Go by Example: Maps

Maps are Go’s built-in associative data type (sometimes called hashes or dicts in other languages).

package main
import (
func main() {

To create an empty map, use the builtin make: make(map[key-type]val-type).

    m := make(map[string]int)

Set key/value pairs using typical name[key] = val syntax.

    m["k1"] = 7
    m["k2"] = 13

Printing a map with e.g. fmt.Println will show all of its key/value pairs.

    fmt.Println("map:", m)

Get a value for a key with name[key].

    v1 := m["k1"]
    fmt.Println("v1:", v1)

If the key doesn’t exist, the zero value of the value type is returned.

    v3 := m["k3"]
    fmt.Println("v3:", v3)

The builtin len returns the number of key/value pairs when called on a map.

    fmt.Println("len:", len(m))

The builtin delete removes key/value pairs from a map.

    delete(m, "k2")
    fmt.Println("map:", m)

To remove all key/value pairs from a map, use the clear builtin.

    fmt.Println("map:", m)

The optional second return value when getting a value from a map indicates if the key was present in the map. This can be used to disambiguate between missing keys and keys with zero values like 0 or "". Here we didn’t need the value itself, so we ignored it with the blank identifier _.

    _, prs := m["k2"]
    fmt.Println("prs:", prs)

You can also declare and initialize a new map in the same line with this syntax.

    n := map[string]int{"foo": 1, "bar": 2}
    fmt.Println("map:", n)

The maps package contains a number of useful utility functions for maps.

    n2 := map[string]int{"foo": 1, "bar": 2}
    if maps.Equal(n, n2) {
        fmt.Println("n == n2")

Note that maps appear in the form map[k:v k:v] when printed with fmt.Println.

$ go run maps.go 
map: map[k1:7 k2:13]
v1: 7
v3: 0
len: 2
map: map[k1:7]
map: map[]
prs: false
map: map[bar:2 foo:1]
n == n2

Next example: Range.