9.2 Algorithms - A-Level Computer Science

Learning Objectives

By the end of this lesson, you will be able to:

Write pseudocode using the three basic constructs of sequence, selection and iteration
Document simple algorithms using pseudocode
Understand and use IF...THEN...ELSE...ENDIF selection statements
Understand and use CASE...OF...OTHERWISE...ENDCASE selection statements
Create nested IF statements for complex decision making
Use appropriate data types (Boolean, Char, String, Real, Integer, Date) in algorithms
Apply selection statements to solve real-world problems

Key Terms

Selection

Testing a condition which determines the sequence of execution in an algorithm

Conditional Statement

A statement that performs different actions based on whether a condition is true or false

IF...THEN...ELSE...ENDIF

A selection statement that executes one block of code if condition is true, another if false

CASE...OF...OTHERWISE...ENDCASE

A selection statement that chooses between multiple values/options

Nested IF Statement

An IF statement that contains another IF statement within it

Pseudocode

A simplified, readable way to represent an algorithm without using actual programming syntax

Boolean

A data type that can only have two values: TRUE or FALSE

Integer

A data type for whole numbers without decimal points

Real

A data type for numbers with decimal points

Char

A data type for single characters (letters, digits, symbols)

String

A data type for sequences of characters (text)

Date

A data type for storing dates and times

Selection Statements

Selection is the term used for testing a condition which determines the sequence of execution. When different actions are performed by an algorithm according to values of variables, conditional statements can be used to decide which action should be taken.

Types of Conditional Statements

IF...THEN...ELSE...ENDIF

For an IF condition, the THEN path is followed if the condition is true and the ELSE path is followed if the condition is false. There may or may not be an ELSE path. The end of the statement is shown by ENDIF.

Example 1: Checking Weight

DECLARE weight : Real

INPUT "Enter your weight", weight

IF weight < 20 THEN

OUTPUT "You are underweight"

ENDIF

Example 2: Age Classification

DECLARE age : Integer

INPUT "Enter your age in years", age

IF age < 18 THEN

OUTPUT "You are a Child"

ELSE

OUTPUT "You are an Adult"

ENDIF

CASE...OF...OTHERWISE...ENDCASE

When there are too many available routes in an algorithm, it requires too many IF...THEN...ELSE...ENDIF statements to make a selection among these routes, which is not an easy task and makes the algorithm difficult to manage. To overcome this issue, the CASE...OF...OTHERWISE...ENDCASE statement is used.

CASE Statement Structure

CASE variable OF

1: Instructions

2: Instructions

OTHERWISE

Instructions

ENDCASE

When to Use Each:

Use IF for true/false conditions, use CASE for multiple specific values/choices.

Visualizing Selection with Flowcharts

IF Statement Flowchart

START

INPUT weight

weight < 20?

TRUE

OUTPUT "Underweight"

FALSE

(Continue Program)

END

How it works: The diamond represents a decision point. Based on whether the condition (weight < 20) is TRUE or FALSE, the program follows different paths. This visual representation helps you understand how selection statements control program flow.

Real-Life Example: Number Guessing Game

A number-guessing game follows different steps depending on certain conditions:

Player inputs a number to guess the secret number stored
If guess was correct, output a congratulations message
If number input was larger than secret number, output message "secret number is smaller"
If number input was smaller than secret number, output message "secret number is greater"

DECLARE SecretNum, Guess : Integer

SecretNum ← 30 // SET a value for secret number

INPUT "Enter your guess number", Guess

IF Guess = SecretNum THEN

OUTPUT "Well done. You have guessed the secret number"

ELSE

IF Guess > SecretNum THEN

OUTPUT "secret number is smaller"

ELSE

OUTPUT "secret number is greater"

ENDIF

This is a nested IF statement - an IF statement inside another IF statement, shown clearly by the use of a second level of indentation.

