graph = append(graph, arr_a)

    //print graph
    fmt.Println(graph)
}

See in playground