Tutorial 1: The `if` Statement – Making a Single Decision

Learning Objective

To be able to use an if statement to run a code block only when a specific condition is True.

1. The Theory (Deep Dive)

What is a Condition?

In programming, a condition is any expression that Python can evaluate as either True or False. This data type is called a Boolean (named after George Boole).

When a condition evaluates to True, the code inside the if block executes.
When it evaluates to False, Python completely ignores that block and continues with the rest of the program.

The Flowchart of an `if` Statement

Visualizing code helps a lot. Here is how an if statement flows:

     [ Start ]
        |
        v
   [Check Condition]
        |
   +----+----+
   |         |
  True      False
   |         |
   v         |
[Run Indented] |
   |         |
   +----+----+
        |
        v
 [Continue with 
  rest of program]

Truthiness (Advanced but Important)

In Python, the if statement doesn't only work with True or False. It also checks for "truthiness":

Values like 0, None, "" (empty string), [] (empty list) are treated as False.
All other values (e.g., 1, -5, "Hello", [1, 2, 3]) are treated as True.

Example:

if "Python":
    print("This will print, because a non-empty string is truthy!")

Beginner Tip: For now, always write clear comparisons (e.g., if age > 12:) rather than relying on truthiness, to make your code easier to read.

The Role of `input()` and Type Conversion

The input() function always returns a string (text), even if the user types a number.
To compare numbers, you must convert the string to an integer using int().

Incorrect (Causes an error or wrong logic):

age = input("Enter age: ")  # age is a string, e.g., "15"
if age > 12:                # ERROR! Can't compare string to int.
    print("Teenager")

Correct:

age = int(input("Enter age: "))  # Converts to integer
if age > 12:
    print("Teenager")

2. Syntax

if condition:
    # Code to run if condition is True
    do_something()
# Code here runs regardless (outside the if block)

3. Expanded Code Examples

Example 1: Checking Weather

temperature = 30

if temperature > 25:
    print("It's a hot day! Drink water.")

print("This line always runs.")

Output:

It's a hot day! Drink water.
This line always runs.

Example 2: Skipping the Block

temperature = 20
if temperature > 25:
    print("It's a hot day!")  # This is completely skipped
print("Goodbye.")             # This runs normally

Output:

Goodbye.

Example 3: Comparing Strings (Case Sensitivity)

Python comparisons are case-sensitive. "hello" is not the same as "Hello".

user_input = "Yes"

if user_input == "yes":   # Checks for lowercase 'yes'
    print("Confirmed!")
print("Program ended.")

Output: Program ended. (Because "Yes" does not equal "yes").

Example 4: Checking if a Number is Even

number = 10

if number % 2 == 0:   # % is the modulo operator (remainder after division)
    print(f"{number} is an even number.")

Output: 10 is an even number.

4. Common Comparison Operators (Cheat Sheet)

Operator	Meaning	True Example	False Example
`>`	Greater than	`10 > 5`	`4 > 8`
`<`	Less than	`3 < 7`	`9 < 2`
`>=`	Greater than or equal to	`5 >= 5`	`4 >= 6`
`<=`	Less than or equal to	`3 <= 3`	`7 <= 2`
`==`	Equal to (VALUE check)	`"Hi" == "Hi"`	`"Hi" == "hi"`
`!=`	Not equal to	`5 != 3`	`5 != 5`

Critical Rule: = is for assigning a value (e.g., x = 10). == is for checking if two things are equal. Never mix them up inside an if!

5. Common Pitfalls

The Missing Colon (:)
- Wrong: if temperature > 25 (No colon)
- Error: SyntaxError: invalid syntax
- Fix: Always add : at the end of the if line.
Inconsistent Indentation
- Python strictly uses indentation to know which code belongs to the if.
- Wrong:
```
if x > 5:
print("Big!")   # Not indented
```
- Error: IndentationError: expected an indented block
- Fix: Use exactly 4 spaces (or 1 Tab, but never mix them) for the code inside the if.
Comparing Different Data Types
- Wrong: if age > "12": (Comparing int to str)
- Error: TypeError: '>' not supported between instances of 'int' and 'str'
- Fix: Use int(input()) to convert user input to a number before comparing.
Floating Point Precision
- Be careful when comparing floats (decimals). Due to how computers store decimals, 0.1 + 0.2 == 0.3 is actually False in Python!
- Fix: For floats, check if numbers are close enough (e.g., abs(0.1 + 0.2 - 0.3) < 0.0001), though for this tutorial, stick to integers.

6. Quizzes (Quick Knowledge Check)

Q1: Which keyword starts a conditional statement in Python?

(A) condition
(B) if
(C) when
(D) loop

Answer

(B) `if`

Q2: The code inside an if statement must be __________ to the right.

(A) centred
(B) indented
(C) aligned with the if
(D) bold

Answer

(B) indented

Q3: The expression "apple" == "Apple" evaluates to:

(A) True
(B) False

Answer

(B) `False`

Q4: What is the output of:

score = 85
if score >= 90:
    print("A")
print("Done")

(A) A
(B) Done
(C) A and Done
(D) Nothing

Answer

(B) `Done`

Q5: What is printed?

x = 0
if x:
    print("Hello")
print("World")

(A) Hello then World
(B) World only
(C) Hello only
(D) Nothing

Answer

(B) `World` only (0 is falsy)

7. Hands-on Practice Exercises

Exercise 1: Positive Numbers
Write a program that asks the user for a number. If the number is greater than 0, print "The number is positive."