Activity 1: Writing IF Statements

Write pseudocode for the following problems:

Take three numbers as input and output the largest number among them
Take a mark as input (0-100) and print "Pass" if marks are greater than or equal to 50, and "Fail" if they are not
Take a number as input and tell whether the number is even, odd, or zero

Solution:

1. Largest of Three Numbers:

DECLARE num1, num2, num3 : Integer

OUTPUT "Enter Three numbers"

INPUT num1, num2, num3

IF num1 > num2 AND num1 > num3 THEN

OUTPUT "The largest Number is", num1

ELSEIF num2 > num1 AND num2 > num3 THEN

OUTPUT "The largest Number is", num2

ELSE

OUTPUT "The largest Number is", num3

ENDIF

2. Pass/Fail Check:

OUTPUT "Please enter a mark "

INPUT PercentageMark

IF PercentageMark < 0 OR PercentageMark > 100 THEN

OUTPUT "Invalid Mark"

ELSE

IF PercentageMark > 49 THEN

OUTPUT "Pass"

ELSE

OUTPUT "Fail"

ENDIF

A rejected percentage mark must be either less than zero or greater than 100. This is a nested IF statement.

3. Even/Odd/Zero Check:

DECLARE num : Integer

INPUT "Enter a number", num

IF num % 2 = 0 THEN

OUTPUT "The number is even"

ELSEIF num % 2 = 1 THEN

OUTPUT "The number is Odd"

ELSE

OUTPUT "The number is zero"

ENDIF

Check Your Understanding: IF Statements

1. What is selection in programming? [2 marks]

Answer

[1 mark] Testing a condition which determines the sequence of execution
[1 mark] When different actions are performed by an algorithm according to values of variables
[Additional] Also called conditional or decision statements

2. When does the ELSE path execute in an IF statement? [1 mark]

Answer

[1 mark] The ELSE path executes when the IF condition is FALSE
[Additional] If there is no ELSE path and condition is false, the program continues after ENDIF

3. What is a nested IF statement? [2 marks]

Answer

[1 mark] An IF statement that contains another IF statement within it
[1 mark] Used for more complex decision making with multiple conditions
[Additional] Shown by indentation levels in pseudocode

4. Write pseudocode to check if a student's grade is A, B, C, or F. [3 marks]

Answer

DECLARE grade : Char

INPUT "Enter grade", grade

IF grade = 'A' THEN

OUTPUT "Excellent"

ELSEIF grade = 'B' THEN

OUTPUT "Well done"

ELSEIF grade = 'C' THEN

OUTPUT "You passed"

ELSEIF grade = 'F' THEN

OUTPUT "Better try again"

ELSE

OUTPUT "Invalid grade"

ENDIF

Note: This could also be done more efficiently with a CASE statement.

5. What is the purpose of ENDIF? [1 mark]

Answer

[1 mark] ENDIF marks the end of an IF...THEN...ELSE statement
[Additional] It shows where the conditional block ends and normal program execution continues

CASE Statements

When there are too many available routes in an algorithm then it requires too many IF...THEN...ELSE...ENDIF statements to make a selection among these routes which is not an easy task and it makes the algorithm difficult to manage. To overcome this issue, CASE...OF...OTHERWISE...ENDCASE statements are used.

CASE Statement Structure

Basic Syntax

CASE variable OF

value1: Instructions

value2: Instructions

OTHERWISE

Instructions

ENDCASE

Example: Grade Evaluation

OUTPUT "Enter Your Computer Science Grade"

INPUT grade

CASE grade OF

'A':

OUTPUT "Excellent!"

'B':

OUTPUT "Well done!"

'C':

OUTPUT "You passed!"

