Maps in Golang - Golang Docs

Maps are one of the most useful data structures. It can store in key-value pairs and doesn’t allow for duplicate keys. Now, we will learn how the Go programming language implements maps.

What is a map?

A map is a key-value pair storage container. It offers fast and efficient lookups. And it doesn’t allow duplicate keys while it can have duplicate values.

Declaration of a Map in Go

Now we will see how to declare a map in Go.

package main

import (
	"fmt"
)

func main() {
	var names map[int]string    // name map has int keys and string values
}

In the above example, the key is of type int while the values are of string type.

Initializing a Map

Let’s see how we can initialize a map with values.

1. Using make() function

The function make(), can be used to initialize a map as shown below.

package main

import (
	"fmt"
)

func main() {
	var names = make(map[int]string)
	names[0] = "John"
	names[1] = "Jane"

	fmt.Println(names)  // map[0:John 1:Jane]
}

2. Using literal syntax

Maps can be initialized using map literal syntax. Below is an example illustrating that. Let’s take the same code before and modify that to initialize with map literal.

package main

import (
	"fmt"
)

func main() {
	var names = map[int]string {
		0: "John",
		1: "Jane",   // last comma is a must
	}
	
	
	fmt.Println(names)  // prints map[0:John 1:Jane]
}

Working with maps in Golang

We can insert, delete, retrieve keys in a map. Let’s see how to do that.

1. Inserting elements in a map

You can insert keys in two ways. Either insert keys when initializing or use index syntax to initialize.

package main

import (
	"fmt"
)

func main() {
	var names = make(map[int]string)
	
	names[0] = "Freddy"    // indexed insertion
	names[1] = "Shawn"
	
	fmt.Println(names)  // prints map[0:Freddy 1:Shawn]
}

2. Getting values from a map

We can get values associated with keys from a map using index notation as shown below.

package main

import (
	"fmt"
)

func main() {
	var names = make(map[int]string)
	
	names[0] = "Freddy"    // indexed insertion
	names[1] = "Shawn"
	
	fmt.Println(names[1])  // prints Shawn
}

3. Check if a key exists

We get a boolean when we try retrieving a value. That helps to check if a key exists.

package main

import (
	"fmt"
)

func main() {
	var names = make(map[int]string)
	
	names[0] = "Freddy"    // indexed insertion
	names[1] = "Shawn"
	
	fred, exists := names[0]
	if(exists) {
		fmt.Printf("%s exists", fred)  // prints "Freddy exists"
	}	
}

4. Delete key from a map

We use the delete function to remove a key from a map. Let’s see an example of how it is done.

package main

import (
	"fmt"
)

func main() {
	var names = make(map[int]string)
	
	names[0] = "Freddy"    // indexed insertion
	names[1] = "Shawn"
	names[2] = "Batman"
	
	// delete shawn
	delete(names, 1) 
	fmt.Println(names)     // prints map[0:Freddy 2:Batman]
}

5. Iterating over a map entries

Using range we can iterate over a map and get keys and values both.

package main

import (
	"fmt"
)

func main() {
	var names = make(map[int]string)
	
	names[0] = "Freddy"    // indexed insertion
	names[1] = "Shawn"
	names[2] = "Batman"
	names[3] = "Spiderman"
	names[4] = "Joker"
	
	for _, name := range names {
		fmt.Println(name)
	}
	
	// prints: 
	    // Joker
        // Freddy
        // Shawn
        // Batman
        // Spiderman
}

This is how we can add or remove items, retrieve keys and values from a map. It is one of the most useful data structures.