Programming Paradigms
Imperative vs Declarative vs Functional

What are Programming Paradigms?

A programming paradigm is a style or way of programming. It defines how we structure and organize code to solve problems. The three major paradigms we'll explore are:

• Imperative Programming
• Declarative Programming
• Functional Programming

Imperative Programming

Imperative programming focuses on how to achieve a result. You write step-by-step instructions that change the program’s state.

Example: Sum of first 5 numbers

total = 0
for i from 1 to 5:
    total = total + i
print total

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This approach tells the computer exactly what to do — initialize a variable, iterate, and update it. This is explicit and state-based.

Question:

Why is it called “imperative”?

Because you give commands (like in imperative sentences in grammar), instructing the computer step-by-step.

Declarative Programming

Declarative programming focuses on what you want, not how to do it. You describe the result and let the system figure out how to compute it.

Example: Sum of first 5 numbers

sum = sumOfNumbers(1 to 5)
print sum

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This hides the control flow (looping, iteration), and focuses only on the end result. You declare your intention.

Question:

What are the benefits of declarative programming?

It leads to cleaner, more readable code and abstracts low-level implementation details.

Functional Programming

Functional programming is a subset of declarative programming that focuses on pure functions and avoiding shared state. It uses concepts like mapping, filtering, and reducing collections of data.

Example: Sum of first 5 numbers using reduce

numbers = [1, 2, 3, 4, 5]
sum = reduce(numbers, (acc, curr) => acc + curr)
print sum

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Here, reduce is a higher-order function. No variables are modified. Everything is based on pure transformations.

Question:

What makes a function “pure”?

A function is pure if:

• It gives the same output for the same input
• It does not change any external state or variable

Paradigm Comparison Table

Paradigm	Focus	State Mutation	Control Flow
Imperative	How to do it	Yes	Explicit (loops, steps)
Declarative	What to do	Hidden or minimal	Implicit
Functional	What to do using functions	No (stateless)	Abstracted via recursion or higher-order functions

Summary

• Imperative: You control every step.
• Declarative: You describe the desired result.
• Functional: You use pure functions and avoid mutable state.

Quick Quiz

Which paradigm would be best for processing a list of items without modifying global variables?

Answer: Functional programming.

Which paradigm gives you the most control over how the task is performed?

Answer: Imperative programming.