← IGCSE Computer Science Paper 2

Comments above a subroutine

Starts a comment. Put one on the line before a procedure or function to say what it does and what its parameters are. The computer ignores comments.

Part of: IGCSE Computer Science 0478 — 8.1c Procedures and Functions — Module 42

Understanding Comments above a subroutine in IGCSE Computer Science Paper 2

What it does: The // comment sits directly above the PROCEDURE line and states its purpose. Everything after // on the line is a comment, so it is ignored when the program runs. Comments are only for people reading the code. They never run and never print. A useful comment is specific. It says what the subroutine does and what the parameter is for. Comments must begin with // so that the computer ignores them.

Common mistakes: A short end-of-line comment must start with //. A single / is not a comment, so the computer tries to run 'doubled value' as code. It should be: OUTPUT D // doubled value. The line before the function has no // at the start, so the computer treats the words as code and gives an error. It should be: // Returns a random number.

Class: _____________________ · Name: _____________________ · Date: _________

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This module's key terms are also in a printable crossword (Modules 429–438) and word search (Modules 419–438).

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Learning

Duo

Which line is a correct comment in Cambridge IGCSE pseudocode?

// Calculates the area of a circle# Calculates the area of a circle

Matches

Calculates the area of a rectangle FUNCTION Area(w : INTEGER, h : INTEGER) RETURNS INTEGER

↓

The comment has no // at the start, so the computer treats it as code and reports an error. It should be // Calculates the area of a rectangle.

Applying

Meanings

Term

Meaning

Starts a comment. Put one on the line before a procedure or function to say what it does and what its parameters are. The computer ignores comments.

Practice

// Adds two whole numbers and returns the total

Testing

Gaps

Shows a welcome message. No parameters.

Check Answer

Spots

3 D ← N * 2

4 OUTPUT D / doubled value

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Quick Questions

1. Which line is a correct comment in Cambridge IGCSE pseudocode?

A. // Calculates the area of a circle ✓

B. # Calculates the area of a circle

Cambridge pseudocode comments start with //. The computer ignores everything after // on that line.

2. Where is the best place to put a comment that explains what a procedure does?

A. After ENDPROCEDURE at the end

B. On the line before PROCEDURE ✓

A comment above the subroutine tells the reader its purpose before they read the code.

3. What does the computer do with a // comment when the program runs?

A. Ignores it ✓

B. Displays it with OUTPUT

Comments are only for people reading the code. They never run and never print.

4. A comment above FUNCTION Square(Num : INTEGER) RETURNS INTEGER should mention...

A. What it does and what Num is ✓

B. Only the name of the programmer

A good comment states the purpose of the subroutine and describes its parameters.

5. Which comment is at the correct indentation for code that is not indented?

A. // Show a greeting PROCEDURE Greet()

B. // Show a greeting PROCEDURE Greet() ✓

A comment should line up with the code it describes. Here the PROCEDURE line has no indent, so the comment has none either.

Match the Pairs

// Calculates the area of a rectangle → A comment placed on the line before a function that states its purpose

// width and height are both INTEGER → A comment that states the parameters of the subroutine below it

A comment is written on the line before the procedure and explains what it does → // Displays a greeting for the name given PROCEDURE Greet(name : STRING)

A short end-of-line comment can explain one line of code, and the computer ignores it → total ← total + 1 // add one to the count

Calculates the area of a rectangle FUNCTION Area(w : INTEGER, h : INTEGER) RETURNS INTEGER → The comment has no // at the start, so the computer treats it as code and reports an error. It should be // Calculates the area of a rectangle.

PROCEDURE ShowScore(score : INTEGER) // Displays the score → The comment is placed after the procedure header instead of on the line before it. It should be written above the PROCEDURE line.

Key Term

//

Starts a comment. Put one on the line before a procedure or function to say what it does and what its parameters are. The computer ignores comments.

Fill in the Blanks

1. // Shows a welcome message. No parameters. PROCEDURE ShowMessage() OUTPUT "Welcome"

Hint: Type the symbol that starts a comment line, which the computer ignores.

2. // Returns N multiplied by 2 FUNCTION Double(N : INTEGER) RETURNS INTEGER RETURN N * 2

Hint: Type the parameter name that the comment above describes.

3. Total ← Price * Qty // cost of all items

Hint: Type the symbol that starts a short end-of-line comment.

4. // Shows Name on the screen. Parameter: Name PROCEDURE ShowName(Name : STRING) OUTPUT Name

Hint: Type the keyword that begins a subroutine which does not return a value.

5. // Purpose: add tax to Price FUNCTION AddTax(Price : REAL) RETURNS REAL RETURN Price * 1.2

Hint: Type the keyword that sends the result back to the code that called the function.

Spot the Error

Question 1

A // Shows N doubled

B PROCEDURE Show(N : INTEGER)

C D ← N * 2

D OUTPUT D / doubled value

E ENDPROCEDURE

Line D — A short end-of-line comment must start with //. A single / is not a comment, so the computer tries to run 'doubled value' as code. It should be: OUTPUT D // doubled value.

Question 2

A OUTPUT "Start"

B Returns a random number

C FUNCTION Roll() RETURNS REAL

D RETURN RANDOM()

E ENDFUNCTION

Line B — The line before the function has no // at the start, so the computer treats the words as code and gives an error. It should be: // Returns a random number.

Question 3

A DECLARE Score : INTEGER

B Score ← 0

C // Takes 1 from the score

D PROCEDURE AddOne()

E Score ← Score + 1

F ENDPROCEDURE

Line C — The comment above a subroutine must state what it really does. AddOne adds 1 to Score, it does not take 1 away. It should be: // Adds 1 to the score.

Question 4

A Total ← 0

B -- Shows the total

C PROCEDURE ShowTotal()

D OUTPUT Total

E ENDPROCEDURE

Line B — Comments in IGCSE pseudocode start with //, not --. It should be: // Shows the total.

Question 5

A // Shows the total price

B // A: price of one item

C // B: number of items

D PROCEDURE Cost(A : INTEGER)

E OUTPUT A * B

F ENDPROCEDURE

Line D — The comments say the procedure has two parameters, A and B, but the header only has A. The header must match the comments and the body: PROCEDURE Cost(A, B : INTEGER).

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