Maps in Go

In Go lang, a map is a data structure that stores key-value pairs where keys must be unique but values can be duplicate.

Maps provide easy retrieval of values. It is a reference to an underlying hash table.

Declare Maps

Maps can be declared using var or shorthand syntax.

syntax: 

// Using var keyword
var mymap = map[TypeOfKey] TypeOfValue { key1:value1, key2:value2,...}

// shorthand syntax
mymap :=  map[TypeOfKey] TypeOfValue { key1:value1, key2:value2,...}

The following example declares maps using var keyword and := syntax:

Example: Map Declaration

package  main
import "fmt"

func main() {
    //using var
    var emplist = map[int] string {1:"Steve", 2:"Roy", 3:"Amar", 4:"Nancy", 5:"Abdul" }
    fmt.Println(emplist)
 
    //shorthand syntax
    empsal := map[string] int {"Steve":60000, "Roy":70000, "Amar":75000, "Nancy":60000, "Abdul":65000} 
    fmt.Println(empsal)
}

Output:

[1:Steve 2:Roy 3:Amar 4:Nancy 5:Abdul]
map[Abdul:65000 Amar:75000 Nancy:60000 Roy:70000 Steve:60000]

In the above example, a map named emplist is created using the var keyword with int type keys and string type values. Keys stores employee ids and the values store names. The map empsal is created using shorthand syntax := with keys of string type and value of type int.

Sometimes the order of declaration of the key-value pairs is different from the order in which they are stored. In the above example, the order of the key-value pairs in the output of empsal is different from the declared order. This is because the data in a map is stored efficiently to make retrieval easier.

Create Map using make() Function

The built-in make() function can be used to create an empty map by specifying the key and value type, as shown below.

Example: Create Map using make()

// create an empty map
var emplist = make(map[int]string) 

fmt.Println(emplist) //output: map[]

// Add members to the map
emplist[1] = "Steve"
emplist[2] = "Roy"
emplist[3] = "Amar"
emplist[4] = "Nancy"
emplist[5] = "Abdul"

fmt.Println(emplist) //output: [1:Steve 2:Roy 3:Amar 4:Nancy 5:Abdul]

Valid Types for Map

The data type of a key element should be a type that can be compared using the == operator.

Valid Key Types: Numbers, string, Boolean, arrays, pointers, struct

Invalid Key Types: functions, slices, and maps

The type of value can be of any type.

Access Maps

The values in a map can be retrieved by passing the key in the square bracket, e.g. map[key].

The following retrieves the map values.

Example: Retrieve Map Values

emplist := map[int] string {1:"Steve", 2:"Roy", 3:"Amar", 4:"Nancy", 5:"Abdul" }

fmt.Println(emplist[1]) // output: Steve
fmt.Println(emplist[2]) // output: Roy
fmt.Println(emplist[3]) // output: Amar
fmt.Println(emplist[4]) // output: Nancy
fmt.Println(emplist[5]) // output: Abdul

fmt.Println(emplist[15]) // output:

Check If Key Exists

The map[key] returns two values, first the value itself, and second, true or false depending upon if the specified key exists or not. It returns true if a key exists; otherwise returns false. If the specified key does not exist then the value will be the default value of the type of value.

Example: Check Key Exist

emplist := map[int] string {1:"Steve", 2:"Roy", 3:"Amar", 4:"Nancy", 5:"Abdul" }

//check for an existing key
val, ok := emplist[3]
fmt.Println(val, ok) //output: Amar  true 
  
//check for a non-existing key
val2, ok := emplist[7]
fmt.Println(val2, ok)

In the above example, key 3 exists in the emplist map and so it returns the value associated with key 3 and returns true. For key value 7, it returns the default value of string which is a blank string "", and also returns false because key 7 does not exist.

Add or Update Elements

You can add a new element or update an existing one in the map using the map[key]. The following updates the value of an existing key 2, and adds a new key 6, and its value to the map.

Example: Add a New Key-Value Pair

emplist := map[int] string {1:"Steve", 2:"Roy", 3:"Amar", 4:"Nancy", 5:"Abdul" }
emplist[2] = "Yash" //update
emplist[6] = "Anita" //add

fmt.Println(emplist) //output:map[1:Steve 2:Yash 3:Amar 4:Nancy 5:Abdul 6:Anita]

Delete Map Element

Use the delete(map, key) function to remove a key value pair from a map.

Example: Delete Element from Map

emplist := map[int] string {1:"Steve", 2:"Roy", 3:"Amar", 4:"Nancy", 5:"Abdul" }
delete(emplist, 1)
delete(emplist, 2)

fmt.Println(emplist) //output: map[3:Amar 4:Nancy 5:Abdul 6:Anita]

Get Count of Map Elements

The len() function returns the total number of elements in a map.

Example: Get Total Count

emplist := map[int] string {1:"Steve", 2:"Roy", 3:"Amar", 4:"Nancy", 5:"Abdul" }

fmt.Println(len(emplist)) //output: 5

Iterating Map using for Loop

Similar to arrays, you can iterate through a map using the for loop.

Example: Map with For Loop

emplist := map[int] string {1:"Steve", 2:"Roy", 3:"Amar", 4:"Nancy", 5:"Abdul" }
   
for keyVar, val := range emplist {
	fmt.Printf("%v : %v, ", keyVar, val) // output 3 : Amar, 4 : Nancy, 5 : Abdul, 1 : Steve, 2 : Roy, 
}