当键不在地图中时不引发错误（例如Python）的好处是什么？ [关闭]

Question

In Go, let m is map object that map string to int, suppose "foo" is not a key of m, then the statement

m["foo"]

returns two value 0 and false which false implies "foo" is not a key of m. Could you give some situations, examples that the go's property is useful?

Answer 1

Go does not raise errors. Go reports errors.

---

A common use of a map element zero value:

package main

import (
    "fmt"
)

func main() {
    counts := map[string]int{}
    fmt.Println(counts)

    // Equivalent:
    // counts["foo"] = counts["foo"] + 1
    // counts["foo"] += 1
    // counts["foo"]++

    counts["foo"]++
    fmt.Println(counts)

    counts["foo"]++
    fmt.Println(counts)
}

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

Output:

map[]
map[foo:1]
map[foo:2]

---

A common use of the comma, ok form map index expression:

package main

import (
    "fmt"
)

func main() {
    m := map[string]map[string]int{}
    fmt.Println(m)

    // panic: assignment to entry in nil map
    // m["K1"]["K2"]++

    _, ok := m["K1"]
    if !ok {
        m["K1"] = map[string]int{}
    }
    m["K1"]["K2"]++
    fmt.Println(m)

    m["K1"]["K2"]++
    fmt.Println(m)
}

Playground: https://play.golang.org/p/9byxSSIWBre

Output:

map[]
map[K1:map[K2:1]]
map[K1:map[K2:2]]