Unit 12.1B · Term 1

Declarative vs Imperative Programming

Programming paradigms describe different ways of expressing a solution. Imperative programming tells the computer how to perform a task through explicit steps and state changes. Declarative programming focuses on what result or relationship is required.

Learning Objective

12.5.1.1 Compare declarative and imperative programming languages

Success Criteria

I can identify imperative and declarative characteristics · explain HOW vs WHAT · identify sequence, selection and iteration · classify code examples · compare two solutions to the same problem · justify which characteristics are suitable for a given scenario.

Lesson Presentation

Declarative vs Imperative · Slides for classroom use

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Conceptual Anchor

The Restaurant Analogy

Imperative: give the chef explicit instructions: “Boil the water, add pasta, cook it, drain it, add sauce.”

Declarative: describe the desired result: “I want pasta bolognese.”

Declarative versus imperative programming — Imperative focuses on HOW; declarative focuses on WHAT.

Imperative — HOW?

The programmer specifies the sequence of actions that should be executed.

EXPLICIT STEPS + CONTROL FLOW

Goal: find all students older than 16

1. Start with an empty result list.
2. Read the first student.
3. Check the student's age.
4. If age > 16, add the student.
5. Move to the next student.
6. Repeat until all students are checked.

Another Analogy: Navigation

Imperative: “Turn right, drive 300 m, turn left, stop at the second building.”
Declarative: “Take me to the school.”

Imperative Programming

Imperative programming describes a computation as a sequence of commands that change the program's state. The programmer explicitly controls what happens and in what order.

Explicit sequence of instructions.
Variables commonly change value during execution.
Uses control structures such as selection and iteration.
The programmer describes the procedure used to obtain the result.

Three Core Control Structures

Sequence

Instructions are executed in order.

→

x = 5
y = x + 2
print(y)

Selection

A condition chooses which path is executed.

x > 0?

YES

if x > 0:
    print("positive")
else:
    print("not positive")

Iteration

A block of instructions is repeated.

repeat

for i in range(5):
    print(i)

1 Imperative Example — Python

numbers = [1, 2, 3, 4, 5, 6, 7, 8]

evens = []

for n in numbers:
    if n % 2 == 0:
        evens.append(n)

print(evens)

The programmer explicitly creates state (evens), loops through the data, tests each value and updates the list.

Declarative Programming

Declarative programming expresses the desired result, relationship or rule without explicitly describing the complete sequence of control-flow steps used to obtain it.

Focuses on the required result or logical relationship.
The execution mechanism decides many procedural details.
Commonly uses queries, expressions, facts or rules.
The programmer usually specifies less explicit control flow.

2 Declarative Example — SQL

SELECT value
FROM numbers
WHERE value % 2 = 0;

The query states what rows are required. It does not specify the search algorithm, loop structure or exact execution plan. The database management system decides how to execute the query.

3 Declarative Logic — Prolog

grandparent(X, Z) :-
    parent(X, Y),
    parent(Y, Z).

The rule describes the logical relationship that makes X a grandparent of Z. It does not provide a step-by-step loop for searching the knowledge base.

Paradigm ≠ Language

Important idea

A programming language does not always belong to exactly one paradigm. Many modern languages support several programming styles. It is more accurate to discuss how a language or piece of code is commonly used.

Python multi-paradigm

Frequently used imperatively, but also supports object-oriented and functional-style constructs.

C / C++ mainly imperative

Explicit statements, state changes, loops and conditions are central.

SQL declarative query language

Describes the result set required from a database.

Prolog declarative logic programming

Describes facts, rules and queries rather than a conventional control-flow algorithm.

What about HTML and CSS?

HTML and CSS are often described as declarative technologies because they describe structure and presentation rather than a step-by-step algorithm. However, HTML itself is a markup language rather than a general-purpose programming language.

Comparison Matrix

Feature	Imperative	Declarative
Main focus	HOW to perform the task	WHAT result or relationship is required
Control flow	Usually specified explicitly by the programmer	Often handled by the execution system
State	Explicit state changes are common	Usually less emphasis on explicit step-by-step state updates
Typical constructs	Assignments, loops, conditions, sequences	Queries, expressions, facts, rules
Examples	Imperative Python/C++ code	SQL queries, Prolog rules
Responsibility	Programmer specifies much of the execution procedure	System determines many details of how the result is produced

Same Problem — Two Styles

Both examples below solve the same problem: find all students whose age is greater than 16. The difference is how responsibility is divided between programmer and execution system.

