英文:
map of sets initialization (Go)
问题
以下是翻译好的内容：
下面的 Golang 代码根据以下形式的输入，为每个食谱收集一组营养物质：
# applepie- 面粉- 苹果- 鸡蛋# pizza- 面粉- 奶酪- 鸡蛋- 番茄
它在 (2) 处报错 assignment to entry in nil map。
但是为什么呢？每个“子映射”在 (1) 处都使用 make 进行了初始化？
package 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 { // 开始食谱currentRecipe := line[2:]fmt.Println("---------", currentRecipe) // 更新当前食谱tot[currentRecipe] = make(map[string]bool) // <== 初始化集合 (1)}if foundnut, _ := regexp.MatchString("^- [[:graph:]]+", line); foundnut { // 新成分nutri := line[2:]tot[currentRecipe][nutri] = true // <===== 这里 (2)}}
英文:
Below Golang code gathers a set of nutriments for each recipe, based on an input of this form:
# applepie- flour- apple- egg# pizza- flour- cheese- egg- tomato
It complains at (2) of assignment to entry in nil map.
But why ? Each "submap" is howhever initialized with the make at (1) ?
package 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 recipecurrentRecipe := line[2:]fmt.Println("---------", currentRecipe) // update currenttot[currentRecipe] = make(map[string]bool) // <== INITIALIZE SET (1)}if foundnut, _ := regexp.MatchString("^- [[:graph:]]+", line); foundnut { // New ingredientnutri := line[2:]tot[currentRecipe][nutri] = true // <===== HERE (2)}}
答案1
得分: 1
根据评论中指出的问题，问题出在对currentRecipe的更新机制上。字符串是按值传递的。
以下是正确的代码：
package 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)var currentRecipe stringfor 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 recipecurrentRecipe = line[2:]fmt.Println("---------", currentRecipe) // update currenttot[currentRecipe] = make(map[string]bool) // prepare set}if foundnut, _ := regexp.MatchString("^- [[:graph:]]+", line); foundnut { // Begin ingredients listnutri := line[2:]tot[currentRecipe][nutri] = true}return currentRecipe}
希望对你有帮助！
英文:
As pointed in comments, problem was lying in the update mechanism for currentRecipe. string was passed by value.
Here is correct code
package 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() {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 recipecurrentRecipe = line[2:]fmt.Println("---------", currentRecipe) // update currenttot[currentRecipe] = make(map[string]bool) // prepare set}if foundnut, _ := regexp.MatchString("^- [[:graph:]]+", line); foundnut { // Begin ingredients listnutri := line[2:]tot[currentRecipe][nutri] = true}return currentRecipe}
通过集体智慧和协作来改善编程学习和解决问题的方式。致力于成为全球开发者共同参与的知识库，让每个人都能够通过互相帮助和分享经验来进步。
评论