为什么Go中的地图最初会根据地图的大小而具有空值？

Question

Hoping to understand maps in Go better.

Given this code:

package main

import "fmt"

type Vertex struct {
    Lat, Long float64
}

var m []map[string]Vertex
var m1 map[string]Vertex

func main() {
    m = make([]map[string]Vertex, 3)
    m1 = make(map[string]Vertex)
    m1["Bell Labs"] = Vertex{
        40.68433, -74.39967,
    }
    m = append(m, m1)
    fmt.Println(m)
    fmt.Println(len(m))
    fmt.Println(m[3]["Bell Labs"])
}

I get an output of

[map[] map[] map[] map[Bell Labs:{40.68433 -74.39967}]]
4
{40.68433 -74.39967}

Why is it that the first 3 elements in the array are empty/null maps, shouldn't it print out [map[Bell Labs:{40.68433 -74.39967}]] instead?

Answer 1

Why is it that the first 3 elements in the array are empty/null maps?

---

The Go Programming Language Specification

Making slices, maps and channels

The built-in function make takes a type T, which must be a slice, map or channel type, optionally followed by a type-specific list of expressions. It returns a value of type T (not *T). The memory is initialized as described in the section on initial values.

Call             Type T     Result

make(T, n)       slice      slice of type T with length n and capacity n
make(T, n, m)    slice      slice of type T with length n and capacity m

---

The slice m of map

m = make([]map[string]Vertex, 3)

is equivalent to

m = make([]map[string]Vertex, 3, 3)

it should be

m = make([]map[string]Vertex, 0, 3)

---

For example,

package main

import "fmt"

type Vertex struct {
    Lat, Long float64
}

var m []map[string]Vertex
var m1 map[string]Vertex

func main() {
    m = make([]map[string]Vertex, 0, 3)
    fmt.Println(len(m), cap(m))
    m1 = make(map[string]Vertex)
    m1["Bell Labs"] = Vertex{
        40.68433, -74.39967,
    }
    m = append(m, m1)
    fmt.Println(m)
    fmt.Println(len(m), cap(m))
    fmt.Println(m[0]["Bell Labs"])
}

Playground: https://play.golang.org/p/i9f0rrCrtY_5

Output:

0 3
[map[Bell Labs:{40.68433 -74.39967}]]
1 3
{40.68433 -74.39967}