'F':

OUTPUT "Better try again"

OTHERWISE

OUTPUT "Invalid grade"

ENDCASE

Calculator Example

Simple Calculator Program

DECLARE Num1, Num2 : Integer

DECLARE Choice : Char

DECLARE Answer : Float

OUTPUT "Enter two Numbers"

INPUT Num1, Num2

OUTPUT "Enter 1 For addition, 2 For Subtraction, 3 For Multiplication, 4 For Division"

INPUT Choice

CASE Choice OF

'1': Answer ← Num1 + Num2

'2': Answer ← Num1 - Num2

'3': Answer ← Num1 * Num2

'4': Answer ← Num1 / Num2

OTHERWISE OUTPUT "Please enter a valid choice"

ENDCASE

OUTPUT "Your result is ", Answer

Note:

CASE statements are cleaner and more readable than multiple IF statements when dealing with multiple specific values.

CASE Statement Simulator

Step through this grade evaluation program to see how a CASE statement works:

CASE grade OF

'A': OUTPUT "Excellent!"

'B': OUTPUT "Well done!"

'C': OUTPUT "You passed!"

'F': OUTPUT "Better try again"

OTHERWISE: OUTPUT "Invalid grade"

ENDCASE

Input grade value

Select a grade to test:

CASE statement evaluation

The program compares the grade with each CASE value

grade = "A"

Execute matching CASE

Program jumps to the matching CASE and executes its instructions

Output: "Excellent!"

Continue after ENDCASE

Program continues with next statement after ENDCASE

Next statement would execute here...

How CASE works: The program compares the variable value with each CASE value. When it finds a match, it executes that CASE's instructions, then jumps to ENDCASE. If no match is found, it executes the OTHERWISE block.

Important: Understanding Data Types

Data types specify the kind of data that can be stored in a variable. Different data types are used for different purposes in algorithms:

Integer

Whole numbers without decimals

Example: 42, -7, 0

Real

Numbers with decimal points

Example: 3.14, -2.5, 0.0

Boolean

True or False values

Example: TRUE, FALSE

Char

Single character

Example: 'A', '7', '$'

String

Sequence of characters

Example: "Hello", "A123"

Date

Date and time values

Example: 2024-12-25

Choosing the Right Data Type

Always choose the most appropriate data type for your variables:

Use Integer for whole numbers (age, count, score)
Use Real for measurements (weight, temperature, average)
Use Boolean for true/false conditions (isPassed, isLoggedIn)
Use Char for single letters (grade, menu choice)
Use String for text (name, address, message)
Use Date for dates and times (birthday, appointment)

Activity 2: CASE Statement Practice

Solve the following problems using CASE statements:

Write pseudocode that takes three numbers, then asks the user to enter "S" to find the smallest number among them and "L" to find the largest number
Write a program that asks the user to enter two numbers and then perform basic arithmetic operations on them based on user choice
Write a program to check whether a triangle is equilateral, isosceles, or scalene based on three side lengths

Solution:

1. Smallest or Largest Finder:

DECLARE Num1, Num2, Num3 : Integer

DECLARE UserChoice : Char

OUTPUT "Enter three Numbers"

INPUT Num1, Num2, Num3

OUTPUT "Enter 'S' to find smallest number and enter 'L' to find largest number"

INPUT UserChoice

CASE UserChoice OF

'S':

IF num1 < num2 AND num1 < num3 THEN

OUTPUT "Smallest Number is", num1

ELSEIF num2 < num1 AND num2 < num3 THEN

OUTPUT "Smallest Number is", num2

ELSE

OUTPUT "Smallest Number is", num3

ENDIF

'L':

IF num1 > num2 AND num1 > num3 THEN

OUTPUT "The largest Number is", num1

ELSEIF num2 > num1 AND num2 > num3 THEN

OUTPUT "The largest Number is", num2

ELSE

OUTPUT "The largest Number is", num3

ENDIF

OTHERWISE: OUTPUT "Invalid Input"

ENDCASE

2. Calculator Program:

(Already shown in the CASE example above)

3. Triangle Type Checker:

DECLARE side1, side2, side3 : Real

OUTPUT "Input length of three sides"

