Programming Paradigms: Imperative vs. Declarative

When you learn your first programming language (usually Python, C, or Java), you are implicitly learning a specific "style" of thinking. This style is called a Programming Paradigm.

There are hundreds of languages, but they almost all fall into two major camps: those that tell the computer how to do something, and those that tell the computer what result you want.

1. Imperative Programming (The "How")

Imperative programming is the oldest and most common paradigm. It maps directly to how the hardware works (The Turing Machine). You give the computer a specific sequence of instructions to change its state.

The Vibe: A detailed recipe. "Preheat oven to 350. Mix flour and sugar. Beat eggs. Pour into pan. Bake for 30 minutes."

Sub-paradigms: - Procedural: Grouping instructions into functions (C, Pascal). - Object-Oriented (OOP): Grouping instructions and data into objects (Java, C++).

Example (Python): Filtering a list Notice how we manually iterate, check, and append. We control the flow.

numbers = [1, 2, 3, 4, 5, 6]
evens = []
for n in numbers:
    if n % 2 == 0:
        evens.append(n)

2. Declarative Programming (The "What")

Declarative programming hides the control flow. You describe the logic of the computation without describing its control flow. You tell the compiler/interpreter the properties of the result you want, and it figures out the steps.

The Vibe: A blueprint or a restaurant order. "I want a pepperoni pizza." (You don't tell the chef how to roll the dough).

Sub-paradigms: - Functional: Building programs by composing pure functions (Haskell, Lisp). - Logic: Defining facts and rules (Prolog). - Query: Requesting data (SQL). - Markup: Defining structure (HTML/CSS).

Example (SQL): Filtering a list Notice we don't say how to search the table. We just say what we want.

SELECT number FROM numbers WHERE number % 2 = 0;

Example (Python List Comprehension): Python supports declarative styles too!

evens = [n for n in numbers if n % 2 == 0]

The Great Convergence

Historically, languages picked a side. C was purely Imperative. Lisp was purely Declarative (Functional).

Modern languages are Multi-Paradigm. - Python/JavaScript: Allow you to write imperative loops OR declarative map/filter functions. - Java: Added declarative Streams in Java 8. - React (JS Library): Shifted UI development from Imperative ("Change this button's text") to Declarative ("The button's text should be X based on state Y").

Practice Problems

Practice Problem 1: Classify the Code

Is HTML Imperative or Declarative?

<h1>Hello World</h1>
<p>This is a paragraph.</p>

Solution

Declarative.

You are telling the browser what to display (a heading, a paragraph). You are not telling it how to render the pixels, manage the layout engine, or flow the text.

Practice Problem 2: The Loop vs. The Map

Which of these two JavaScript snippets is Imperative?

A:

const doubled = numbers.map(n => n * 2);

B:

const doubled = [];
for (let i = 0; i < numbers.length; i++) {
    doubled.push(numbers[i] * 2);
}

Solution

Snippet B is Imperative.

It specifies the control flow (the loop, the index i, the push operation). Snippet A is Declarative (Functional)—it just says "Map these numbers to their double."

Key Takeaways

Feature	Imperative	Declarative
Focus	How to do it (Steps).	What output to get (Result).
State	Mutable (Changeable).	Immutable (New values created).
Analogy	Recipe.	Blueprint.
Examples	C, Java, Python (Loops).	SQL, HTML, Haskell, Excel.

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Understanding paradigms is like understanding musical genres. You might be a rock musician (Imperative C coder), but learning jazz (Functional Haskell) will make you a better musician overall.