Tutorial 2: The `if-else` Statement – Two Paths, Two Outcomes

Learning Objective

Use if-else to run one code block if a condition is True, and a completely different block if it is False.

1. The Theory (Deep Dive)

The "Fork in the Road"

In Tutorial 1, you learned that an if statement can either run a block or skip it entirely (doing nothing). But in many real-world programs, you must choose between two actions. This is where if-else comes in.

An if-else statement creates a guaranteed fork:

Path A (The if block): Runs only if the condition is True.
Path B (The else block): Runs only if the condition is False.

Crucial Concept – Mutual Exclusivity: The if block and the else block are mutually exclusive. This means exactly one of the two blocks will always execute. There is no scenario where both run, and no scenario where neither runs. The program must choose one of the two paths.

The Flowchart of `if-else`

     [ Start ]
        |
        v
   [Check Condition]
        |
    +---+---+
    |       |
   True    False
    |       |
    v       v
 [If Block] [Else Block]
    |       |
    +---+---+
        |
        v
 [Continue with rest 
  of the program]

Real-World Analogy

Imagine a security gate at a parking lot:

If you have a valid ticket, the gate opens.
Else (you don't have a valid ticket), the gate stays closed and a red light flashes. There is no third option. One of these two actions must happen every time a car approaches.

Comparison to Tutorial 1

if alone: "If it rains, take an umbrella." (If it doesn't rain, you do nothing with the umbrella).
if-else: "If it rains, take an umbrella. Else, put on sunglasses." (You are guaranteed to do one of these two things).

2. Syntax

if condition:
    # Code block for when condition is True
    action_a()
else:
    # Code block for when condition is False
    action_b()
# Code here runs regardless of which block executed

Grammar Rules to Memorize:

The if line ends with a colon :.
The else line ends with a colon :.
The else keyword must be aligned vertically with the if keyword (same indentation level).
The code blocks inside must be indented (4 spaces).

3. Expanded Code Examples

Example 1: The Classic Even/Odd Checker

(This is the practice task from the prompt, now shown as a full solution)

number = int(input("Enter a number: "))

if number % 2 == 0:
    print(f"{number} is an even number.")
else:
    print(f"{number} is an odd number.")

If the user enters 4 → 4 % 2 == 0 is True → Prints "4 is an even number."
If the user enters 7 → 7 % 2 == 0 is False → Prints "7 is an odd number."

Example 2: The Password Checker (Case-Insensitive)

password = input("Enter password: ")

if password.lower() == "python123":  # .lower() converts to lowercase to ignore case
    print("Access Granted. Welcome!")
else:
    print("Access Denied. Incorrect password.")

"Python123".lower() becomes "python123", so the check passes.

Example 3: Checking if a Number is Positive (or Not)

number = float(input("Enter a number: "))

if number > 0:
    print("Positive number.")
else:
    print("Not a positive number.")  # This catches zero AND negative numbers!

Takeaway: The else block catches everything that isn't > 0. This includes 0, -5, -100.5, etc. Be careful—if you specifically wanted to handle negatives and zeros differently, you'd need elif (Tutorial 3).

Example 4: Discount Eligibility

total_purchase = float(input("Enter total purchase amount: $"))

if total_purchase >= 100:
    discounted_price = total_purchase * 0.9  # 10% discount
    print(f"Discount applied! You pay: ${discounted_price:.2f}")
else:
    print(f"No discount. You pay: ${total_purchase:.2f}")

Example 5: Check if a String is Empty

user_name = input("Enter your name: ")

if user_name:  # An empty string "" is falsy. A non-empty string is truthy.
    print(f"Hello, {user_name}!")
else:
    print("Hello, stranger! (You didn't type anything).")

4. Important Rules (Reinforced)

Rule	Explanation
`else` has NO condition	`else:` is always written alone. You cannot write `else (x > 5):`.
`else` must follow an `if`	A lone `else` without a preceding `if` causes a `SyntaxError`.
Indentation determines ownership	The `else` must be indented exactly at the same level as its matching `if`.
Only ONE block runs	Because the condition is binary (`True`/`False`), Python will never run both blocks.

5. Common Pitfalls

Putting a Condition on else (The #1 Mistake)
- Wrong: else (x > 5): or else x > 5:
- Error: SyntaxError: invalid syntax
- Fix: else stands for "everything else." If you need another condition, wait for Tutorial 3 (elif).
Forgetting the Colon (:) on else
- Wrong: else (no colon)
- Error: SyntaxError: invalid syntax
- Fix: Always write else:.
Indentation Mismatch
- The else must align with its if. If the if is indented, the else must be indented to the exact same level.
- Wrong:
```
if x > 5:
    print("Big")
  else:   # Indented incorrectly!
    print("Small")
```
- Error: IndentationError: unindent does not match any outer indentation level
Assuming else runs for a specific "other" case
- If you check if score > 80 and use else, that else runs for every score <= 80 (including negative scores, strings if they got past, etc.). Remember, else is a catch-all, not a specific alternative.
Nested if Confusion (Dangling else)
- In Python, else always belongs to the nearest unpaired if at the same indentation level. This is rarely a bug if you indent correctly.

