How to Merge Maps in Golang and Handle Duplicate Values?

Merge Maps in Golang

Merging Maps

Merging multiple maps with the same key type into a single map requires merging their values for matching keys. In Golang, map values can be any type, including slices.

Custom Merge Function

A simple approach to merge maps is to create a function that iterates over the input maps and appends values to a slice for matching keys in the result map.

<code class="go">func merge(ms ...map[string]string) map[string][]string {
    res := map[string][]string{}
    for _, m := range ms {
        for k, v := range m {
            res[k] = append(res[k], v)
        }
    }
    return res
}</code>

Avoiding Duplicates

If duplicate values for the same key should be avoided, the merge function can be extended with a check before appending:

<code class="go">func merge(ms ...map[string]string) map[string][]string {
    res := map[string][]string{}
    for _, m := range ms {
    srcMap:
        for k, v := range m {
            for _, v2 := range res[k] {
                if v == v2 {
                    continue srcMap
                }
            }
            res[k] = append(res[k], v)
        }
    }
    return res
}</code>

Example Usage

Consider the following maps:

<code class="go">m1 := map[string]string{"id_1": "val_1"}
m2 := map[string]string{"id_2": "val_2"}
m3 := map[string]string{"id_1": "val_3"}</code>

Using the merge function to merge them produces the following result map:

<code class="go">res := merge(m1, m2, m3)
fmt.Println(res)</code>

Output:

map[id_1:[val_1 val_3] id_2:[val_2]]

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