Imperative — Python

The programmer describes the procedure.

result = []

for student in students:
    if student["age"] > 16:
        result.append(student)

print(result)

Explicit iteration.
Explicit condition.
Explicit state update with append().

Declarative — SQL

The programmer describes the required result.

SELECT *
FROM students
WHERE age > 16;

No explicit loop is written.
The required condition is declared.
The DBMS chooses an execution plan.

Exam-quality comparison

Do not write only “Python is imperative and SQL is declarative”. Compare the characteristics: explicit control flow, state changes, procedure, desired result and responsibility of the execution system.

Imperative or Declarative?

Classify each example. Do not rely only on the language name — look for explicit steps/state changes versus a description of the required result.

Choose a paradigm and explain your reason before checking the feedback.

Example 1 of 8

When Are These Characteristics Useful?

Detailed Algorithmic Control

When a programmer needs explicit control over sequence, conditions, loops and state changes.

Often associated with imperative solutions.

Database Query

When the programmer wants to specify which data is required while the DBMS chooses how to retrieve it.

Declarative SQL is well suited to this pattern.

Rule-Based Knowledge

When a problem can be expressed using logical relationships, facts and rules.

Declarative logic programming is a natural fit.

Common Misconceptions

1. “Declarative means no algorithm exists”

An algorithm still runs underneath. The difference is that the programmer usually does not explicitly describe every control-flow step.

2. “Every language belongs to only one paradigm”

Many languages are multi-paradigm. Classify the programming style or construct being used rather than relying only on the language name.

3. “SQL is imperative because the DBMS performs steps”

SQL is declarative because the query specifies the desired data, while the database engine decides the detailed execution strategy.

4. “Declarative means there are no variables”

Declarative systems can use variables. Prolog variables, for example, represent values that can be matched during logical inference.

Where This Topic Leads Next

Expert Systems & Prolog

Use declarative facts, rules and queries to build a simple expert system.

Program Compilation Stages

Study how source code is analysed and transformed into target code.

Practice Tasks

Remember

Define imperative programming and declarative programming.

Understand

Explain the phrase HOW vs WHAT in the context of programming paradigms.

Understand

Explain the difference between sequence, selection and iteration.

Apply

Classify six short code fragments as imperative or declarative and justify each answer using programming characteristics.

Apply

Identify sequence, selection and iteration in a short Python program.

Analyze

Compare the Python and SQL examples that find students older than 16. Give at least three differences.

Analyze

Explain why SQL is considered declarative even though the database engine still executes algorithms internally.

Analyze

For each scenario — detailed algorithmic control, database query, rule-based knowledge system — explain which paradigm characteristics are useful and why.

Exam Practice

Q1: State one difference between imperative and declarative programming. [2]

Reveal marking points

Imperative code explicitly describes the procedure/control flow used to solve the problem; declarative code describes the desired result or relationship without explicitly specifying all control-flow steps.

Q2: Explain why a SQL SELECT query is an example of declarative programming. [2]

Reveal marking points

The query specifies what data should be returned, while the DBMS determines the detailed search/access/execution strategy.

Q3: Describe two features of imperative programming. [2]

Reveal marking points

Examples: explicit sequence of statements; use of selection/iteration; explicit state or variable updates; programmer-controlled procedure.

Q4: Compare an imperative and a declarative solution to the same data-filtering problem. [4]

Reveal marking points

Credit comparative points such as: explicit loop vs no explicit loop; explicit state updates vs result specification; programmer-defined procedure vs execution plan chosen by the system; HOW vs WHAT.

Self-Check Quiz

Q1: What is the main focus of imperative programming?

HOW to perform the task through explicit instructions, control flow and state changes.

Q2: What is the main focus of declarative programming?

WHAT result, relationship or condition is required.

Q3: What are the three core imperative control structures?

Sequence, selection and iteration.

Q4: Why is SQL considered declarative?

It describes the data/result required while the DBMS chooses the detailed execution strategy.

Q5: Why is Prolog considered declarative?

It describes facts, rules and logical relationships and allows the Prolog engine to search for solutions.

Q6: Can one programming language support several paradigms?

Yes. Many modern languages are multi-paradigm; for example, Python supports imperative, object-oriented and functional-style programming.

Exit Ticket — 3–2–1

Write three characteristics of imperative programming.

Write two characteristics of declarative programming.

Give one clear HOW vs WHAT example of your own.