← IGCSE Computer Science Paper 2

Difference between a procedure and a function

Only a function has RETURNS: it gives back a value used where it is called. A procedure returns nothing and is run on its own with CALL.

Part of: IGCSE Computer Science 0478 — 8.1c Procedures and Functions — Module 6

Understanding Difference between a procedure and a function in IGCSE Computer Science Paper 2

What it does: The call Total(3, 4) is replaced by the value it returns, so that value can be stored in Cost. A function returns a value to the place where it was called. A procedure does not return anything. A procedure is run as its own statement using CALL. It has no value to store in a variable. A function call is used inside an expression, such as an assignment. CALL is only for procedures.

Common mistakes: A function must give its value back to the place it was called, so it needs RETURN Result. OUTPUT only displays the value and returns nothing. A procedure starts with PROCEDURE, so it must finish with ENDPROCEDURE. ENDFUNCTION is only used to close a function.

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This module's key terms are also in a printable crossword (Modules 399–408) and word search (Modules 399–418).

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Learning

Duo

Which one gives back a value that can be used where it is called?

A procedureA function

Matches

CALL Total(3, 4)

↓

Total is a function, and CALL is only for procedures. The returned value is lost. Write Answer ← Total(3, 4) or OUTPUT Total(3, 4).

Applying

Meanings

Term

RETURNS

Meaning

Only a function has RETURNS: it gives back a value used where it is called. A procedure returns nothing and is run on its own with CALL.

Practice

PROCEDURE ShowLine()

Testing

Gaps

FUNCTION Twice(N : INTEGER) INTEGER

Check Answer

Spots

2 Result ← 5 + 3

3 OUTPUT Result

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Quick Questions

1. Which one gives back a value that can be used where it is called?

A. A function ✓

B. A procedure

A function returns a value to the place where it was called. A procedure does not return anything.

2. Which header is a function?

A. FUNCTION Total(a, b) RETURNS INTEGER ✓

B. PROCEDURE Total(a, b) RETURNS INTEGER

Only a function has RETURNS in its header. A procedure header never has RETURNS.

3. How do you run a procedure called DrawLine?

A. total ← CALL DrawLine()

B. CALL DrawLine() ✓

A procedure is run as its own statement using CALL. It has no value to store in a variable.

4. Which statement uses a function correctly?

A. total ← AddUp(4, 5) ✓

B. CALL AddUp(4, 5)

A function call is used inside an expression, such as an assignment. CALL is only for procedures.

5. Which subroutine would you use to draw a line of 20 stars on the screen and return nothing?

A. A function

B. A procedure ✓

Drawing a line just does a job. No value needs to come back, so a procedure is the right choice.

Match the Pairs

FUNCTION AddUp(X : INTEGER) RETURNS INTEGER → A function header: RETURNS gives the data type of the value the function sends back.

PROCEDURE DrawLine() → A procedure header with no RETURNS part, because a procedure gives nothing back.

A function's returned value is used where it is called, for example stored in a variable → Cost ← Total(3, 4)

A procedure is called as its own statement using CALL → CALL DrawLine()

CALL Total(3, 4) → Total is a function, and CALL is only for procedures. The returned value is lost. Write Answer ← Total(3, 4) or OUTPUT Total(3, 4).

Result ← DrawLine() → DrawLine is a procedure and returns nothing, so there is no value to store. It should be run with CALL DrawLine().

Key Term

RETURNS

Only a function has RETURNS: it gives back a value used where it is called. A procedure returns nothing and is run on its own with CALL.

Fill in the Blanks

1. FUNCTION Twice(N : INTEGER) RETURNS INTEGER

Hint: Type the keyword in a function header that states the data type of the value given back.

2. CALL DrawLine(5)

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

3. FUNCTION Twice(N : INTEGER) RETURNS INTEGER RETURN N * 2 ENDFUNCTION

Hint: Type the statement inside a function that sends a value back to where it was called.

4. PROCEDURE DrawLine(Size : INTEGER) OUTPUT "-----" ENDPROCEDURE

Hint: Type the keyword that starts a subroutine that returns nothing.

5. Answer ← Twice(4) OUTPUT Answer

Hint: Type the assignment symbol that stores the value returned by the function call in a variable.

Spot the Error

Question 1

A FUNCTION Sum() RETURNS INTEGER

B Result ← 5 + 3

C OUTPUT Result

D ENDFUNCTION

Line C — A function must give its value back to the place it was called, so it needs RETURN Result. OUTPUT only displays the value and returns nothing.

Question 2

A PROCEDURE DrawLine()

B OUTPUT "----------"

C ENDFUNCTION

D CALL DrawLine()

Line C — A procedure starts with PROCEDURE, so it must finish with ENDPROCEDURE. ENDFUNCTION is only used to close a function.

Question 3

A PROCEDURE Hello()

B OUTPUT "Hi"

C ENDPROCEDURE

D Hello()

Line D — A procedure is run as its own statement using CALL. The line should be CALL Hello().

Question 4

A FUNCTION Pi() RETURNS REAL

B RETURNS 3.14

C ENDFUNCTION

D OUTPUT Pi()

Line B — RETURNS belongs in the function header to give the data type. Inside the function the keyword that sends a value back is RETURN 3.14.

Question 5

A PROCEDURE Show(N : INTEGER)

B RETURN N * 2

C ENDPROCEDURE

Line B — A procedure returns nothing, so it cannot use RETURN. It should display the value with OUTPUT N * 2.

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