Maps

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

package main

import "fmt"

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)

    // 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)

    // 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)
}

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
len: 2
map: map[k1:7]
prs: false
map: map[bar:2 foo:1]