Procedures & Functions (Cambridge (CIE) O Level Computer Science)

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Robert Hampton

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What are functions and procedures?

Functions and procedures are a type of sub program, a sequence of instructions that perform a specific task or set of tasks
Procedures and functions are defined at the start of the code
Sub programs are often used to simplify a program by breaking it into smaller, more manageable parts
Sub programs can be used to:
- Avoid duplicating code and can be reused throughout a program
- Improve the readability and maintainability of code
- Perform calculations, to retrieve data, or to make decisions based on input
Parameters are values that are passed into a sub program
- Parameters can be variables or values and they are located in brackets after the name of the sub program
- Example: FUNCTION TaxCalculator(pay,taxcode) OR PROCEDURE TaxCalculator(pay,taxcode)
Sub programs can have multiple parameters
To use a sub program you 'call' it from the main program

What's the difference between a function and procedure?

A Function returns a value whereas a procedure does not

Procedures

Procedures are defined using the PROCEDURE keyword in pseudocode or def keyword in Python
A procedure can be written as:

PROCEDURE <identifier>
<statements>
ENDPROCEDURE
Or if parameters are being used:

PROCEDURE <identifier>(<param1>:<data type>, <param2>:<data type>...)
<statements>
ENDPROCEDURE

Creating a procedure
Pseudocode
`PROCEDURE calculate_area(length: INTEGER, width: INTEGER)` `area ← length * width` `OUTPUT "The area is " + area` `END PROCEDURE`
Python
`def ageCheck(age):` `if age > 18:` `print("You are old enough")` `else:` `print("You are too young")`

To call a procedure, it can be written as:

CALL <identifier>
CALL <identifier>(Value1,Value2...)

Calling a procedure
Pseudocode
`CALL Calculate_area` `CALL Calculate_area(5,3)`
Python
`ageCheck(21)` `print(ageCheck(21))`

Examples

A Python program using procedures to display a menu and navigate between them
Procedures are defined at the start of the program and the main program calls the first procedure to start
In this example, no parameters are needed

Procedures
# Procedure definition `def main_menu():` # Outputs the option `print("1. Addition")` `print("2. Subtraction")` `print("3. Multiplication")` `print("4. Division")` `print("5. Exit")` # Asks the user to enter their choice `choice = int(input("Enter your choice: "))` `if choice == 1:` `addition()` `elif choice == 2:` `subtraction()` `elif choice == 3:` `multiplication()` `elif choice == 4:` `division()` `elif choice == 5:` `exit()` # Procedure definition `def addition():` `num1 = int(input("Enter the first number: "))` `num2 = int(input("Enter the second number: "))` `print(num1 + num2)` `def subtraction():` `num1 = int(input("Enter the first number: "))` `num2 = int(input("Enter the second number: "))` `print(num1 - num2)` `def multiplication():` `num1 = int(input("Enter the first number: "))` `num2 = int(input("Enter the second number: "))` `print(num1 * num2)` `def division():` `num1 = int(input("Enter the first number: "))` `num2 = int(input("Enter the second number: "))` `print(num1 / num2)` # Main program `main_menu()` # Calls the main_menu procedure

Procedures

# Procedure definition

def main_menu():

# Outputs the option
print("1. Addition")
print("2. Subtraction")
print("3. Multiplication")
print("4. Division")
print("5. Exit")

# Asks the user to enter their choice

choice = int(input("Enter your choice: "))
if choice == 1:
addition()
elif choice == 2:
subtraction()
elif choice == 3:
multiplication()
elif choice == 4:
division()
elif choice == 5:
exit()

# Procedure definition

def addition():
num1 = int(input("Enter the first number: "))
num2 = int(input("Enter the second number: "))
print(num1 + num2)

def subtraction():
num1 = int(input("Enter the first number: "))
num2 = int(input("Enter the second number: "))
print(num1 - num2)

def multiplication():
num1 = int(input("Enter the first number: "))
num2 = int(input("Enter the second number: "))
print(num1 * num2)

def division():
num1 = int(input("Enter the first number: "))
num2 = int(input("Enter the second number: "))
print(num1 / num2)

# Main program
main_menu() # Calls the main_menu procedure

Functions

Functions are defined using the FUNCTION keyword in pseudocode or def keyword in Python
Functions always return a value so they do not use the CALL function, instead they are called within an expression
A function can be written as:

FUNCTION <identifier> RETURNS <data type>
<statements>
ENDFUNCTION
Or if parameters are being used:

FUNCTION <identifier>(<param1>:<data type>, <param2>:<data type>...) RETURNS <data type>
<statements>
ENDFUNCTION

Creating and using a function
Pseudocode
`FUNCTION calculate_area(length: INTEGER, width: INTEGER)` `area ← length * width` `RETURN area` `ENDFUNCTION` // Output the value returned from the function `OUTPUT(calculate_area(5,3))`
Python
`def squared(number):` `squared = number^2` `return squared` // Output the value returned from the function `print(squared(4))`

Examples

A Python program using a function to calculate area and return the result
Two options for main program are shown, one which outputs the result (# 1) and one which stores the result so that it can be used at a later time (# 2)

Functions
# Function definition, length and width are parameters `def area(length, width):` `area = length * width` # Calculate area `return area` # Return area # Main program #1 `length = int(input("Enter the length: "))` `width = int(input("Enter the width: "))` `print(area(length, width))` # Main program #2 `length = int(input("Enter the length: ")) length` `width = int(input("Enter the width: "))` `area = area(length, width)` # Stores the result of the function in a variable `print("The area is " + str(area) + " cm^2")`

Functions

# Function definition, length and width are parameters

def area(length, width):
area = length * width # Calculate area
return area # Return area

# Main program #1
length = int(input("Enter the length: "))
width = int(input("Enter the width: "))
print(area(length, width))

# Main program #2
length = int(input("Enter the length: ")) length
width = int(input("Enter the width: "))
area = area(length, width) # Stores the result of the function in a variable
print("The area is " + str(area) + " cm^2")

Worked Example

An economy-class airline ticket costs £199. A first-class airline ticket costs £595.

(A) Create a function, flightCost(), that takes the number of passengers and the type of ticket as parameters, calculates and returns the price to pay.

You do not have to validate these parameters

You must use :

a high-level programming language that you have studied [4]

(B) Write program code, that uses flightCost(), to output the price of 3 passengers flying economy.

You must use :

a high-level programming language that you have studied [3]

How do I answer this question?

(A)

Define the function, what parameters are needed? where do they go?
How do you calculate the price?
Return the result

(B)

How do you call a function?
What parameters does the function need to return the result?

Answers

Part

Python

def flightCost(passengers, type):

if type == "economy":

cost = 199 * passengers

elif type == "first":

cost = 595 * passengers

return cost

print(flightCost("economy", 3)

x = flightCost("economy", 3)

print(x)

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Procedures & Functions (Cambridge (CIE) O Level Computer Science)

Revision Note

What are functions and procedures?

What's the difference between a function and procedure?

Procedures

Examples

Functions

Examples

Worked Example

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Author: Robert Hampton