Why Does Assigning to a Map in Go Result in a 'runtime error: assignment to entry in nil map' and How Can It Be Fixed?

Runtime Error in Map Assignment

Question:

While attempting to convert a map to a YAML file, an error is encountered stating "runtime error: assignment to entry in nil map." Review the following code snippet:

package main

import (
    "fmt"
    "gopkg.in/yaml.v2"
)

type T struct {
    cn     string
    street string
}

func main() {
    names := []string{"kasi", "remya", "nandan"}

    m := make(map[string]map[string]T, len(names))
    for _, name := range names {

        //t := T{cn: "Chaithra", street: "fkmp"}

        m["uid"][name] = T{cn: "Chaithra", street: "fkmp"}

    }
    fmt.Println(m)

    y, _ := yaml.Marshal(&m)

    fmt.Println(string(y))
    //fmt.Println(m, names)
}

What is causing this error and how can it be resolved?

Answer:

The issue lies in the initialization of the inner map within the outer map. In the example provided, the map m["uid"] is not initialized, resulting in a nil map when attempting to assign the name to it.

To initialize the inner map, add the following line before the for loop:

m["uid"] = make(map[string]T)

This creates the inner map and allows the name to be assigned to it without encountering a nil map error.

The corrected code becomes:

func main() {
    names := []string{"kasi", "remya", "nandan"}

    m := make(map[string]map[string]T, len(names))
    m["uid"] = make(map[string]T) // Initialize the inner map

    for _, name := range names {
        m["uid"][name] = T{cn: "Chaithra", street: "fkmp"}
    }
    fmt.Println(m)

    y, _ := yaml.Marshal(&m)

    fmt.Println(string(y))
    //fmt.Println(m, names)
}

The above is the detailed content of Why Does Assigning to a Map in Go Result in a 'runtime error: assignment to entry in nil map' and How Can It Be Fixed?. For more information, please follow other related articles on the PHP Chinese website!