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Learn X By Example

    Go by Example: Maps

    Go by Example : Maps

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

    package main
    import (
    "fmt"
    "maps"
)
    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.

        clear(m)
    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: Functions .

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