6. Quiz (Quick Knowledge Check)

Test your understanding before moving to the exercises.

Q1: What keyword do you use to run code when the if condition is False?

(A) elif
(B) otherwise
(C) else
(D) then

Answer

Q2: In an if-else statement, it is possible for both blocks to run.

(A) True
(B) False

Answer

(B) False

Q3: The else statement cannot have a __________ attached to it.

(A) colon
(B) condition
(C) indentation
(D) block

Answer

(B) condition

Q4: What is the output?

value = 0
if value:
    print("Truthy")
else:
    print("Falsy")

(A) Truthy
(B) Falsy

Answer

(B) `Falsy`

Q5: If the condition in an if-else is True, which block runs?

(A) The else block
(B) The if block
(C) Both
(D) Neither

Answer

(B) The `if` block

7. Hands-on Practice Exercises

Write a Python script for each task. Test them with different inputs.

Exercise 1: Pass or Fail
Ask for a score (0–100). Print "Pass" if 50 or higher, else "Fail".

Sample Solution

score = float(input("Enter score: "))
if score >= 50:
    print("Pass")
else:
    print("Fail")

Exercise 2: Minimum Purchase
Ask for number of items. If 5 or more, print "Bulk discount applied!", else "No discount available."

Sample Solution

items = int(input("How many items? "))
if items >= 5:
    print("Bulk discount applied!")
else:
    print("No discount available.")

Exercise 3: Name Length
Ask for a name. If length > 5, print "Long name!", else "Short name."

Sample Solution

name = input("Enter name: ")
if len(name) > 5:
    print("Long name!")
else:
    print("Short name.")

Exercise 4: Guessing Game
Set secret = 7. Ask user to guess. If correct, print "You win!", else "Wrong guess!"

Sample Solution

secret = 7
guess = int(input("Guess the number: "))
if guess == secret:
    print("You win!")
else:
    print("Wrong guess!")

Exercise 5: Leap Year (simplified)
Ask for a year. If divisible by 4, print "Leap year", else "Not a leap year".

Sample Solution

year = int(input("Enter year: "))
if year % 4 == 0:
    print("Leap year")
else:
    print("Not a leap year")

Homework Assignment: The `if-else` Statement (Tutorial 2 Review)

Instructions: Complete all parts below. After finishing, click the "Answer" sections to check your work.

Homework Part A: Code Predictor (No Computer!)

Write down the exact output for each snippet.

Snippet 1:

x = 10
if x % 2 == 1:
    print("Odd")
else:
    print("Even")

Answer

Even

Explanation: 10 % 2 equals 0. The condition 0 == 1 is False, so the else block runs and prints "Even".

Snippet 2:

city = "New York"
if city == "new york":
    print("Welcome to the Big Apple!")
else:
    print("You are in a different city.")

Answer

You are in a different city.

Explanation: The condition city == "new york" compares "New York" (capital 'N' and 'Y') to "new york" (lowercase). In Python, string comparisons are case‑sensitive, so they are not equal. Therefore, the else block runs.

Snippet 3:

age = 25
if age < 18:
    ticket_price = 5
else:
    ticket_price = 15
print(f"Ticket price: ${ticket_price}")

Answer

Ticket price: $15

Explanation: The condition age < 18 is False (25 is not less than 18). So the else block runs and sets ticket_price = 15. The final print outputs the string.

Snippet 4:

data = ""
if data:
    print("Data received.")
else:
    print("No data.")

Answer

No data.

Explanation: An empty string "" is considered falsy in Python. So if data: evaluates to False, and the else block runs.

Homework Part B: Bug Hunter

The following snippets contain errors. Identify the error, explain why it's wrong, and write the corrected code.

Buggy Code 1:

score = 85
if score >= 60
    print("Pass")
else:
    print("Fail")

Answer

Error: Missing colon : at the end of the if line.

Explanation: In Python, every if statement must end with a colon. Without it, Python raises a SyntaxError.

Corrected Code:

score = 85
if score >= 60:
    print("Pass")
else:
    print("Fail")

Buggy Code 2:

temperature = 30
if temperature > 25:
    print("It's hot.")
else temperature <= 25:   # Something is wrong here!
    print("It's cool.")

Answer

Error: Putting a condition on the else block. The else keyword does not accept a condition.

