转到映射键循环的++运算符

Question

I am following the Go tutorial here https://tour.golang.org/moretypes/23 and have modified the exercise a little bit to try to dig deeper.

package main

import (
    "fmt"
    "strings"
)

func WordCount(s string) map[string]int {
    m := make(map[string]int)
    x := strings.Fields(s)
    for _, e := range x {
        m[e]++
    }

    return m
}

func main() {
    phrase := "The quick brown fox"
    fmt.Println(WordCount(phrase), "length:", len(WordCount(phrase)))
}

What doesn't make sense to me is how the ++ operator works in this context when adding new elements to the map.

Definition of ++ operator: Increment operator. It increases the integer value by one.

In this context, the ++ operator increasing the integer value of the LENGTH of the map and then adding the e element to the new map length?

Answer 1

The default value of int values in a map is 0. So, when you iterate through x and call m[e]++, the expanded version would be

m[e] = m[e] + 1

In other words:

m[e] = 0 + 1

Of course, if a field repeats, it will already be in the map (with some value > 0).

When you check the length of the map after the loop, it gives the number of unique fields in the string.