Golang: Map

By Xah Lee. Date: . Last updated: .

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

If you have a question, put $5 at patreon and message me.

Golang

  1. Compile, Run
  2. Source Encoding
  3. Package, Import
  4. Comment
  5. Print
  6. String
  7. Print String
  8. String Backslash Escape
  9. Rune
  10. Variable
  11. Zero Value
  12. Constant
  13. If Then Else
  14. Switch/Case
  15. Loop
  16. Basic Types
  17. Array
  18. Slice
  19. Map
  20. Struct
  21. Function
  22. Closure
  23. Pointer
  24. String, Byte Slice, Rune Slice
  25. regexp
  26. Read File
  27. Write to File
  28. Walk Dir
  29. Check File Exist
  30. System Call
  31. Get Script Path
  32. Defer
  33. Random Number

Examples

  1. Match Any Regexp
  2. Find Replace
  3. Validate Links
  4. Generate Sitemap

Reference

  1. Go Spec