INPUT side1, side2, side3

IF (side1 = side2) AND (side2 = side3) THEN

OUTPUT "Equilateral triangle"

ELSEIF (side1 = side2) OR (side2 = side3) OR (side1 = side3) THEN

OUTPUT "Isosceles triangle"

ELSE

OUTPUT "Scalene Triangle"

ENDIF

Check Your Understanding: CASE Statements

1. When should you use a CASE statement instead of multiple IF statements? [2 marks]

Answer

[1 mark] When there are multiple specific values to check against a single variable
[1 mark] When the algorithm becomes difficult to manage with too many IF statements
[Additional] CASE statements are cleaner, more readable, and more efficient for multiple specific value checks

2. What does the OTHERWISE clause do in a CASE statement? [2 marks]

Answer

[1 mark] Executes when none of the CASE values match the variable
[1 mark] Acts as a default or fallback option
[Additional] Similar to ELSE in an IF statement, but for CASE statements

3. Write a CASE statement that outputs days of the week based on numbers 1-7. [3 marks]

Answer

DECLARE dayNumber : Integer

INPUT dayNumber

CASE dayNumber OF

1: OUTPUT "Monday"

2: OUTPUT "Tuesday"

3: OUTPUT "Wednesday"

4: OUTPUT "Thursday"

5: OUTPUT "Friday"

6: OUTPUT "Saturday"

7: OUTPUT "Sunday"

OTHERWISE: OUTPUT "Invalid day number"

ENDCASE

4. What happens after a matching CASE is executed? [2 marks]

Answer

[1 mark] The program executes the instructions for that CASE
[1 mark] Then it jumps to the statement after ENDCASE (doesn't check other cases)
[Additional] This is called "fall-through" - once a match is found, only that CASE executes

5. List six common data types used in pseudocode. [3 marks]

Answer

[0.5 mark each] Boolean, Char, String, Real, Integer, Date
[Additional] These data types are used to specify the type of data that is stored in computer memory

Practice Problems

These practice problems combine both IF and CASE statements to solve real-world scenarios. Try to solve them on your own before checking the solutions.

Problem

Problem 1: Even/Odd/Zero Check

Write a program that will ask the user to input a number and print whether the number is even, odd, or zero.

Solution:

DECLARE num : integer

OUTPUT "enter your number "

INPUT num

IF num MOD 2 = 0 THEN

OUTPUT "Number is even"

ELSEIF num MOD 2 <> 0 THEN

OUTPUT "Number is odd"

ELSE OUTPUT "number is zero"

ENDIF

Note:

MOD gives the remainder after division. If num MOD 2 = 0, the number is even.

Problem 2: Smallest/Largest Finder

Write pseudocode that takes three numbers, then asks the user to enter "S" to find the smallest number among them and "L" to find the largest number.

Solution:

DECLARE Num1, Num2, Num3 : Integer

DECLARE UserChoice : Char

OUTPUT "Enter three Numbers"

INPUT Num1, Num2, Num3

OUTPUT "Enter 'S' to find smallest number and 'L' to find largest number"

INPUT UserChoice

CASE UserChoice OF

'S': // Find smallest logic

'L': // Find largest logic

OTHERWISE: OUTPUT "Invalid Input"

ENDCASE

Interactive Problem Solver: Triangle Classifier

Enter three side lengths to classify the triangle. The program will determine if it's equilateral, isosceles, or scalene.

