What are Maps / Dictionaries?

A map or dictionary is a data structure that stores data in the form of key-value pairs.

In a map or dictionary, each key is connected to a specific value. Instead of accessing data by position like a list or array, we access data by using a meaningful key.

Example:

student = {
    "name": "Aman",
    "age": 20,
    "course": "Programming"
}

Here, "name", "age", and "course" are keys. Their related values are "Aman", 20, and "Programming".

Easy Real-Life Example

contacts = {
    "Aman": "9876543210",
    "Riya": "9123456780",
    "Sohan": "9988776655"
}

To find Riya's number, we do not need to search by position. We can directly use the key "Riya".

contacts["Riya"] → "9123456780"

Why are Maps / Dictionaries Used?

Maps and dictionaries are used when data should be accessed by a meaningful name, ID, code, or label instead of a numeric index.

A list stores data by position:

student = ["Aman", 20, "Programming"]

student[0] → "Aman"
student[1] → 20

But a dictionary stores data by keys:

student = {
    "name": "Aman",
    "age": 20,
    "course": "Programming"
}

student["name"] → "Aman"
student["age"] → 20

Important Terms Related to Maps / Dictionaries

General Syntax of Maps / Dictionaries

The exact syntax differs across programming languages, but the general structure is:

dictionaryName = {
    key1: value1,
    key2: value2,
    key3: value3
}

Example:

student = {
    "id": 101,
    "name": "Riya",
    "marks": 92
}

Here, each key is followed by its value.

Keys Must Usually Be Unique

In maps and dictionaries, keys are usually unique. This means the same key cannot normally store two different values at the same time.

student = {
    "name": "Aman",
    "name": "Riya"
}

If duplicate keys are used, many languages keep the latest value or overwrite the previous value.

Final value of "name" may become "Riya"

Values Can Be Different Types

Values in a dictionary can often be numbers, text, lists, sets, other dictionaries, objects, or any supported data type.

student = {
    "name": "Aman",
    "age": 20,
    "isActive": true,
    "skills": ["Programming", "Database", "Communication"]
}

This makes dictionaries very useful for storing structured real-world data.

Accessing Values Using Keys

We access values in a dictionary using their keys.

/*
This program accesses values from a dictionary.
*/

ENTRY POINT
    DECLARE student AS DICTIONARY = {
        "name": "Aman",
        "age": 20,
        "course": "Programming"
    }

    DISPLAY student["name"]
    DISPLAY student["age"]
    DISPLAY student["course"]
END ENTRY POINT

Expected Output

Aman
20
Programming

Adding a New Key-Value Pair

We can add a new key-value pair to a dictionary by assigning a value to a new key.

/*
This program adds a new key-value pair.
*/

ENTRY POINT
    DECLARE student AS DICTIONARY = {
        "name": "Riya",
        "age": 21
    }

    SET student["course"] = "Database"

    DISPLAY student
END ENTRY POINT

Expected Output

{
    "name": "Riya",
    "age": 21,
    "course": "Database"
}

Updating an Existing Value

If a key already exists, assigning a new value to that key updates the old value.

/*
This program updates an existing value.
*/

ENTRY POINT
    DECLARE product AS DICTIONARY = {
        "name": "Keyboard",
        "price": 500
    }

    SET product["price"] = 450

    DISPLAY product["price"]
END ENTRY POINT

Expected Output

450

Removing a Key-Value Pair

We can remove a key-value pair when it is no longer needed.

/*
This program removes a key-value pair.
*/

ENTRY POINT
    DECLARE user AS DICTIONARY = {
        "name": "Sohan",
        "email": "sohan@example.com",
        "temporaryCode": "ABC123"
    }

    REMOVE KEY "temporaryCode" FROM user

    DISPLAY user
END ENTRY POINT

Expected Output

{
    "name": "Sohan",
    "email": "sohan@example.com"
}

Checking Whether a Key Exists

Before accessing a value, it is often a good idea to check whether the key exists.

/*
This program checks whether a key exists.
*/

ENTRY POINT
    DECLARE settings AS DICTIONARY = {
        "theme": "dark",
        "language": "English"
    }

    IF "theme" EXISTS IN settings THEN
        DISPLAY settings["theme"]
    ELSE
        DISPLAY "Theme setting not found"
    END IF
END ENTRY POINT

Expected Output

dark

Traversing a Map / Dictionary

Traversing a dictionary means visiting its keys, values, or key-value pairs one by one.

