Procedures, functions and maintainable programs Cambridge IGCSE Computer Science revision
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In plain words
As programs grow, the same few lines turn up again and again. Sooner than copy them out, you give them a name and call them by that name whenever they are needed. A named block of code like this is a procedure or a function.
The difference between the two is small but important: a function hands a value back.
6 things to know
- A procedure is a named block of code that carries out a task. It is run with CALL: for example, CALL ShowMenu.
- A function is a named block of code that carries out a task and returns a value. It is used inside an expression: Area ← RectangleArea(4, 5).
- A parameter is a value passed into a procedure or function when it is called, so that the same code can work on different data.
- A local variable is declared inside a procedure or function and can only be used there. A global variable is declared in the main program and can be used anywhere.
- A program is maintainable when someone else can read it and change it. That means meaningful identifiers for variables, constants, arrays, procedures and functions.
- It also means comments that explain what the code does, and procedures and functions that keep each task in one place.
Worked example
A function is defined as FUNCTION Double(Number : INTEGER) RETURNS INTEGER, and its body is RETURN Number * 2. What does OUTPUT Double(7) + 1 display?
- Double is called with the parameter 7.
- Inside the function, Number is 7, so it returns 7 × 2 = 14.
- The expression becomes 14 + 1, so 15 is displayed.
Tips and tricks
- A function returns a value and a procedure does not. If the question says "returns", it is a function.
- Use local variables where you can. A global variable can be changed from anywhere, which makes errors hard to track down.
It lands in your notebook with its questions as flashcards.
Procedures, functions and maintainable programs: 6 questions and answers
These are the quiz’s questions. Do the quiz first, then come back here for the ones that got you.
What is the difference between a function and a procedure?
Both are named blocks of code.
What is a parameter?
It lets the same code work with different data.
Where can a local variable be used?
Outside that block it does not exist.
Which keyword runs a procedure?
For example, CALL DrawLine(10).
Which of these makes a program easier to maintain?
Other programmers can then follow what the code does.
A function body is RETURN Length * Width. It is called as Area(3, 6). What does it return?
3 × 6 = 18.
Quiz
6 questions
Tap an answer and you’ll see straight away whether it’s right, and why.
Worksheet
3 questions, 7 marks. Write your answers on paper, then check them.
Procedures, functions and maintainable programs
Cambridge IGCSE Computer Science 0478 · 7 marks · papermunch.org
Name ______________________________ Date ______________
Describe the difference between a procedure and a function.[2]
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Both are named blocks of code that carry out a task. A function returns a value to the code that called it, and a procedure does not.
Give three ways of making a program easier to maintain.[3]
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Use meaningful identifiers. Add comments to explain the code. Use procedures and functions.
Describe the difference between a local variable and a global variable.[2]
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A local variable can only be used inside the procedure or function where it is declared. A global variable can be used anywhere in the program.
Answers: Procedures, functions and maintainable programs
- 1. Both are named blocks of code that carry out a task. A function returns a value to the code that called it, and a procedure does not.
- 2. Use meaningful identifiers. Add comments to explain the code. Use procedures and functions.
- 3. A local variable can only be used inside the procedure or function where it is declared. A global variable can be used anywhere in the program.