map is a data structure for storing data that comes as key/value pairs. In key/value data, we assign a value to a key, and later, using that key, we can access its value.

To define a map, we need to use the map keyword:

var ftruits map[string]string

Inside the square brackets, [ ], we specify the keys' data type, and after the brackets, we specify the values' data type. In the example above, we created a map whose keys and values can be of string type.

The keys' data type must be comparable. You can't use a data type that can't be compared with the == operator as the key type. For example, you can't use a slice as a key.

var m map[string]int

In the example above, the keys' data type is string and the values' data type is int.

The map data type is a reference type (similar to a pointer or slice); after declaring a map, its zero value will be nil. In this state, you can read a value from the map, but you can't write anything onto the map! Take a look at the example below:

package main

import "fmt"

func main() {
	var countryCodes map[string]int
	
	fmt.Println(countryCodes)
	fmt.Println(countryCodes == nil)
	fmt.Println(countryCodes["germany"])
	fmt.Println(countryCodes["iran"])
}

// output:
// map[]
// true
// 0
// 0

If a map's value is nil, reading a value from it returns the default value (zero value) corresponding to the value type; in this state, trying to write to the map results in a runtime panic. So you should never try to write data into a map before allocating memory for it (initializing the map).

So far, we've gotten familiar with declaring a map and learned that its initial value is nil. There are two ways to allocate memory for a map:

1. A map literal

2. Using the make function

package main

import "fmt"

func main(){
	// 1. using map literal
	countryCodes_1 := map[string]int{"iran": 98, "germany": 49}
	
	fmt.Println(countryCodes_1)

	// 2. using make to allocate memory for map
	countryCodes_2 := make(map[string]int)
	countryCodes_2["iran"] = 98
	countryCodes_2["germany"] = 49
	
	fmt.Println(countryCodes_2)
}

// output:
// map[germany:49 iran:98]
// map[germany:49 iran:98]

A map literal means using the map keyword to create a value of map type and put it into your variable.