Traversing Keys

/*
This program displays all keys.
*/

ENTRY POINT
    DECLARE student AS DICTIONARY = {
        "name": "Aman",
        "age": 20,
        "course": "Programming"
    }

    FOR EACH key IN student
        DISPLAY key
    END FOR
END ENTRY POINT

Expected Output

name
age
course

Traversing Values

/*
This program displays all values.
*/

ENTRY POINT
    DECLARE student AS DICTIONARY = {
        "name": "Aman",
        "age": 20,
        "course": "Programming"
    }

    FOR EACH key IN student
        DISPLAY student[key]
    END FOR
END ENTRY POINT

Expected Output

Aman
20
Programming

Common Map / Dictionary Operations

Example: Student Record Using Dictionary

/*
This program stores a student record using a dictionary.
*/

ENTRY POINT
    DECLARE student AS DICTIONARY = {
        "id": 101,
        "name": "Meera",
        "marks": 88,
        "grade": "A"
    }

    DISPLAY "Student ID: " + student["id"]
    DISPLAY "Name: " + student["name"]
    DISPLAY "Marks: " + student["marks"]
    DISPLAY "Grade: " + student["grade"]
END ENTRY POINT

Expected Output

Student ID: 101
Name: Meera
Marks: 88
Grade: A

Example: Count Frequency Using Dictionary

Dictionaries are very useful for counting how many times each value appears.

/*
This program counts frequency of numbers.
*/

ENTRY POINT
    DECLARE numbers AS LIST = [10, 20, 10, 30, 20, 10]
    DECLARE frequency AS DICTIONARY = {}

    FOR EACH number IN numbers
        IF number EXISTS IN frequency THEN
            SET frequency[number] = frequency[number] + 1
        ELSE
            SET frequency[number] = 1
        END IF
    END FOR

    DISPLAY frequency
END ENTRY POINT

Expected Output

{
    10: 3,
    20: 2,
    30: 1
}

Example: Product Price Lookup

/*
This program uses a dictionary as a price lookup table.
*/

ENTRY POINT
    DECLARE prices AS DICTIONARY = {
        "Pen": 10,
        "Notebook": 50,
        "Bag": 700
    }

    DECLARE productName AS TEXT = "Notebook"

    IF productName EXISTS IN prices THEN
        DISPLAY productName + " price is " + prices[productName]
    ELSE
        DISPLAY "Product not found"
    END IF
END ENTRY POINT

Expected Output

Notebook price is 50

Nested Dictionaries

A dictionary can contain another dictionary as a value. This is called a nested dictionary.

students = {
    "101": {
        "name": "Aman",
        "marks": 85
    },
    "102": {
        "name": "Riya",
        "marks": 92
    }
}

Nested dictionaries are useful for storing structured data such as student records, product details, user profiles, and API responses.

students["102"]["name"] → "Riya"

Map / Dictionary vs List

Map / Dictionary vs Set

Why Maps / Dictionaries are Fast for Lookup

Many map and dictionary implementations use hashing or similar techniques internally.

This means they are usually designed to quickly find a value using its key.

contacts["Riya"] → quickly returns Riya's phone number

Advantages of Maps / Dictionaries

Limitations of Maps / Dictionaries

Common Beginner Mistakes

Best Practices for Maps / Dictionaries

Prerequisites Before Learning Maps / Dictionaries

Students should already understand:

Trace Table Example: Frequency Count

Let us trace how a dictionary counts repeated values.

numbers = [2, 3, 2, 4, 3]
frequency = {}

FOR EACH number IN numbers
    IF number EXISTS IN frequency THEN
        frequency[number] = frequency[number] + 1
    ELSE
        frequency[number] = 1
    END IF
END FOR

Practice Activity: Work with a Dictionary

Study the following dictionary:

book = {
    "title": "Programming Basics",
    "author": "Rumman",
    "price": 499
}

Questions

1. What is the value of book["title"]?
2. What is the value of book["price"]?
3. Add a new key "pages" with value 250.
4. Update "price" to 399.
5. Remove the key "author".

Sample Answers

1. Programming Basics
2. 499
3. book["pages"] = 250
4. book["price"] = 399
5. REMOVE KEY "author" FROM book

Mini Quiz

Interview Questions on Maps / Dictionaries

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