Lab 09: Python Selections

Pull and Update in VS Code

Before starting any lab, you need to make sure that the repo in your GitHub is the latest one. Sync the repo if the upstream repo have been updated.

Once the online repo is in-sync, bring those changes down to your PC by clicking Source Control and then ... beside Changes and click Pull.

Selection

Launch VS Code and open the exercise.py file in /labs/lab09/. We'll start with a simple question and evolve it into a more complex grading system.

Basic Problem

A student has scored 85 marks out of 100 in their Python Programming exam. Calculate and display their percentage.

Copy this code into your exercise.py file:

# Stage 1: Basic grade calculation
marks = 85
total_marks = 100

percentage = (marks / total_marks) * 100
print(f"Student scored: {percentage}%")

Run this code. It works perfectly for this specific case. But what if we want to tell the student whether they passed or failed?

TIP

This is where we realize we need decision-making in our programs. Our code can calculate, but it can't make decisions based on the results.

Adding Decision Making

The university wants to automatically tell students if they passed. The passing grade is 60%. Let's improve our program to make this decision.

In Python, the keyword for selection is if. You can see the example of usage below.

if conditions:
    # Do things when the conditions is True

Lets implement this techniques in our previous code. Add this improved version to your exercise.py file:

# Adding basic decision making
marks = 85
total_marks = 100

percentage = (marks / total_marks) * 100
print(f"Student scored: {percentage}%")

# conditional statement
if percentage >= 60:
    print("Congratulations! You passed!")

Run this code. Now it tells students when they pass. But wait, what happens when a student fails? Our program stays silent about failure.

Try changing marks = 85 to marks = 45 and run it again. Do you see the problem?

PROBLEM

Our program only handles the success case. Students who fail get no feedback at all. This is clearly incomplete.

Handling Both Cases

We need to handle both passing and failing students. Python handles this case using the else statement. The usage of it is shown below.

if conditions:
    # Do things when the conditions is True
else:
    # Do things when the conditions is False

Update your code to include the else statement:

# Stage 3: Complete pass/fail system
marks = 45  # Try both passing and failing grades
total_marks = 100

percentage = (marks / total_marks) * 100
print(f"Student scored: {percentage}%")

# Complete conditional with else
if percentage >= 60:
    print("Congratulations! You passed!")
else:
    print("Sorry, you failed. Better luck next time!")

Great, now our program handles both cases. Test it with different values: marks = 85 (should pass) and marks = 45 (should fail).

But now, there is a new requirement: "We don't just need pass/fail. We need to assign letter grades: A, B, C, D, and F."

Try adding this with just if and else:

# This doesn't work well for multiple grades
if percentage >= 90:
    print("Grade: A")
else:
    print("Grade: ?")  # What about B, C, D, F?

LIMITATION

With only if and else, we can only handle two cases. But letter grades require multiple categories!

Multiple Selection

Python provides elif (else-if) to handle multiple conditions cleanly. The usage is as below.

if conditions:
    # Do things when the conditions is True
elif conditions :
    # Do things when conditions is True
elif conditions :
    # Do things when conditions is True
else:
    # Do things when everything else is False

Now lets improve our previous code using the elif statement:

# multiple conditions with elif
marks = 87
total_marks = 100
percentage = (marks / total_marks) * 100

print(f"Student scored: {percentage}%")

if percentage >= 90:
    print("Grade: A - Excellent!")
elif percentage >= 80:
    print("Grade: B - Good!")
elif percentage >= 70:
    print("Grade: C - Satisfactory!")
elif percentage >= 60:
    print("Grade: D - Pass!")
else:
    print("Grade: F - Fail!")

Much cleaner! Test this with different values: marks = 95, marks = 82, marks = 73, marks = 65, marks = 45.

HOW ELIF WORKS

Python checks each condition in order. Once it finds a True condition, it executes that block and skips the rest. This is why we can use >= 80 instead of >= 80 and < 90.

More Comparison Operators

The university now wants special handling for perfect scores and failing grades. Let's explore different comparison operators

Operator	Description	Example
==	Equal to	x == y
!=	Not equal to	x != y
>	Greater than	x > y
<	Less than	x < y
>=	Greater than or equal	x >= y
<=	Less than or equal	x <= y

Operators can only be used with 2 values at one time. For example the code below is valid:

x = 10

if x > 5:
    print("x is greater than 5")

But the code below is not valid:

x = 10

if 2< x > 5:
    print("x is between 2 and x")

Now that we have learned new types of operator, lets again improve our code.

