Using Subroutines (AQA GCSE Computer Science)

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Written by: James Woodhouse, Computer Science

Reviewed by: Lucy Kirkham, Head of STEM

Using Subroutines

Why do we use subroutines?

Subroutines are used to be able to decompose a big problem into smaller parts to make it easier to solve
Subroutines are split into 2 types of subprogram
- Functions
- Procedures
Functions and procedures are a type of sub-program, a sequence of instructions that perform a specific task or set of tasks
Subprograms are often used to simplify a program by breaking it into smaller, more manageable parts
The advantages of using subprograms are:
- 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 def taxCalculator(pay,taxcode)
Sub programs can have multiple parameters
To use a sub program you 'call' it from the main program

Functions and Procedures

What's the difference between a function and procedure?

A Function returns a value whereas a procedure does not

Concept	Pseudocode	Python
Creating a function	`SUBROUTINE squared(number)` `squared ← number^2` `RETURN squared` `ENDSUBROUTINE`	`def squared(number):` `squared = number^2` `return squared`
Calling a function	`SquNum ← squared(4)` `print(SquNum)` OR `OUTPUT(SquNum(4))`	`SquNum = squared(4)` `print(SquNum)` OR `print(SquNum(4))`
Creating a procedure	`SUBROUTINE ageCheck(age)` `IF age > 18 THEN` `OUTPUT("You are old enough")` `ELSE` `OUTPUT("You are too young")` `ENDIF` `ENDSUBROUTINE`	`def ageCheck(age):` `if age > 18:` `print("You are old enough")` `else:` `print("You are too young")`
Calling a procedure	`ageCheck(21)`	`ageCheck(21)`

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

Functions

def area(length, width): # Function definition, length and width are parameters
area = length * width # Calculate area
return area # Return area

# Main program #1
length = int(input("Enter the length: ")) # Asks the user to enter the length
width = int(input("Enter the width: ")) # Asks the user to enter the width
print(area(length, width)) # Outputs the result of the function

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

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
`def main_menu(): # Function definition` `print("1. Addition") # Outputs the option` `print("2. Subtraction")` `print("3. Multiplication")` `print("4. Division")` `print("5. Exit")` `choice = int(input("Enter your choice: ")) # Asks the user to enter their choice` `if choice == 1: # If the user chooses 1` `addition() # Calls the addition function` `elif choice == 2:` `subtraction()` `elif choice == 3:` `multiplication()` `elif choice == 4:` `division()` `elif choice == 5:` `exit()` `def addition(): # Function definition` `num1 = int(input("Enter the first number: ")) # Asks the user to enter the first number` `num2 = int(input("Enter the second number: ")) # Asks the user to enter the second number` `print(num1 + num2) # Outputs the result of the addition` `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 function`

Procedures

def main_menu(): # Function definition
print("1. Addition") # Outputs the option
print("2. Subtraction")
print("3. Multiplication")
print("4. Division")
print("5. Exit")
choice = int(input("Enter your choice: ")) # Asks the user to enter their choice
if choice == 1: # If the user chooses 1
addition() # Calls the addition function
elif choice == 2:
subtraction()
elif choice == 3:
multiplication()
elif choice == 4:
division()
elif choice == 5:
exit()

def addition(): # Function definition
num1 = int(input("Enter the first number: ")) # Asks the user to enter the first number
num2 = int(input("Enter the second number: ")) # Asks the user to enter the second number
print(num1 + num2) # Outputs the result of the addition

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 function

What is a local variable?

A local variable is a variable declared within a specific scope, such as a function or a code block
Local variables are accessible only within the block in which they are defined, and their lifetime is limited to that particular block
Once the execution of the block ends, the local variable is destroyed, and its memory is released

Python example

In this python code, you can see that the localVariable (with the value 10) is declared inside of the function printValue. This means that only this function can access and change the value in the local variable. It cannot be accessed by other modules in the program.

Local variables
`def printValue():` `localVariable = 10 # Defines a local variable inside the function` `print("The value of the local variable is:", localVariable)` `printValue() # Call the function`

Local variables

def printValue():
localVariable = 10 # Defines a local variable inside the function
print("The value of the local variable is:", localVariable)

printValue() # Call the function

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 either:

Pseudocode, or
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 either:

Pseudocode, or
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	Pseudocode	Python
A	`SUBROUTINE flightCost(passengers, type)` `IF type == "economy" then` `cost ← 199 * passengers` `ELSE IF type == "first" then` `cost ← 595 * passengers` `ENDIF` `RETURN cost` `ENDSUBROUTINE`	`def flightCost(passengers, type):` `if type == "economy":` `cost = 199 * passengers` `elif type == "first":` `cost = 595 * passengers` `return cost`
B	`OUTPUT(flightCost("economy", 3)` OR `x ← flightCost("economy", 3)` `OUTPUT(x)`	`print(flightCost("economy", 3)` OR `x = flightCost("economy", 3)` `print(x)`

Part

Pseudocode

Python

SUBROUTINE flightCost(passengers, type)

IF type == "economy" then

cost ← 199 * passengers

ELSE IF type == "first" then

cost ← 595 * passengers

ENDIF

RETURN cost

ENDSUBROUTINE

def flightCost(passengers, type):

if type == "economy":

cost = 199 * passengers

elif type == "first":

cost = 595 * passengers

return cost

OUTPUT(flightCost("economy", 3)

x ← flightCost("economy", 3)

OUTPUT(x)

print(flightCost("economy", 3)

x = flightCost("economy", 3)

print(x)

Last updated: 13 August 2024

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