Unit 12.1B · Term 1

Declarative vs Imperative Programming

Programming paradigms dictate how we solve problems with code. Imperative languages explicitly instruct the computer how to do something step by step. Declarative languages describe what the end result should be.

Learning Objectives

  • 12.5.1.1 Compare declarative and imperative programming languages
  • 12.5.1.2 Create a simple expert system
  • 12.5.1.3 Describe program compilation stages: lexical and syntactic analysis, code generation, and optimization

Lesson Presentation

Declarative vs Imperative · Slides for classroom use

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Video Explanation

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Conceptual Anchor

The Restaurant Analogy

Imperative = giving the chef step-by-step cooking instructions: "Boil water, add pasta, cook 10 min, drain, add sauce."
Declarative = just ordering from the menu: "I want pasta bolognese." You describe WHAT you want, not HOW to make it.


Declarative vs Imperative
Declarative vs Imperative.

Rules & Theory

Imperative Programming

  • Definition: A paradigm that uses statements that change a program's state. It relies on explicit control flow using sequence, selection (if/else), and iteration (loops).
  • Where it is applied: System programming, game engine development, and general-purpose software (e.g., Python, C++, Java, Pascal).
  • Benefits:
    • Direct control over each step of execution.
    • Programmers can optimize for specific hardware or constraints.
    • Can handle a wide range of problems and domains.
  • Drawbacks:
    • Can become complex and hard to maintain, especially for large programs.
    • Managing state and control flow explicitly can lead to errors.
    • Focuses more on how things are done rather than what is achieved.

Declarative Programming

  • Definition: A paradigm that expresses the logic of a computation without describing its control flow. It relies on facts, rules, and queries rather than sequential steps.
  • Where it is applied: Database management (SQL), artificial intelligence and logic programming (Prolog), and web page structuring (HTML).
  • Benefits:
    • Code is often easier to read and understand.
    • Often fewer lines of code are required for the same task.
    • Easier to write programs that take advantage of multiple cores.
  • Drawbacks:
    • Abstracts away control flow, which may not allow for fine-grained optimization.
    • Best suited for specific types of tasks (e.g., data querying, configuration).
    • May not be as performant in low-level, resource-constrained environments like embedded systems.

Comparison Matrix

Feature Imperative Declarative
Focus HOW to achieve the result WHAT the result should be
Control flow Explicitly detailed by the programmer (loops, conditions, sequences) Handled automatically by the system or interpreter
State Requires explicit control of variable updates and state changes Programmer does not manage low-level details like memory allocation or variable assignments

Code Comparison

# IMPERATIVE (Python) — HOW to find even numbers numbers = [1, 2, 3, 4, 5, 6, 7, 8] evens = [] for n in numbers: # Iteration if n % 2 == 0: # Selection evens.append(n) # Sequence & State change print(evens) # Output: [2, 4, 6, 8]
-- DECLARATIVE (SQL) — WHAT you want SELECT * FROM numbers WHERE value % 2 = 0;

Expert Systems

Component Purpose
Knowledge Base A collection of facts and rules about a specific domain.
Inference Engine Applies the rules to the known facts to derive new conclusions.
User Interface Allows users to input queries and receive answers.
% Simple Expert System in Prolog (Declarative) % Knowledge Base — Facts: animal(dog). animal(cat). has_fur(dog). has_fur(cat). says(dog, woof). says(cat, meow). % Rules: pet(X) :- animal(X), has_fur(X). % Query (Asking the engine WHAT we want to know): ?- pet(dog). % Result: true ?- says(cat, Y). % Result: Y = meow

Compilation Stages

Stage What It Does Example
1. Lexical Analysis Breaks source code into tokens (keywords, identifiers, operators). Removes whitespace and comments. x = 5 + 3 → tokens: [x] [=] [5] [+] [3]
2. Syntax Analysis Checks if tokens follow language grammar rules; builds a parse tree. Detects x = 5 + as a syntax error (missing operand).
3. Code Generation Converts the validated parse tree into machine code (binary instructions) or intermediate object code. ADD R1, R2 → 00101001...
4. Optimization Improves code efficiency (execution speed/memory usage) without changing the final results. Removes unused variables or redundant loops.

Common Pitfalls

SQL is NOT Imperative

SQL says what data you want, not how to find it. SELECT * FROM students WHERE age > 16 doesn't specify searching algorithms (like linear or binary search) — the database engine decides the optimal path.

Tasks

Remember

List 3 imperative and 3 declarative programming languages.

Understand

Explain the difference between Sequence, Selection, and Iteration in an imperative language.

Apply

Write a simple expert system (facts + rules) in Prolog for diagnosing 3 common computer faults based on error symptoms.

Self-Check Quiz

Q1: What is the main difference between imperative and declarative?

Imperative explicitly describes HOW to solve a problem step-by-step using control flows. Declarative describes WHAT the result should be using logic, facts, or queries.

Q2: What are the 3 main components of an Expert System?

1. Knowledge Base (facts & rules). 2. Inference Engine (applies rules to derive answers). 3. User Interface (handles input/output).

Q3: At which compilation stage are comments and whitespaces removed?

Lexical Analysis. This stage cleans the code and breaks it down into individual tokens.