`Maps.Copy`

How can we duplicate a map, copying all the keys and values to a new map? In Go we can use a for-range loop and build up another map, or we can use maps.Copy.

A generic method, maps.Copy can simplify programs by doing all the work in copying a map on its own. Each map will be separate in memory—adding to one will not affect the other.

Example

This program creates 2 maps (dictionaries) with string keys and string values. It calls maps.Copy to add the keys and values of one to the other.

Step 1 We create a map with 2 keys, each with an associated value. The keys and values are all strings.

Step 2 We create an empty map. It is important that the 2 maps have the same type keys and values.

Step 3 With maps.Copy, the first argument is the target map we want to copy to, and the second is the source we want to copy from.

Step 4 After calling maps.Copy, the two maps have the same lengths—the same key counts.

Step 5 We add a new key to the second map. This does not affect the first map, so now the 2 maps have different key counts.

package main

import (
    "fmt"
    "maps"
)

func main() {
    // Step 1: create a map of string keys and string values.
    first := map[string]string {
        "bird": "blue",
        "cat": "yellow",
    }
    // Step 2: create an empty map of the same type keys and values.
    second := map[string]string{}

    // Step 3: use maps.Copy to copy the first map to the second map.
    maps.Copy(second, first)

    // Step 4: the maps have the same keys and values.
    fmt.Println("Len of maps:", len(first), len(second))

    // Step 5: add a key and value to one map, and this does not affect the other map.
    second["snake"] = "green"
    fmt.Println("Len of maps:", len(first), len(second))
}Len of maps: 2 2
Len of maps: 2 3

The "maps" package in Go is helpful for commonly-needed operations on maps. As long as the key and value types are equal in the maps, we can use maps.Copy to duplicate maps.