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This article describes how to merge two maps in Golang.

Golang does not provide any in-built functionality to merge mappings, but this can be achieved using Copy method or by following the corresponding procedure. Let’s try both the methods.

Merge two maps using Copy method in Golang

By using the Copy method, we can copy the contents of one map to another map, thereby realizing the map merging function. The Copy method was introduced in Golang version 1.18.

The Copy method takes two parameters which are maps. The contents of the second map will be copied into the first map.

Let's try an example:

package main

import (
    "fmt"

    "golang.org/x/exp/maps"
)

func main() {
    map1 := map[string]int{
        "JiyikOne":   10,
        "JiyikTwo":   20,
        "JiyikThree": 30,
    }
    map2 := map[string]int{
        "JiyikTwo":   20,
        "JiyikThree": 30,
        "JiyikFour":  40,
        "JiyikFive":  50,
    }
    // Print the map before merging
    fmt.Println("The First Map: ", map1)
    fmt.Println("The Second Map: ", map2)

    maps.Copy(map1, map2)
    fmt.Println("The merged Map is: ", map1)
}

The above code will use the Copy method to merge two maps based on union. The only disadvantage of the Copy method is that it modifies our first Map.

See the output:

The First Map:  map[JiyikOne:10 JiyikThree:30 JiyikTwo:20]
The Second Map:  map[JiyikFive:50 JiyikFour:40 JiyikThree:30 JiyikTwo:20]
The merged Map is:  map[JiyikFive:50 JiyikFour:40 JiyikOne:10 JiyikThree:30 JiyikTwo:20]

User Defined Method to Merge Two Maps in Golang

We can devise a way to merge two maps based on a union, which will exclude entries with the same value. Here is an example:

package main

import     "fmt"

type Jiyik struct {
    Id     int
    Name   string
    Salary int
}

type EmployeeMaps map[int]Jiyik

func MapUnion(map1, map2 EmployeeMaps) EmployeeMaps {
    for x, y := range map1 {
        if m, n := map2[x]; n {
            y.Salary += m.Salary
        }
        map2[x] = y

    }
    return map2
}

func main() {
    map1 := EmployeeMaps{2: {30, "Jack", 2000}, 5: {50, "Samuel", 1500}, 6: {60, "John", 1800}}
    map2 := EmployeeMaps{1: {10, "Pamela", 3000}, 2: {20, "Connor", 2000}, 3: {30, "Jack", 2000}, 4: {40, "Joe", 4000}}
    result := MapUnion(map2, map1)
    fmt.Println(result)

}

The above code merges two maps by excluding entries with the same value, basically a union of the maps. The code first creates a structure and maps with int keys and structure values; then, it uses the user-defined method MapUnion to merge them based on the union of the values.

See the output:

map[1:{10 Pamela 3000} 2:{20 Connor 4000} 3:{30 Jack 2000} 4:{40 Joe 4000} 5:{50 Samuel 1500} 6:{60 John 1800}]