为什么Go中的地图最初根据地图的大小有空值?

为什么数组中的前3个元素是空/空映射?

The Go Programming Language Specification

Making slices, maps and channels

内置函数make采用T类型,它必须是slice、map 或通道类型,可选后跟 表达。它返回一个T类型的值(不是*T)。记忆是

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

切片 m 属于 map

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

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

应该是

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

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"])
}

游乐场: https://play.golang.org/p/i9f0rrCrtY_5

输出:

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