# Different comparison operators
marks = 100  # Try different values: 100, 0, 75, 60
total_marks = 100
percentage = (marks / total_marks) * 100

print(f"Student scored: {percentage}%")

# Using different comparison operators
if percentage == 100:
    print("Perfect Score! Outstanding achievement!")
elif percentage > 90:
    print("Grade: A+ - Exceptional!")
elif percentage >= 80:
    print("Grade: A - Excellent!")
elif percentage >= 70:
    print("Grade: B - Good!")
elif percentage >= 60:
    print("Grade: C - Satisfactory!")
elif percentage > 0:
    print("Grade: F - Fail, but you tried!")
else:  # percentage == 0
    print("Grade: F - No marks scored!")

Notice how we use different operators:

• == for exact equality (perfect 100%)
• > for greater than (A+ grade)
• >= for greater than or equal (other grades)

Boolean Operator

The university introduces a new policy: "Honour students must have grades of A or B AND perfect attendance." Now we need to check multiple conditions together.

Boolean operators allows us to test multiple conditions inside on statments. There are 3 types of boolean operator.

Operator	Example
and	A and B
or	B or C
not	not B

The usage of boolean operators slightly differs from comparison operator in a way that we can use to with more that 2 values.

if condition1 and condition2:
    # Do things is both conditions are True

if condition1 or condition2:
    # Do things if either are True

if not condition:
    # Do things if condition is False

if condition1 and condition2 and condition3:
    # Do things if all are True

if condition1 or condition2 or condition3:
    # Do things if any are True

Boolean operators have strict data type usage, it can only be uses with a Boolean data type. Any value that want to use the boolean operator must be either True or False.

condition1 = True
condition2 = False

if condition1 and condition2:
    # Do things is both conditions are True

if condition1 or 1:
    # This will cause error because 1 is an integer, not a boolean

Now, again lets improve our code with boolean operators:

# Complete system with boolean logic
marks = 85
total_marks = 100
attendance_percentage = 100
extra_credit = 5  # Bonus points

percentage = (marks / total_marks) * 100
print(f"Student scored: {percentage}%")
print(f"Attendance: {attendance_percentage}%")
print(f"Extra credit: {extra_credit} points")

# Enhanced grading with boolean logic
if percentage >= 90:
    grade = "A"
elif percentage >= 80:
    grade = "B"
elif percentage >= 70:
    grade = "C"
elif percentage >= 60:
    grade = "D"
else:
    grade = "F"

print(f"Base Grade: {grade}")

# Honor Roll: (Grade A or B) AND perfect attendance
honor_roll = (grade == "A" or grade == "B") and attendance_percentage == 100
print(f"Honor Roll: {'Yes!' if honor_roll else 'No'}")

BOOLEAN OPERATORS

• and : Both conditions must be True
• or : At least one condition must be True
• not : Reverses the truth value
• Use parentheses () to group conditions clearly

Precedence Table

When we combine boolean and comparison operators together, how does python know what to evaluate first?

if percentage >= 80  or extra_credit > 0:
    print("Honor Roll")

From the example above, obviously we want to check percentage >= 80 first. But how do we know that Python will not evaluate 80 or extra_credit first? Even though we have established earlier that for a boolean operator to work, both values need to be boolean, this still does not ensure that 80 or extra_credit will not be evaluated first.

This is where the precedence table comes into picture:

Precedence	Type	Operator
1 (Highest)	Arithmetic	() (Parentheses)
2	Arithmetic	** (Exponentiation)
3	Arithmetic	*, /, //, % (Multiply, Divide, Floor Division, Modulus)
4	Arithmetic	+, - (Addition, Subtraction)
5	Comparison	<, <=, >, >=, ==, != (All comparison operators)
6	Boolean	not (Boolean NOT)
7	Boolean	and (Boolean AND)
8 (Lowest)	Boolean	or (Boolean OR)

Python evaluates expressions from highest to lowest precedence. This means:

# Example: percentage >= 80 or extra_credit > 0
# Step 1: Evaluate comparisons first (>= and >)
# Step 2: Then evaluate boolean operator (or)

if percentage >= 80 or extra_credit > 0:
    print("Honor Roll")

When in doubt, use parentheses () to make your intentions clear:

# Clear grouping with parentheses
if (percentage >= 80) or (extra_credit > 0):
    print("Honor Roll")

Selection Structure Positioning and Usage

Python provides three selection structure components: if, elif, and else. Understanding where each can be positioned and when to use each is essential for controlling program flow.

