← IGCSE Computer Science Paper 2

Choosing a procedure or a function for a task

Use a FUNCTION when a task produces one value the program needs, such as an area; use a PROCEDURE when it only does something, such as OUTPUT a menu.

Part of: IGCSE Computer Science 0478 — 8.1c Procedures and Functions — Module 25

Understanding Choosing a procedure or a function for a task in IGCSE Computer Science Paper 2

What it does: The call to the function is used in an assignment because the function returns a value that is stored in Area. Displaying a menu is an action, not a value the program needs. Actions like this are written as procedures. Functions return a value with RETURN. Procedures do not return a value. Showing a menu only does something. Calculating a total gives one value back, which suits a function. A good justification uses 'because' and links the choice to a value being returned. Returning the value lets the program store it or use it in other calculations. Outputting it only shows it on screen.

Common mistakes: Area produces one value the program needs, so it is a FUNCTION. A FUNCTION must be closed with ENDFUNCTION, not ENDPROCEDURE. Showing a menu only does something, so it is a PROCEDURE. A procedure displays with OUTPUT and does not RETURN a value. The line should be OUTPUT "1. Play".

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Learning

Duo

A subroutine works out the area of a rectangle and the program uses that number later. Which should you write?

A function, because it produces one value that the program needsA procedure, because it produces one value that the program needs

Matches

PROCEDURE CalcArea(L, W) RETURN L * W ENDPROCEDURE

↓

A procedure cannot return a value. The task produces one value the program needs, so it should be a FUNCTION with RETURNS and ENDFUNCTION.

Applying

Meanings

Term

FUNCTION

Meaning

Use a FUNCTION when a task produces one value the program needs, such as an area; use a PROCEDURE when it only does something, such as OUTPUT a menu.

Practice

// Procedure because it only displays

Testing

Gaps

ShowMenu()

Check Answer

Spots

2 RETURN S * S

3ENDPROCEDURE

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Quick Questions

1. A subroutine works out the area of a rectangle and the program uses that number later. Which should you write?

A. A procedure, because it produces one value that the program needs

B. A function, because it produces one value that the program needs ✓

The area is a single value used later in the program, so a function RETURNs it. A procedure does not return a value.

2. A subroutine only displays a menu with OUTPUT and gives nothing back. Which should you write?

A. A procedure, because it only does something ✓

B. A function, because it only does something

Displaying a menu is an action, not a value the program needs. Actions like this are written as procedures.

3. Which statement is correct?

A. A function uses RETURN to send a value back to the line that called it ✓

B. A procedure uses RETURN to send a value back to the line that called it

Functions return a value with RETURN. Procedures do not return a value.

4. Which task suits a function?

A. Calculating the area of a circle from its radius ✓

B. Printing a welcome menu on the screen

The area is one value the program can store or use. Printing the menu is just an action, so it suits a procedure.

5. Which task suits a procedure?

A. Calculating the total price of items in a basket

B. Using OUTPUT to show a list of menu options ✓

Showing a menu only does something. Calculating a total gives one value back, which suits a function.

Match the Pairs

FUNCTION CalcArea(L : INTEGER, W : INTEGER) RETURNS INTEGER → Use a function because the task produces one value, the area, that the program needs.

PROCEDURE ShowMenu() → Use a procedure because the task only does something, such as displaying a menu, and gives no value back.

A function returns a value to the program, so the result can be stored in a variable. → Area ← CalcArea(5, 3)

A procedure only carries out actions such as OUTPUT, so it is run with its own CALL statement. → CALL ShowMenu()

PROCEDURE CalcArea(L, W) RETURN L * W ENDPROCEDURE → A procedure cannot return a value. The task produces one value the program needs, so it should be a FUNCTION with RETURNS and ENDFUNCTION.

FUNCTION ShowMenu() OUTPUT "1. Play" ENDFUNCTION → Displaying a menu only does something and returns no value, so it should be a PROCEDURE and not a FUNCTION.

Key Term

FUNCTION

Use a FUNCTION when a task produces one value the program needs, such as an area; use a PROCEDURE when it only does something, such as OUTPUT a menu.

Fill in the Blanks

1. PROCEDURE ShowMenu() OUTPUT "1. Play" ENDPROCEDURE

Hint: Type the keyword for a subroutine that only does something, such as displaying a menu, and gives no value back.

2. FUNCTION Area(L : INTEGER) RETURNS INTEGER RETURN L * L ENDFUNCTION

Hint: Type the keyword that states which data type the function sends back.

3. FUNCTION Area(L, W) RETURNS INTEGER RETURN L * W ENDFUNCTION

Hint: Type the keyword that sends the calculated value back to the caller instead of displaying it.

4. CALL ShowMenu() area ← Area(4, 5)

Hint: Type the keyword used to run a procedure as a statement on its own line.

5. area ← Area(3, 6) OUTPUT area

Hint: Type the assignment symbol that stores the value the function returns in a variable.

Spot the Error

Question 1

A FUNCTION Area(S) RETURNS REAL

B RETURN S * S

C ENDPROCEDURE

D OUTPUT Area(4)

Line C — Area produces one value the program needs, so it is a FUNCTION. A FUNCTION must be closed with ENDFUNCTION, not ENDPROCEDURE.

Question 2

A PROCEDURE ShowMenu()

B RETURN "1. Play"

C OUTPUT "2. Quit"

D ENDPROCEDURE

Line B — Showing a menu only does something, so it is a PROCEDURE. A procedure displays with OUTPUT and does not RETURN a value. The line should be OUTPUT "1. Play".

Question 3

A FUNCTION Area(S) RETURNS REAL

B RETURN S * S

C ENDFUNCTION

D CALL Area(5)

E OUTPUT Total

Line D — A function returns a value, so the call must be used in an expression or stored in a variable. CALL on its own throws the area away. It should be Total ← Area(5).

Question 4

A FUNCTION Perim(S) RETURNS REAL

B OUTPUT S * 4

C ENDFUNCTION

D OUTPUT Perim(3)

Line B — A function must RETURN its value so the program can use it. OUTPUT only displays it and gives nothing back. The line should be RETURN S * 4.

Question 5

A PROCEDURE ShowMenu()

B OUTPUT "1. Play"

C ENDPROCEDURE

D ShowMenu()

Line D — A procedure produces no value, so it is run with a CALL statement. Writing just the name is not a valid call. It should be CALL ShowMenu().

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