Side 1:

Side 2:

Side 3:

Result will appear here

Enter values and click "Classify Triangle"

Algorithm Logic:

IF (side1 = side2) AND (side2 = side3) THEN

OUTPUT "Equilateral triangle"

ELSEIF (side1 = side2 OR side2 = side3 OR side1 = side3) THEN

OUTPUT "Isosceles triangle"

ELSE

OUTPUT "Scalene Triangle"

ENDIF

Real-Life Example: Mobile App Menu System

Mobile apps use selection statements extensively for menu systems and user interactions:

// Mobile banking app menu

DECLARE userChoice : Char

DECLARE balance : Real

balance ← 1000.00

OUTPUT "Banking Menu:"

OUTPUT "1. Check Balance"

OUTPUT "2. Deposit Money"

OUTPUT "3. Withdraw Money"

OUTPUT "4. Exit"

INPUT userChoice

CASE userChoice OF

'1': OUTPUT "Your balance is: $", balance

'2': // Deposit logic

'3': // Withdrawal logic

'4': OUTPUT "Thank you for banking with us!"

OTHERWISE: OUTPUT "Invalid choice"

ENDCASE

This shows how CASE statements are perfect for menu-driven programs where users select from multiple options.

Home Task

Take three numbers as input and output the smallest number.

Write pseudocode that takes three numbers as input and outputs the smallest number among them. This is similar to the "largest number" example but you need to find the smallest instead.

Hint:

Modify the "largest number" algorithm by changing the comparison operators from > (greater than) to < (less than). The logic structure remains the same.

Check Your Understanding: Mixed Problems

1. What is the difference between IF and CASE statements? [3 marks]

Answer

[1 mark] IF tests true/false conditions; CASE tests for specific values
[1 mark] IF can test complex conditions with AND/OR; CASE tests single variable against multiple values
[1 mark] CASE is cleaner for multiple specific value checks; IF is better for range checks and complex conditions
[Additional] IF uses THEN/ELSE/ENDIF; CASE uses OF/OTHERWISE/ENDCASE

2. Write pseudocode to calculate a student's grade based on marks. [4 marks]
A: 90-100, B: 80-89, C: 70-79, D: 60-69, F: Below 60

Answer

DECLARE marks : Integer

INPUT "Enter marks (0-100)", marks

IF marks >= 90 AND marks <= 100 THEN

OUTPUT "Grade: A"

ELSEIF marks >= 80 AND marks <= 89 THEN

OUTPUT "Grade: B"

ELSEIF marks >= 70 AND marks <= 79 THEN

OUTPUT "Grade: C"

ELSEIF marks >= 60 AND marks <= 69 THEN

OUTPUT "Grade: D"

ELSE

OUTPUT "Grade: F"

ENDIF

3. When would you use nested IF statements? [2 marks]

Answer

[1 mark] When you need to make decisions within decisions
[1 mark] For complex conditions that depend on multiple factors
[Additional] Example: First check if a student passed (marks >= 50), then if passed, check if they got distinction (marks >= 80)

4. What is the MOD operator and how is it used? [2 marks]

Answer

[1 mark] MOD gives the remainder after integer division
[1 mark] Used to check if a number is even (num MOD 2 = 0) or odd (num MOD 2 = 1)
[Additional] Example: 7 MOD 2 = 1, 10 MOD 3 = 1, 8 MOD 4 = 0

5. Write pseudocode for a traffic light system. [3 marks]
Red: Stop, Yellow: Get ready, Green: Go

Answer

DECLARE lightColor : Char

INPUT "Enter traffic light color (R/Y/G)", lightColor

CASE lightColor OF

'R': OUTPUT "STOP"

'Y': OUTPUT "GET READY"

'G': OUTPUT "GO"

OTHERWISE: OUTPUT "Invalid color"

ENDCASE

This could also be written using IF statements, but CASE is cleaner for specific values.