Positioning Rules for Selection Structures

Selection structures in Python follow strict positioning rules that determine how conditions are evaluated:

Component	Position Rules	Can Repeat?	Purpose
if	Always first in a selection block	No	Initial condition check
elif	Only after if or another elif	Yes (multiple allowed)	Additional condition checks
else	Always last in a selection block	No	Default action when all conditions fail

The positioning creates a decision chain where Python evaluates conditions in order from top to bottom, executing only the first True condition it encounters.

Understanding If Statement Placement

The if statement must always be positioned first in any selection structure. It establishes the beginning of the decision-making process.

Copy this code into your exercise.py file and run it:

# if statement - always at the beginning
student_grade = 85

if student_grade >= 90:
    print("Excellent performance")

What output do you see? Try changing student_grade to 95 and run it again. What happens now?

The if statement serves as the entry point for conditional logic. Python encounters the if first and evaluates whether its condition is True or False. Without an if statement, you cannot create conditional logic in Python.

Understanding Elif Statement Placement

The elif statement can only be positioned after an if statement or after another elif statement. It provides additional conditions to check when previous conditions are False.

Run this code in your exercise.py file:

# elif positioning - can have multiple elif statements
student_grade = 85

if student_grade >= 90:
    print("Excellent performance")
elif student_grade >= 80:    # After if
    print("Good performance") 
elif student_grade >= 70:    # After elif
    print("Satisfactory performance")
elif student_grade >= 60:    # After elif
    print("Minimum performance")

Which message appears? Try changing student_grade to different values like 95, 75, and 55. What happens each time?

The elif statement creates a chain of alternatives. Python only checks an elif condition if all previous conditions (if and any preceding elif statements) were False. Once any condition evaluates to True, Python executes that block and skips all remaining elif and else statements.

Understanding Else Statement Placement

The else statement can only be positioned as the final component in a selection structure. It executes when all previous conditions (if and any elif statements) evaluate to False.

Copy this code into your exercise.py file and run it:

# else positioning - always last
student_grade = 45

if student_grade >= 90:
    print("Excellent performance")
elif student_grade >= 80:
    print("Good performance")
elif student_grade >= 70:
    print("Satisfactory performance") 
elif student_grade >= 60:
    print("Minimum performance")
else:                        # Always positioned last
    print("Below minimum performance")

What message appears? Try changing student_grade to 75, then to 65. Which conditions are being checked each time?

The else statement serves as the default action. It provides a guaranteed execution path when none of the conditional statements above it are True. The else statement does not have a condition because it represents "everything else" - all cases not handled by the if and elif conditions.

When to Use Multiple If vs If-Elif-Else

Understanding when to use multiple independent if statements versus connected if-elif-else chains is crucial for correct program logic.

Use multiple independent if statements when:

• Multiple conditions can be True simultaneously
• Each condition represents a separate, independent check
• You want all applicable conditions to execute their code blocks

Use if-elif-else chains when:

• Only one condition should execute its code block
• Conditions represent mutually exclusive choices
• You want Python to stop checking once it finds the first True condition

Practical Example: Multiple Independent Checks

Consider a health monitoring system that provides multiple warnings. Each health recommendation is independent - a person might need multiple recommendations simultaneously:

Copy this code into your exercise.py file and run it:

# Multiple independent health checks - use multiple if statements
weight = 70  # kg
height = 1.75  # meters
age = 25
exercise_hours = 2  # per week

bmi = weight / (height * height)

# Each check is independent - multiple can be True
if bmi >= 25:
    print("Recommendation: Consider weight management")
    
if age >= 40:
    print("Recommendation: Schedule regular health checkups")
    
if exercise_hours < 3:
    print("Recommendation: Increase physical activity")
    
if bmi < 18.5:
    print("Recommendation: Consider increasing caloric intake")

How many recommendations appear? Now try changing age = 45 and exercise_hours = 1. Run it again - how many recommendations do you see now?

In this example, a person with exercise_hours = 2 will see the exercise recommendation. If they're also over 40, they'll see the health checkup recommendation. Multiple recommendations make sense because they're independent health concerns.

