Maps

Maps

Maps are used to store key-value pairs, where the keys are unique and are used to access the corresponding values. So instead of accessing its elements/values using an index (starting from index 0) like in Arrays and Slices, we access the values using the keys. When we add a key-value pair with key that already exists in the map, the new value we set will replace the old value of the corresponding key. While arrays have a fixed size, in maps, we can add as many as key-value pairs as we want.

In other programming languages, maps are also known as dictionaries or hashmaps. For example, in Python or C#, we can use dictionaries to store key-value pairs. In Java, we can also use hashmap to store them.

Declaration

Below are syntax for defining maps. We can also use a 'make' function for defining a map. The function's purpose is to return a map of the specified type, which is initialised and prepared for use.

var <map_name> map[<keys_datatype>]<values_datatype>
<map_name> := make(map[<keys_datatype>]<values_datatype>)

This is how we use map literal for assigning directly some key-value pairs to a map.

<map variable name> := map[<keys_datatype>]<values_datatype>{
    <key_1>: <value_1>,
    <key_2>: <value_2>,
    ...
    <key_n>: <value_n>,
}

For example, we have some information of a user, we can use map to store it.

var user map[string]string = map[string]string{
    "name": "John Doe",
    "phone": "123456789",
    "email": "[email protected]",
}

Accessing and Modifying Maps

To access or retrieve an element.

username := user["name"]
fmt.Println(user["address"])

If the key we are accessing for doesn't exist, it will return the default value for that value type, such as "" for string, false for boolean, and 0 for number.

We can also check whether the key-value pairs exists or not in the map below. First return value is the value of corresponding key and the second one tells us about the key-value pairs presents.

userAddress, ok := user["address"]

The value of ok will be true if key "address" is in "user" map. Vice versa.

To add a new key-value pairs or replace existing one in a map.

user["country"] = "Singapore"

If key country has already been in the map, the existing value will be replaced by new value "Singapore". In contrary, if it doesn't exist yet, a new key-value pairs will be added.

To delete a key-value pairs in the map, we can utilize a 'delete' function.

delete(user, "country")

Exercise

Create a new map called studentGrades that maps a student's name (string) to their numerical grade (integer). You will need to populate the map, verify if a specific student exists, and remove an entry.

Here is the task specification:

1. Create the map and add the following entries:

   • "Danny" with a grade of 90

   • "John" with a grade of 85

   • "Belle" with a grade of 70

2. Change "Belle"'s grade to 75.

3. Remove the entry for "John" from the map.

4. Use a conditional check to see if "Danny", "John", "Belle" exists in the map:

   • If the name exists in the map, print "{name} with a grade of {grade}"

   • If the name doesn't exists in the map, print "{name} doesn't have a grade in map"