集合初始化映射（转围）

Question

下面的Gorang代码根据以下形式的输入为每个食谱收集了一组营养：# applepie- flour- apple- egg# pizza- flour- cheese- egg- tomato它在 （2） 中抱怨。assignment to entry in nil map但是为什么？然而，每个“子映射”都初始化为 at （1） ？makepackage mainimport (    "bufio"    "fmt"    "os"    "regexp")func main() {    nutriments := map[string][]string{            "flour":{"sugars"},            "egg":{"protein", "fat"},            "tomato":{"water", "viamins"},            "cheese":{"calcium", "protein"},            "apple":{"sugars", "fiber", "vitamin"} }    total := make(map[string]map[string]bool)    f := bufio.NewScanner(os.Stdin)    currentRecipe := ""    for f.Scan() {            Line(f.Text(), currentRecipe, nutriments, total)    }    fmt.Println(total)}func Line (line, currentRecipe string, nut map[string][]string, tot map[string]map[string]bool) {    if foundrec, _ := regexp.MatchString("^# [[:graph:]]+", line); foundrec { // Begin recipe            currentRecipe := line[2:]            fmt.Println("---------", currentRecipe) // update current            tot[currentRecipe] = make(map[string]bool) // <== INITIALIZE SET (1)    }    if foundnut, _ := regexp.MatchString("^- [[:graph:]]+", line); foundnut { // New ingredient            nutri := line[2:]            tot[currentRecipe][nutri] = true   // <===== HERE (2)    }}

Answer 1

正如评论所指出的，问题在于当前Recipe的更新机制。字符串按值传递。

这是正确的代码

package main

import (

    "bufio"

    "fmt"

    "os"

    "regexp"

)

func main() {

    nutriments := map[string][]string{

            "flour":{"sugars"},

            "egg":{"protein", "fat"},

            "tomato":{"water", "viamins"},

            "cheese":{"calcium", "protein"},

            "apple":{"sugars", "fiber", "vitamin"} }

    total := make(map[string]map[string]bool)

    f := bufio.NewScanner(os.Stdin)

    currentRecipe := ""

    for f.Scan() {

            currentRecipe = Line(f.Text(), currentRecipe, nutriments, total)

    }

    fmt.Println(total)

}

func Line (line, currentRecipe string, nut map[string][]string, tot map[string]map[string]bool) string {

    if foundrec, _ := regexp.MatchString("^# [[:graph:]]+", line); foundrec { // Begin recipe

            currentRecipe = line[2:]

            fmt.Println("---------", currentRecipe) // update current

            tot[currentRecipe] = make(map[string]bool) // prepare set

    }

    if foundnut, _ := regexp.MatchString("^- [[:graph:]]+", line); foundnut { // Begin ingredients list

            nutri := line[2:]

            tot[currentRecipe][nutri] = true

    }

    return currentRecipe

}