Golang: Map

Map is a unordered collection of key value pairs. Similar to dictionary, hashtable, associative array, in other languages.

Each key value pair is called an element.

Syntax of Map Type

Syntax of the map type is:

map[key_type_name]value_type_name

example:

var mm map[string]int → declare a map, with key of string and value of int.

package main

import "fmt"

func main() {

	// declare mm to be map type
	var mm map[string]int

	fmt.Printf("%T\n", mm)          // map[string]int
}

Create Map, Literal Expression

To create a map, with keys and values, do like this:

map[key_type]value_type{ key1: val1, key2: val2 …}

example:

map[string]int{"a": 1, "b": 2}

package main

import "fmt"

func main() {

	var mm = map[string]int{"a": 1, "b": 2}

	fmt.Println(mm) // map[a:1 b:2]
}

Create Map with make

you can create a empty map with make.

make(map[key_type]value_type)

example:

make(map[string]int)

package main

import "fmt"

func main() {

	var mp = make(map[string]int)

	mp["a"] = 1
	mp["b"] = 2

	fmt.Println(mp) // map[a:1 b:2]
}

set value, add key

m[k] = v → sets value v to key k in map m. Create it if the key does not exist.

package main

import "fmt"

func main() {

	var mm = map[string]int{"a": 1, "b": 2}

	// set value to existing key
	mm["a"] = 9
	fmt.Println(mm) // map[a:9 b:2]

	// if key don't exist, add it
	mm["c"] = 8
	fmt.Println(mm) // map[a:9 b:2 c:8]

}

get value

m[k] → returns 2 values. First value is the value of key k in map m. Second value is true if the key exist, else false.

You can get both values by var a, b = m[k]

When a key does not exist, the first value is map's value's type's “zero value”. (example: if the map's value's type is integer, it return 0 if the key doesn't exist.)

[see Zero Value]

package main

import "fmt"

func main() {

	var mm = map[string]int{"a": 1, "b": 2}

	// get
	var x = mm["a"]
	fmt.Println(x) // 1

	// returns 2 values. second is true if exist, else false
	var y, z = mm["b"]
	fmt.Println(y, z) // 2 true

	// returns 2 values. second is true if exist, else false
	var h1, h2 = mm["c"]
	fmt.Println(h1, h2) // 0 false

}

Delete Key

delete(m, k) → delete the key k in m. If key doesn't exist, do nothing.

package main

import "fmt"

func main() {

	var mm = map[string]int{"a": 1, "b": 2}

	// delete a key
	delete(mm, "a")
	fmt.Println(mm) // map[b:2]

	// if key doesn't exist, do nothing
	delete(mm, "9")
	fmt.Println(mm) // map[b:2]

}

Loop Thru Map

example:

for key, val := range map { fmt.Printf("%v, %v\n", kk, vv) }

If one of the variable you don't need, name it _, else compiler will complain. The _ is called “blank identifier”.

package main

import "fmt"

func main() {

	var mm = map[string]int{"a": 1, "b": 2}

	for kk, vv := range mm {
		fmt.Printf("%v, %v\n", kk, vv)
	}
	// prints
	// a, 1
	// b, 2

}

[see Golang: loop]

Reference

The Go Programming Language Specification - The Go Programming Language#Map_types

The Go Programming Language Specification - The Go Programming Language#Deletion_of_map_elements