变异映射中不存在的键与数组中相似的键

Question

I'm a newbie in golang and this part of a tour of Go about mutating maps is killing my world of pink ponies.

Here is an example of the problem:

package main

import "fmt"

func main() {
    a := make([]int, 10)
    fmt.Println(a[123]) //panic: runtime error: index out of range, good

    b := make(map[int]int, 10)
    fmt.Println(b[123]) // 0 ? WHAAAT? Not nil, not panic?
}

I know, that I can check element existence with second value, like this:

v, ok := b[11]
if (!ok) {
    // do something strange
}

But why I need to do this every time in every place where I need to get something from map? I consider maps like associative arrays, but this behavior is really strange.

How can I make a map with panic-level checking of keys?

Answer 1

The only way to be certain that a zero value from a map isn't due to the key not existing is the "comma, ok" idiom (v, ok := m[k]). If you need a panic response to a non-existent key, the best route is to write a helper function.

func mapSafe(m map[string]int, key string) (i int) {
    i, ok: = m[key]
    if (!ok) {
        panic("Key not in map")
    }
    return
}