Key Takeaways

Selection is testing a condition to determine the sequence of execution in an algorithm
IF...THEN...ELSE...ENDIF is used for true/false conditions - THEN path if true, ELSE if false
CASE...OF...OTHERWISE...ENDCASE is used for multiple specific value checks - cleaner than multiple IFs
Nested IF statements are IF statements inside other IF statements, used for complex decision making
Indentation is crucial in pseudocode to show structure and nesting levels
Data types (Boolean, Char, String, Real, Integer, Date) specify what kind of data a variable can store
MOD operator gives the remainder after division, used to check even/odd numbers
Use IF for range checks and complex conditions with AND/OR operators
Use CASE for menu systems and when checking a variable against multiple specific values
Always validate input to ensure it's within expected ranges before processing
Real-world applications include: grading systems, calculators, traffic lights, menu systems, game logic
Practice is essential - write algorithms for different scenarios to master selection statements

Question Bank

1. Explain the purpose of selection statements in algorithms. Provide two examples. [4 marks]

Marking Scheme & Answer

[2 marks] Purpose: Selection statements test conditions to determine which instructions to execute. They allow algorithms to make decisions and follow different paths based on variable values or user input.
[1 mark] Example 1: Checking if a student passed an exam (IF marks >= 50 THEN OUTPUT "Pass" ELSE OUTPUT "Fail")
[1 mark] Example 2: A calculator program choosing operation based on user input (CASE choice OF '1': addition, '2': subtraction, etc.)
[Additional] Selection is one of the three basic programming constructs along with sequence and iteration.

2. Write pseudocode for a program that calculates electricity bill based on units consumed. [6 marks]
First 100 units: $0.50 per unit
Next 200 units: $0.75 per unit
Above 300 units: $1.00 per unit

Marking Scheme & Answer

DECLARE units, bill : Real

INPUT "Enter units consumed", units

IF units <= 100 THEN

bill ← units * 0.50

ELSEIF units <= 300 THEN

bill ← (100 * 0.50) + ((units - 100) * 0.75)

ELSE

bill ← (100 * 0.50) + (200 * 0.75) + ((units - 300) * 1.00)

ENDIF

OUTPUT "Total bill: $", bill

Mark allocation: [1] Variable declaration, [1] Input, [1] First condition, [1] Second condition, [1] Else condition, [1] Output result

3. Compare and contrast IF statements and CASE statements. [5 marks]

Marking Scheme & Answer

IF Statements	CASE Statements
Tests true/false conditions	Tests for specific values
Uses THEN/ELSE/ENDIF	Uses OF/OTHERWISE/ENDCASE
Can test complex conditions with AND/OR	Tests single variable against multiple values
Better for range checks (e.g., 0-100)	Better for exact value checks (e.g., 'A', 'B', 'C')
Can be nested for complex decisions	Cleaner for multiple specific options

Key insight: Use IF when testing conditions (true/false), use CASE when choosing between multiple specific options.

4. Write pseudocode for a simple login system. [5 marks]
Check username and password. If both correct: "Access granted".
If username correct but password wrong: "Wrong password".
If username doesn't exist: "User not found".

Marking Scheme & Answer

DECLARE storedUser, storedPass, inputUser, inputPass : String

storedUser ← "admin"

storedPass ← "password123"

OUTPUT "Enter username:"

INPUT inputUser

OUTPUT "Enter password:"

INPUT inputPass

IF inputUser = storedUser THEN

IF inputPass = storedPass THEN

OUTPUT "Access granted"

ELSE

OUTPUT "Wrong password"

ENDIF

ELSE

OUTPUT "User not found"

ENDIF

Note: This uses nested IF statements to first check username, then check password if username is correct.

5. What are the advantages of using pseudocode over actual programming code when designing algorithms? [4 marks]

Marking Scheme & Answer

[1 mark] Language independent - can be understood by programmers who know different languages
[1 mark] Focuses on logic rather than syntax - no need to worry about semicolons, brackets, etc.
[1 mark] Easy to read and understand - uses plain English with some structured keywords
[1 mark] Quick to write and modify - easy to test algorithms without writing full code
[Additional] Helps in planning before coding, reduces errors, facilitates communication between team members

6. Write pseudocode for a vending machine that sells drinks. [6 marks]
Options: 1-Coke ($1), 2-Pepsi ($1), 3-Water ($0.5), 4-Juice ($1.5)
Ask user to select drink and insert money. Calculate change or ask for more money.