Explanation: else is a catch‑all for everything that didn't match the preceding if. If you need another condition, you must use elif (covered in Tutorial 3). This code raises a SyntaxError.

Corrected Code (using if-else): Since the condition temperature <= 25 is the exact opposite of temperature > 25, we can simply remove the condition:

temperature = 30
if temperature > 25:
    print("It's hot.")
else:
    print("It's cool.")

(If you wanted a different condition, you would use elif).

Buggy Code 3:

num = 7
if num == 7:
    print("Lucky number!")
else:
print("Not lucky.")   # Check the indentation!

Answer

Error: The print("Not lucky.") statement is not indented correctly.

Explanation: In Python, the code inside the else block must be indented (usually 4 spaces) to show that it belongs to the else. Incorrect indentation raises an IndentationError.

Corrected Code:

num = 7
if num == 7:
    print("Lucky number!")
else:
    print("Not lucky.")

Homework Part C: Write Complete Programs

Write a Python script for each task. Test your code with different inputs.

Task 1: The Voter Validator Ask the user for their age.

If they are 18 or older, print "You are eligible to vote."
Otherwise, print "You are too young to vote."

Sample Solution

age = int(input("Enter your age: "))
if age >= 18:
    print("You are eligible to vote.")
else:
    print("You are too young to vote.")

Task 2: Positive or Not Ask the user for a number. If it is greater than 0, print "Positive". If it is 0 or less, print "Non-positive".

Sample Solution

num = float(input("Enter a number: "))
if num > 0:
    print("Positive")
else:
    print("Non-positive")

Task 3: Temperature Suitability Ask the user for the current temperature in Fahrenheit.

If the temperature is between 60 and 80 (inclusive), print "Perfect weather!".
Otherwise, print "Weather is not ideal."

Sample Solution

temp = float(input("Enter temperature (°F): "))
if temp >= 60 and temp <= 80:
    print("Perfect weather!")
else:
    print("Weather is not ideal.")

Task 4: The Empty Input Guard Ask the user to type a message.

If they type something (the string is not empty), print "You said: " followed by their message.
If they just press Enter without typing, print "You didn't say anything!".

Sample Solution

message = input("Type a message: ")
if message:          # Non-empty strings are truthy
    print(f"You said: {message}")
else:
    print("You didn't say anything!")

Challenge Task: The FizzBuzz Warm-up Ask the user for an integer.

If the number is divisible by 3, print "Fizz".
Otherwise (if not divisible by 3), print the number itself.

Sample Solution

num = int(input("Enter an integer: "))
if num % 3 == 0:
    print("Fizz")
else:
    print(num)

Summary Checklist

Before submitting, ensure you can:

Predict the output of an if-else statement.
Identify and fix common syntax errors (missing colons, incorrect indentation).
Write programs that handle two mutually exclusive outcomes.
Use logical operators (and) to check ranges.
Understand truthiness in Python (e.g., empty strings are False).

Great work completing this homework! You are now ready for Tutorial 3 (if-elif).

9. Quick Reference Card

# Basic structure:
if CONDITION:
    # Do this when True
else:
    # Do this when False

# Real example:
age = int(input("Age: "))
if age >= 18:
    print("Adult")
else:
    print("Minor")

Key Takeaway for Tutorial 2: Always ask yourself: "What are the two possible outcomes I need to handle?" If you have exactly two mutually exclusive paths, if-else is your tool. If you have more than two, or you need a "do nothing" option, you'll need if (Tutorial 1) or elif (Tutorial 3).

You are now ready to move on to Tutorial 3: The if-elif Statement – Multiple Selective Checks. Keep up the great work!

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Tutorial 2: The if-else Statement – Two Paths, Two Outcomes

Learning Objective

1. The Theory (Deep Dive)

The "Fork in the Road"

The Flowchart of if-else

Real-World Analogy

Comparison to Tutorial 1

2. Syntax

3. Expanded Code Examples

Example 1: The Classic Even/Odd Checker

Example 2: The Password Checker (Case-Insensitive)

Example 3: Checking if a Number is Positive (or Not)

Example 4: Discount Eligibility

Example 5: Check if a String is Empty

4. Important Rules (Reinforced)

5. Common Pitfalls

6. Quiz (Quick Knowledge Check)

7. Hands-on Practice Exercises

Homework Assignment: The if-else Statement (Tutorial 2 Review)

Homework Part A: Code Predictor (No Computer!)

Homework Part B: Bug Hunter

Homework Part C: Write Complete Programs

Summary Checklist

9. Quick Reference Card

Tutorial 2: The `if-else` Statement – Two Paths, Two Outcomes

The Flowchart of `if-else`

Homework Assignment: The `if-else` Statement (Tutorial 2 Review)