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 a 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