Runtime error: assignment to entry in nil map

Question

I am trying to generate a map and then convert that to a yaml file like this:

uid :
      kasi:
        cn: Chaithra
        street: fkmp
      nandan:
        cn: Chaithra
        street: fkmp
      remya:
        cn: Chaithra
        street: fkmp

I think I am missing something important while creating the map. My code is below.

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)
}

It is giving the following error:

panic: runtime error: assignment to entry in nil map

possible duplicate of [Runtime error: "assignment to entry in nil map"](http://stackoverflow.com/questions/15117513/runtime-error-assignment-to-entry-in-nil-map) — Jonathan Hall, Jul 03 '15 at 20:18

score 192 · Accepted Answer · answered Dec 03 '14 at 09:33

192

You have not initialized your inner map. Before your for loop you can add m["uid"] = make(map[string]T) and then assign the name.

answered Dec 03 '14 at 09:33

Makpoc

3,921
1
19
19

sharno · Answer 2 · 2020-07-15T14:51:26.733

35

You should check if the map is nil and initialize one if it's nil inside the for loop:

if m["uid"] == nil {
    m["uid"] = map[string]T{}
}

edited Jul 15 '20 at 14:51

answered Sep 12 '17 at 14:26

sharno

630
8
14

score 21 · Answer 3 · answered May 21 '21 at 11:26

21

Probably the map you have define is by using variable var m map[string]interface{}

Instead use m := make(map[string]interface{}) to avoid respective error

answered May 21 '21 at 11:26

Aman Agarwal

396
3
5

score 10 · Answer 4 · answered Feb 02 '18 at 19:03

There is thing as per the error

assignment to entry in nil map

For nested maps when assign to the deep level key we needs to be certain that its outer key has value. Else it will say that the map is nil. For eg in your case

m := make(map[string]map[string]T, len(names))

m is a nested map which contains string key with map[string]T as value. And you are assign the value

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

here you can see the m["uid"] is nil and we are stating it contains a value [name] which is a key to nested value of type T. So first you need to assign value to "uid" or initialise it as

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

score 3 · Answer 5 · edited Aug 18 '23 at 10:45

3

@Makpoc already answered the question. Here is some extra info from the Go Blog. (In the quote, m refers to an example from the blogpost. In this case, the problem is not m but m["uid"].)

Map types are reference types, like pointers or slices, and so the value of m above is nil; it doesn't point to an initialized map. A nil map behaves like an empty map when reading, but attempts to write to a nil map will cause a runtime panic; don't do that.

edited Aug 18 '23 at 10:45

Telemachus

19,459
7
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79

answered Sep 25 '19 at 16:41

dj1986

362
2
9

m is not nil, it is m["uid"] that is nil – Nic Jun 21 '21 at 21:41
@Nic (and for future Googlers): Yes and no. `m` in the quote refers to something from the quoted article. But yes, for this question, `m["uid"]` is the real problem rather than `m`. – Telemachus Aug 18 '23 at 10:46

Runtime error: assignment to entry in nil map

5 Answers5

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