创建新映射时是否可以初始化映射的值？

Question

I am working with a map that goes from byte arrays to 2D lists whose separate list elements are unsigned 32 bit integers. i.e. map[[8]byte][][]uint32

Currently, I have logic that checks to see if the 2D list is populated, and if it isn't, I add two empty lists. From there I can begin to actually fill these lists. Like so:

my_map := make(map[[8]byte][][]uint32)
/* Some logic to define x and i */
if len(my_map[x]) == 0 {
    /* Create two fresh inner-lists */
}
my_map[x][0] = append(my_map[x][0], uint32(i))

However, that isn't the most elegant, nor efficient seeming solution. I was wondering if GO had a way to automatically populate a map's 2D list values with inner lists upon creation of a new mapping. Alternatively, if there is a better datatype to be using, I am all ears.

Essentially, here is what I want to be able to do:

my_map := make(map[[8]byte][][]uint32)
/* Some logic to define x and i */
my_map[x][0] = append(my_map[x][0], uint32(i))

Answer 1

The default value for slices is nil, which append can work with. You can do this:

package main

import "fmt"

func main() {
    m := make(map[string][]int)
    m["test0"] = append(m["test0"], 10)
    m["test1"] = append(m["test1"], 10)
    m["test2"] = append(m["test2"], 10)
    fmt.Println(m)
}

However, your case is trickier because your values are slices of slices. The default nil will not do in this case, because you're dereferencing a nil with my_map[x][0]. So you'll have to explicitly check if the [0] index was already allocated. There's nothing particularly inefficient about it, IMHO. It has the advantage of being explicit, so you can see exactly where things get allocated.

If you wish, you can encapsulate this functionality into a function.