Maps

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

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)
---
map: map[k1:7 k2:13]

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

v1 := m["k1"]
fmt.Println("v1:", v1)
---
v1: 7

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

v3 := m["k3"]
fmt.Println("v3:", v3)
---
v3: 0

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")

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

clear(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}

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.