How to copy a map?

You are not copying the map, but the reference to the map. Your delete thus modifies the values in both your original map and the super map. To copy a map, you have to use a for loop like this:

for k,v := range originalMap {
  newMap[k] = v
}

Here's an example from the now-retired SO documentation:

// Create the original map
originalMap := make(map[string]int)
originalMap["one"] = 1
originalMap["two"] = 2

// Create the target map
targetMap := make(map[string]int)

// Copy from the original map to the target map
for key, value := range originalMap {
  targetMap[key] = value
}

Excerpted from Maps - Copy a Map. The original author was JepZ. Attribution details can be found on the contributor page. The source is licenced under CC BY-SA 3.0 and may be found in the Documentation archive. Reference topic ID: 732 and example ID: 9834.


I'd use recursion just in case so you can deep copy the map and avoid bad surprises in case you were to change a map element that is a map itself.

Here's an example in a utils.go:

package utils

func CopyMap(m map[string]interface{}) map[string]interface{} {
    cp := make(map[string]interface{})
    for k, v := range m {
        vm, ok := v.(map[string]interface{})
        if ok {
            cp[k] = CopyMap(vm)
        } else {
            cp[k] = v
        }
    }

    return cp
}

And its test file (i.e. utils_test.go):

package utils

import (
    "testing"

    "github.com/stretchr/testify/require"
)

func TestCopyMap(t *testing.T) {
    m1 := map[string]interface{}{
        "a": "bbb",
        "b": map[string]interface{}{
            "c": 123,
        },
    }

    m2 := CopyMap(m1)

    m1["a"] = "zzz"
    delete(m1, "b")

    require.Equal(t, map[string]interface{}{"a": "zzz"}, m1)
    require.Equal(t, map[string]interface{}{
        "a": "bbb",
        "b": map[string]interface{}{
            "c": 123,
        },
    }, m2)
}

It should easy enough to adapt if you need the map key to be something else instead of a string.


You have to manually copy each key/value pair to a new map. This is a loop that people have to reprogram any time they want a deep copy of a map.

You can automatically generate the function for this by installing mapper from the maps package using

go get -u github.com/drgrib/maps/cmd/mapper

and running

mapper -types string:aStruct

which will generate the file map_float_astruct.go containing not only a (deep) Copy for your map but also other "missing" map functions ContainsKey, ContainsValue, GetKeys, and GetValues:

func ContainsKeyStringAStruct(m map[string]aStruct, k string) bool {
    _, ok := m[k]
    return ok
}

func ContainsValueStringAStruct(m map[string]aStruct, v aStruct) bool {
    for _, mValue := range m {
        if mValue == v {
            return true
        }
    }

    return false
}

func GetKeysStringAStruct(m map[string]aStruct) []string {
    keys := []string{}

    for k, _ := range m {
        keys = append(keys, k)
    }

    return keys
}

func GetValuesStringAStruct(m map[string]aStruct) []aStruct {
    values := []aStruct{}

    for _, v := range m {
        values = append(values, v)
    }

    return values
}

func CopyStringAStruct(m map[string]aStruct) map[string]aStruct {
    copyMap := map[string]aStruct{}

    for k, v := range m {
        copyMap[k] = v
    }

    return copyMap
}

Full disclosure: I am the creator of this tool. I created it and its containing package because I found myself constantly rewriting these algorithms for the Go map for different type combinations.

Tags:

Go

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