nf.id.au

A Go map is a typed hash map data structure. A map’s type signature is of the form map[keyType]valueType where keyType and valueType are the types of the keys and values respectively.

To initialize a map, you must use the make function:

m := make(map[string]int)

An uninitialized map is equal to nil, and if read from or written a panic will occur at runtime.

The syntax for storing values is much the same as doing so with arrays or slices:

m["Alice"] = 21
m["Bob"] = 17

Similarly, retrieving values from a map is done like so:

a := m["Alice"]
b := m["Bob"]

You can use the range keyword to iterate over a map with a for loop:

for k, v := range m {
    fmt.Println(k, v)
}

This code will print:

Alice 21
Bob 17

Retrieving a value for a key that is not in the map will return the value type’s zero value:

c := m["Charlie"]
// c == 0

By reading multiple values from a map, you can test for a key’s presence. The second value will be a boolean indicating the key’s presence:

a, ok := m["Alice"]
// a == 21, ok == true
c, ok := m["Charlie"]
// c == 0, ok == false

To remove a key/value entry from a map, you flip it around and assign false as the second value:

m["Bob"] = 0, false
b, ok := m["Bob"]
// b == 0, ok == false

You can store arbitrary types in a map by using the empty interface type interface{}:

n := make(map[string]interface{})
n["One"] = 1
n["Two"] = "Two"

The only proviso is that when retrieving those values you must perform a type assertion to use them in their original form:

a := n["One"].(int)
b := n["Two"].(string)

You can use a type switch to determine the types of the values you’re pulling out, and deal with them appropriately:

for k, v := range n {
    switch u := v.(type) {
        case int:
            fmt.Printf("Key %q is an int with the value %v.\n", k, u)
        case string:
            fmt.Printf("Key %q is a string with the value %q.\n", k, u)
    }
}

Inside each of those case blocks, u will be of the type specified in the case statement; no explicit type assertion is necessary.

This code will print:

Key "One" is an int with the value 1.
Key "Two" is a string with the value "Two".

The key can be of any type for which the equality operator is defined, such as integers, floats, strings, and pointers. Interface types can also be used, as long as the underlying type supports equality. (Structs, arrays and slices cannot be used as map keys, because equality is not defined on those types.)

For example, the map o can take keys of any of the above types:

o := make(map[interface{}]int)
o[1] = 1
o["Two"] = 2

And that’s maps in a nutshell.