Marking Scheme & Answer

DECLARE choice : Integer

DECLARE price, money, change : Real

OUTPUT "Select drink: 1-Coke($1), 2-Pepsi($1), 3-Water($0.5), 4-Juice($1.5)"

INPUT choice

CASE choice OF

1: price ← 1.00

2: price ← 1.00

3: price ← 0.50

4: price ← 1.50

OTHERWISE: OUTPUT "Invalid choice"

ENDCASE

OUTPUT "Price is $", price, ". Insert money:"

INPUT money

IF money >= price THEN

change ← money - price

OUTPUT "Dispensing drink. Change: $", change

ELSE

OUTPUT "Insufficient money. Need $", price - money, " more."

ENDIF

7. Explain what nested IF statements are and provide an example. [4 marks]

Marking Scheme & Answer

[2 marks] Definition: Nested IF statements are IF statements that contain other IF statements within them. They are used when decisions depend on multiple conditions in a hierarchical manner.
[2 marks] Example:

IF age >= 18 THEN

    IF hasLicense = TRUE THEN

        OUTPUT "Can drive"

    ELSE

        OUTPUT "Needs license"

    ENDIF

ELSE

    OUTPUT "Too young to drive"

ENDIF

8. Write pseudocode to find the roots of a quadratic equation. [6 marks]
ax² + bx + c = 0. Calculate discriminant D = b² - 4ac.
If D > 0: two real roots, D = 0: one real root, D < 0: no real roots.

Marking Scheme & Answer

DECLARE a, b, c, D, root1, root2 : Real

OUTPUT "Enter coefficients a, b, c:"

INPUT a, b, c

D ← (b * b) - (4 * a * c)

IF D > 0 THEN

root1 ← (-b + SQRT(D)) / (2 * a)

root2 ← (-b - SQRT(D)) / (2 * a)

OUTPUT "Two real roots: ", root1, " and ", root2

ELSEIF D = 0 THEN

root1 ← -b / (2 * a)

OUTPUT "One real root: ", root1

ELSE

OUTPUT "No real roots (complex roots)"

ENDIF

9. What is the purpose of the OTHERWISE clause in a CASE statement? [2 marks]

Marking Scheme & Answer

[1 mark] The OTHERWISE clause provides a default action to execute when none of the CASE values match the variable being tested
[1 mark] It handles unexpected or invalid input values gracefully
[Additional] Similar to the ELSE clause in an IF statement, but specifically for CASE statements when no matching case is found

10. Design an algorithm for a simple ATM machine. [8 marks]
Options: 1-Check Balance, 2-Withdraw, 3-Deposit, 4-Exit
Include validation for insufficient funds during withdrawal.

Marking Scheme & Answer

DECLARE balance, amount : Real

DECLARE choice : Integer

balance ← 1000.00 // Initial balance

REPEAT

OUTPUT "ATM Menu: 1-Check Balance, 2-Withdraw, 3-Deposit, 4-Exit"

INPUT choice

CASE choice OF

1: OUTPUT "Current balance: $", balance

OUTPUT "Enter amount to withdraw:"

INPUT amount

IF amount <= balance THEN

balance ← balance - amount

OUTPUT "Withdrawal successful. New balance: $", balance

ELSE

OUTPUT "Insufficient funds"

ENDIF

OUTPUT "Enter amount to deposit:"

INPUT amount

balance ← balance + amount

OUTPUT "Deposit successful. New balance: $", balance

4: OUTPUT "Thank you for using our ATM"

OTHERWISE: OUTPUT "Invalid choice"

ENDCASE

UNTIL choice = 4

Note: This uses a REPEAT...UNTIL loop (iteration) to keep showing the menu until user chooses to exit. This combines selection with iteration.