Maps #

// _Maps_ are Go's built-in [associative data type](https://en.wikipedia.org/wiki/Associative_array)
// (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](https://go.dev/ref/spec#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