📘 Week 11 – User Input & Conditionals in Python

Period 1 – User Input & Conditionals

Objective: Learn to get input from the user and use if/elif/else statements to make decisions.

1. The input() Function

• Purpose: Pauses the program, waits for the user to type, and returns the input as a string.
• Syntax: variable = input("Prompt: ")
• Example:

  name = input("What is your name? ")
  print("Hello, " + name + "!")

🔁 Converting Input (Type Casting)

Because input() always returns a string, you must convert it to a number if you need to do math.

age   = int(input("How old are you? "))    # → integer
price = float(input("Enter a price: "))  # → float (decimal)

2. Conditional Statements (if, elif, else)

Conditionals let the program choose different paths based on whether a condition is True or False.

Basic structure:

if condition:
    # code runs if condition is True
elif another_condition:
    # runs if first condition was False and this one is True
else:
    # runs if all above conditions are False

3. Comparison Operators

Used to compare values – they always return True or False.

Operator	Meaning	Example
==	equal to	age == 18
!=	not equal to	age != 0
>	greater than	score > 90
<	less than	temp < 0
>=	greater than or equal	age >= 18
<=	less than or equal	price <= 10

4. Logical Operators

Combine multiple conditions:

• and – both conditions must be True
• or – at least one condition must be True
• not – reverses the truth value

if age >= 18 and has_id:
    print("Welcome!")

5. Example – Voting Age Checker (improved)

age = int(input("Enter your age: "))

if age >= 18:
    print("You are old enough to vote!")
elif age > 0:
    print("Sorry, you are too young to vote.")
else:
    print("Invalid age (must be positive).")

6. Bonus – Nested Conditionals

You can put an if inside another if:

age = int(input("Your age: "))
if age >= 18:
    country = input("Which country? ")
    if country.lower() == "usa":
        print("You can vote in the USA!")
    else:
        print("Check local voting laws.")
else:
    print("Too young to vote.")

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Period 2 – Exercises: Build Small Python Programs

Objective: Combine input, type conversion, conditionals, and operators to solve practical problems.

📋 Practical Tasks

1️⃣ Greeting Bot

• Ask the user for their name and age.
• Print a personalised greeting that includes both.
• Extra challenge: If the user is 100 or older, congratulate them!

Sample run:

2️⃣ Simple Calculator

• Ask for two numbers (use float to allow decimals).
• Ask for an operator (+, -, *, /).
• Use if/elif/else to decide the operation.
• Print the result.
• Handle division by zero: if the operator is / and the second number is 0, print "Error: Cannot divide by zero."

Sample run:

3️⃣ Grade Checker

• Ask for a numerical score (0–100).
• Print the corresponding letter grade:

Score Range	Grade
90 – 100	A
80 – 89	B
70 – 79	C
60 – 69	D
Below 60	F

• Bonus: If the score is outside 0–100, print "Invalid score."
• Bonus 2: Add a plus/minus distinction (e.g., 97+ → A+, 93–96 → A, 90–92 → A-).

4️⃣ Temperature Converter (optional challenge)

• Ask the user for a temperature and a scale (C or F).
• If Celsius → Fahrenheit: F = C * 9/5 + 32
• If Fahrenheit → Celsius: C = (F - 32) * 5/9
• Print the result with a suitable message.

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💬 Wrap‑up discussion points

• Why do we need to convert input with int() or float()?
• What happens if we use = inside an if condition?
• How can we check for multiple conditions at once? (Logical operators)
• Why is indentation so important in Python?