Programming paradigms: procedural and declarative

Procedural paradigm

In the procedural paradigm (a kind of imperative programming) a program is a sequence of instructions that say how to solve a problem, step by step.

• Built from sequence, selection and iteration.
• Code is organised into procedures and functions (subroutines) that can be called and reused, with parameters and return values.
• Uses variables whose values (the program's state) change as it runs; local and global scope.
• Examples: Python (used procedurally), C, Pascal.
• Benefits: easy to follow for many problems, reusable subroutines, close to how the CPU works, good for algorithms. Drawbacks: large programs can become hard to maintain; changing global state can cause bugs; the programmer must describe every step.

Declarative paradigm

In the declarative paradigm the programmer states what result is wanted, and the language system works out how to get it.

• Examples: SQL (SELECT Name FROM Student WHERE Form = '12A' says what data is wanted, not how to search the table), and logic languages such as Prolog (facts and rules, then queries).
• Benefits: shorter code; the system can choose an efficient method; less chance of step-by-step errors; good for databases, querying and rule-based problems. Drawbacks: less control over how the work is done; can be harder to predict performance; not suited to every kind of task.

Comparing

Worked example

Explain why the SQL statement SELECT Title FROM Book WHERE Year > 2020 is declarative.

1. It describes the result wanted: titles of books after 2020.
2. It does not say how to find them (no loop, no order of checking).
3. The database system decides how to search the table.

Answer: It states what is required, not the steps to get it.

Check yourself

1. SQL is an example of which paradigm?
   • Procedural
   • Machine code
   • Declarative
   • Assembly
   Show answer

   Declarative. SQL describes the data wanted, not how to retrieve it.

2. Which describes the procedural paradigm?
   • A program is a sequence of instructions, grouped into subroutines, saying how to solve a problem
   • A program states facts and rules and the system finds answers
   • A program describes only the result wanted
   • A program has no variables
   Show answer

   A program is a sequence of instructions, grouped into subroutines, saying how to solve a problem. Procedural = step by step, with procedures and functions.

3. Which is a benefit of declarative programming?
   • The programmer controls exactly how memory is used
   • It is always faster than procedural code
   • The programmer does not need to specify the steps, so code can be shorter
   • It is the only way to write loops
   Show answer

   The programmer does not need to specify the steps, so code can be shorter. The system works out the method.

4. Which is a drawback of the procedural paradigm in very large programs?
   • It cannot use functions
   • Shared, changing global state can make programs hard to maintain and debug
   • It cannot use selection
   • It can only query databases
   Show answer

   Shared, changing global state can make programs hard to maintain and debug. Many subroutines changing shared variables can cause hard-to-find bugs.

5. Explain the main difference between the procedural and declarative paradigms.
   Show answer

   Procedural code tells the computer how to solve the problem, step by step; declarative code states what result is wanted and the system decides how to achieve it.. How versus what.

6. In the procedural paradigm, why are subroutines used?
   • To make every variable global
   • To break a program into reusable, named blocks of code
   • To remove the need for selection
   • To turn the program into SQL
   Show answer

   To break a program into reusable, named blocks of code. Subroutines can be called many times and tested separately.

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Sources

• Pearson Edexcel International Advanced Subsidiary/Advanced Level in Computer Science specification (2026)

Last updated 8 November 2026. Spotted a mistake? Tell us.