Practical Example: Mutually Exclusive Classification

Consider the same health data but now classifying BMI into categories. A person can only belong to one BMI category:

Test this code in your exercise.py file:

# BMI classification - use if-elif-else chain
weight = 85  # kg  
height = 1.75  # meters
bmi = weight / (height * height)

print(f"BMI: {bmi:.1f}")

# Only one classification should apply
if bmi >= 30:
    print("BMI Category: Obese")
elif bmi >= 25:
    print("BMI Category: Overweight")  
elif bmi >= 18.5:
    print("BMI Category: Normal weight")
else:
    print("BMI Category: Underweight")

Which category appears? Try changing the weight to 95 kg, then to 60 kg. How many categories does each person get assigned?

With weight = 85 and height = 1.75, the BMI is approximately 27.8. The person is classified as "Overweight" and only as "Overweight". Python stops checking conditions after finding the first True condition (bmi >= 25).

Why Positioning Matters for Program Logic

Understanding the evaluation order helps you write correct conditional logic:

# Correct: Most specific conditions first
score = 95

if score == 100:
    print("Perfect score!")
elif score >= 90:  
    print("A grade")
elif score >= 80:
    print("B grade")
else:
    print("Below B grade")

The positioning ensures that a perfect score (100) gets the special recognition before being categorized as just an "A grade". Python's top-to-bottom evaluation makes the order of conditions crucial for correct results.

CHOOSING THE RIGHT STRUCTURE

• Independent checks that can all happen → Use multiple if
• Categories where only one applies → Use if-elif-else
• Binary choice with two outcomes → Use if-else
• Single optional check → Use if only

Testing Your Code with pytest

Now that you understand selection statements, let's learn how to test your code using pytest in VS Code. Testing ensures your selection logic works correctly for all possible cases.

Exercise 1: Student Classification System Task

A university needs a student classification system that determines scholarship eligibility and academic standing. Navigate to /labs/lab09/exercise1/ and open exercise1.py.

Your task is to create a program that takes a student's GPA and credit hours as input, then displays their classification:

Requirements:

• Dean's List: GPA ≥ 3.8 AND credit hours ≥ 12
• Honor Roll: GPA ≥ 3.5 AND credit hours ≥ 12
• Good Standing: GPA ≥ 2.0
• Academic Probation: GPA < 2.0
• Part-time students (credit hours < 12) cannot qualify for Dean's List or Honor Roll

You'll find this starter code in exercise1.py:

# Student Classification System
# Get student information
student_name = input("Enter student name: ")
gpa = float(input("Enter GPA (0.0-4.0): "))
credit_hours = int(input("Enter credit hours: "))

# TODO: Add your classification logic here using if-elif-else statements
# Use the requirements above to determine the student's classification
# Store the result in a variable called 'classification'

# Display results
print(f"\nStudent: {student_name}")
print(f"GPA: {gpa}")
print(f"Credit Hours: {credit_hours}")
print(f"Classification: {classification}")

Complete the program by adding selection statements between the input and output sections. DO NOT change the input and print statment given, it will mess up the testing.

Running Tests with pytest in VS Code

The test file test_exercise1.py is located in /labs/lab09/exercise1/test/test_exercise1.py and has already been created for you. The tests will check your program by running it with different inputs and checking if the output matches the expected classification.

How to run your tests:

1. Open Test Explorer:

   • Look for the Test Explorer in the VS Code sidebar (flask/beaker icon)
   • You'll see your test functions listed under test_exercise1.py

2. Run your tests:

   • Click "Run All Tests" button to test your implementation

3. Read the test results:
   • After you click the run button for the test , the Test Result tab will automatically open at the bottom of the window.
   • On the right side, you will see which test pass or failed. Green mean pass and red mean failed.
   • To know why you test failed, scroll down in the Test Result tab until you see the error message marked by the letter E on the left side. The message can be pretty long but ignore the rest and just read the error message.
   • In my example, it failed because variable classification is not defined.

Your Workflow

1. Complete your selection logic in exercise1.py
2. Run all tests to see which cases work
3. Fix the failing cases by improving your if-elif-else statements
4. Run tests again until all are green
5. Success - your program handles all scenarios correctly.

Exercise 2: Tax Calculator Task

Calculate employee net salary after taxes. Overtime pay is RM35 per hour. Tax rates depend on filing status and total income:

Tax Rates:

• Single: 22% if total income ≥ RM5000, otherwise 18%
• Married: 20% if total income ≥ RM6000, otherwise 15%
• Head: 25% if total income ≥ RM5500, otherwise 19%

Additional deductions: EPF 11%, SOCSO 0.5%

Create /labs/lab09/exercise2/exercise2.py:

employee_name = input()
base_salary = float(input())
overtime_hours = int(input())
tax_status = input()

# TODO: Your code here

print(employee_name)
print(tax_rate)
print(f"{net_salary:.2f}")

Exercise 3: Movie Ticket Pricing Task

Calculate movie ticket prices based on day, time, and customer type.

