How to add into map in range loop

问题

package main

import (
    "fmt"
)

func main() {
    m := make(map[int]int, 4)
    m[1] = 0
    m[2] = 0
    for k, _ := range m {
        i := 10 + k
        m[i] = 0
    }
    fmt.Println(m)
    fmt.Println("len:", len(m))
}

This code returns: 8 or 10 or 6 as length of map after loop. Video is here, playgroud here.

I see that new added elements go into range, but can't explain why this loop stops randomly?

回答1:

Spec: For statements:

The iteration order over maps is not specified and is not guaranteed to be the same from one iteration to the next. If a map entry that has not yet been reached is removed during iteration, the corresponding iteration value will not be produced. If a map entry is created during iteration, that entry may be produced during the iteration or may be skipped. The choice may vary for each entry created and from one iteration to the next. If the map is nil, the number of iterations is 0.

The spec states that if you add entries to the map you are ranging over, the elements you add may or may not be visited by the loop, and moreover, which is visited is not even deterministic (may change when doing it again).

回答2:

You are modifying map you are iterating over. This is the cause.

来源：https://stackoverflow.com/questions/55399243/how-to-add-into-map-in-range-loop