Overview

Maps are golang builtin datatype similar to the hash table which maps a key to a value.

Below is the format for a map:

map[key_type]value_type

Both key_type and value_type can be of different type or same type. For below example the key type is string and value type is int

map[string]int

Declare a Map

A map can also be declared with var keyword, but it creates a nil map as default zero value of map is nil. Adding any key value pair to that map will cause a panic. Let’s see an example for this

package main

func main() {
    var employeeSalary map[string]int
    employeeSalary["Tom"] = 2000
}

Output

panic: assignment to entry in nil map

Above program run into panic as the map is nil.

One use case of having a map declared with var keyword is when an already existing map needs to be assigned to it or when we want to assign the result of a function.

Creating a Map

Two ways of creating a map

• Using the map[<key_type>]<value_type>{} format also called map literal

• Using make

Let’s look at each of the above methods one by one.

Using the map[<key_type>]<value_type> format

One of the most common way of creating a map is using the map literal:

map[key_type]value_type{}

An example of above where key type is string and value type is integer

employeeSalary := map[string]int{}

A map can also be created with some key values initialized

employeeSalary := map[string]int{
"John": 1000
"Sam": 2000
}

A key-value pair can also be added to the map

employeeSalary["Tom"] = 2000

Let’s see a program

package main

import "fmt"

func main() {
    //Declare
    employeeSalary := map[string]int{}
    fmt.Println(employeeSalary)
    
    //Intialize using map lieteral
    employeeSalary = map[string]int{
        "John": 1000,
        "Sam":  1200,
    }

    //Adding a key value
    employeeSalary["Tom"] = 2000
    fmt.Println(employeeSalary)
}

Output

map[]
map[John:1000 Sam:1200 Tom:2000]

In above program we created a map literal intialized with some values. Then we added another key value pair in it. Then we printed it using fmt.Println which prints all the key value pairs in format map[key:value key:value]

Using Make

This is another way of creating the map. The builtin function make can be used to create a map. It returns an initialized map. Hence key-value pairs can be added to it.

package main

import "fmt"

func main() {
    //Declare
    employeeSalary := make(map[string]int)
    //Adding a key value
    employeeSalary["Tom"] = 2000
    fmt.Println(employeeSalary)
}

Output

map[Tom:2000]

In above program we created a map using make function. Then we added a key value pair in it. Then we printed it using fmt.Println which prints all the key value pairs.