Pricing Rules:

• Weekends: Adult RM18, Child RM12, Senior RM15
• Weekdays: Adult RM15, Child RM10, Senior RM12
• Evening shows (after 6pm): Add RM3 to base price
• Student discount: 10% off final price on weekdays only

Create /labs/lab09/exercise3/exercise3.py:

day_type = input()
show_time = int(input())
customer_type = input()
is_student = input()

# TODO: Your code here

print(base_price)
print(final_price)

Exercise 4: Water Bill Calculator Task

Calculate water bills with tiered pricing based on consumption.

Rate Structure:

• First 20 cubic meters: RM0.57 per cubic meter
• Next 15 cubic meters (21-35): RM1.03 per cubic meter
• Above 35 cubic meters: RM1.40 per cubic meter

Additional charges: Service charge RM8, Sewerage RM2

Create /labs/lab09/exercise4/exercise4.py:

current_reading = int(input())
previous_reading = int(input())

# TODO: Your code here

print(consumption)
print(water_cost)
print(total_bill)

Exercise 5: Restaurant Billing System Task

Calculate restaurant bills with menu items and discounts.

Menu Pricing:

• Main Course: Chicken RM10, Beef RM12, Fish RM11
• Drink: Soft Drink RM2, Coffee RM3
• Dessert: Ice Cream RM4, Cake RM5

Additional Charges:

• Service charge: 10% on total food cost
• Senior citizen discount (age more than 60): 15% off final bill
• Student discount (age less than 18): 10% off final bill

Create /labs/lab09/exercise5/exercise5.py:

main_course = input()
drink = input()
dessert = input()
customer_age = int(input())

# TODO: Your code here


print(f"{final_bill:.2f}")

Exercise 6: Employee Overtime Calculator Task

Calculate employee pay with overtime rates based on position.

Base Hourly Rates:

• Manager: RM30/hour
• Supervisor: RM20/hour
• Staff: RM15/hour
• Intern: RM8/hour

Overtime Rules:

• First 8 hours of overtime: 1.5x base rate
• Above 8 hours overtime: 2.0x base rate
• Weekend work: Additional RM5/hour bonus

Create /labs/lab09/exercise6/exercise6.py:

position = input()
overtime_hours = int(input())
is_weekend = input()

# TODO: Your code here


print(overtime_pay)

Exercise 7: College Admission System Task

Determine college admission status based on multiple academic requirements.

Admission Requirements:

• Minimum GPA: 3.0 or higher
• Total Score: 1200 or higher
• Extracurricular activities: At least one activity
• Interview: Must be completed

Admission Status:

• Accepted: All 4 requirements met
• Waitlisted: 3 requirements met
• Rejected: Less than 3 requirements met

Create /labs/lab09/exercise7/exercise7.py:

student_gpa = float(input())
total_score = int(input())
number_extracurricular = input()
completed_interview = input()

# TODO: Your code here


print(admission_status)

Exercise 8: Loan Approval System Task

Evaluate loan application based on financial criteria and employment status.

Financial Requirements:

• Monthly income: RM3000 or higher
• Employment status: Must be permanent or contract
• Credit score: Good or excellent rating
• Employment duration: 2 years or more at current job

Approval Status:

• Approved: All 4 criteria met
• Conditionally Approved: 3 criteria met
• Rejected: Less than 3 criteria met

Create /labs/lab09/exercise8/exercise8.py:

monthly_income = int(input())
employment_status = input()
credit_rating = input()
employment_years = int(input())

# TODO: Your code here

print(approval_status)

Exercise 9: Driver License Classification Task

Determine driving license class based on test results and requirements.

License Requirements:

• Age: 18 or older for regular license, 21+ for commercial
• Vision test: Must pass
• Written test: Score 80% or higher
• Driving test: Score 85% or higher
• Medical clearance: Must pass

License Classes:

• Class A (Commercial): All 5 requirements met + age 21+
• Class B (Regular): First 4 requirements met + age 18+
• Restricted License: 2-3 requirements met
• Application Denied: Less than 2 requirements met

Create /labs/lab09/exercise9/exercise9.py:

applicant_age = int(input())
vision_test = input()
written_score = int(input())
driving_score = int(input())
medical_clearance = input()

# TODO: Your code here


print(license_class)