Sample Solution

num = float(input("Enter a number: "))
if num > 0:
    print("The number is positive.")

Exercise 2: Letter Hunter
Ask for a word. If it contains the letter "z", print "This word has a 'z'!".

Sample Solution

word = input("Enter a word: ")
if "z" in word:
    print("This word has a 'z'!")

Exercise 3: Divisible by 5
Ask for an integer. If it is divisible by 5, print "This number is a multiple of 5."

Sample Solution

num = int(input("Enter an integer: "))
if num % 5 == 0:
    print("This number is a multiple of 5.")

Exercise 4: Freezing Alert
Ask for a temperature in Celsius. If it is below 0, print "Warning: Freezing conditions!"

Sample Solution

temp = float(input("Enter temperature (°C): "))
if temp < 0:
    print("Warning: Freezing conditions!")

Exercise 5: Adult Check
Ask for age. If 18 or older, print "You are eligible to vote."

Sample Solution

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

8. Homework Assignment

This homework combines code prediction, bug fixing, and writing original programs. Submit your .py files or written answers.

Part A: Code Predictor (No Computer!)

Write down the exact output of the following snippets without running them.

Snippet 1:

price = 100
if price > 50:
    print("Expensive")
print("Checkout")

Answer

``` Expensive Checkout ``` **Explanation:** The condition `price > 50` is `True` (100 > 50), so the `if` block runs and prints `"Expensive"`. The line after the `if` block is not indented, so it always runs and prints `"Checkout"`.

Snippet 2:

name = "Alice"
if name == "alice":
    print("Welcome back, Alice!")
print("System ready.")

Answer

``` System ready. ``` **Explanation:** The condition `name == "alice"` is `False` because `"Alice"` (capital 'A') is not equal to `"alice"` (lowercase 'a'). Therefore, the `if` block is skipped, and only the final `print` executes.

Snippet 3:

count = 5
if count:
    print("There are items.")
else:
    # Wait, there is no else! Let's just see what happens.
    pass 
print("Done")

Answer

``` There are items. Done ``` **Explanation:** The variable `count` is `5`, which is a non‑zero number and therefore **truthy**. So the `if` block runs and prints `"There are items."`. The `else` block is skipped entirely (the `pass` is a placeholder that does nothing). Finally, `print("Done")` runs.

Part B: Bug Hunter

The following programs have syntax errors or logic errors. Fix them so they run correctly. Explain what was wrong and write the corrected version.

Buggy Code 1:

user_age = input("Enter age: ")
if user_age >= 18
    print("Adult")

Answer

Problems:

Missing colon : after the if condition.
input() returns a string, but we are comparing it to an integer (18) – this causes a TypeError.

Corrected Code:

user_age = int(input("Enter age: "))
if user_age >= 18:
    print("Adult")

Buggy Code 2:

temperature = 30
if temperature > 25
print("It's warm.")

Answer

Problems:

Missing colon : after the if condition.
The print statement is not indented – Python will raise an IndentationError because it expects a block inside the if.

Corrected Code:

temperature = 30
if temperature > 25:
    print("It's warm.")

Buggy Code 3:

answer = "Yes"
if answer = "Yes":
    print("Confirmed")

Answer

Problem: Using the assignment operator = inside the if condition instead of the equality comparison operator ==. This will raise a SyntaxError.

Corrected Code:

answer = "Yes"
if answer == "Yes":
    print("Confirmed")

Part C: Write a Program

Write complete Python programs for each of the following tasks. Use only the concepts covered so far (variables, input(), int(), and only if statements – no else or elif are allowed in these tasks).

Task 1: The Lucky Number Write a program that asks the user to guess a secret number. If the user guesses exactly 42, print "You guessed the secret number!". If they guess anything else, the program should simply end quietly (do nothing).

Sample Solution

guess = int(input("Guess the secret number: "))
if guess == 42:
    print("You guessed the secret number!")

Task 2: Password Strength Checker (Basic) Ask the user to create a password.

If the password is exactly "admin123", print "Secure password set."
If the password is an empty string (the user just presses Enter without typing), print "Password cannot be empty."
Both conditions must use separate if statements (not elif).

Sample Solution

password = input("Create a password: ")
if password == "admin123":
    print("Secure password set.")
if password == "":
    print("Password cannot be empty.")

Task 3: The Weekend Checker Ask the user for today's date as a number (e.g., 1 for Monday, 7 for Sunday). Using only if statements (one for Saturday, one for Sunday), print "It's the weekend!" if the date is 6 or 7. Otherwise, print nothing.

Sample Solution

date = int(input("Enter today's date (1=Mon, 7=Sun): "))
if date == 6:
    print("It's the weekend!")
if date == 7:
    print("It's the weekend!")

Bonus Challenge (Optional)

Add a third if statement to Task 3 that prints "Invalid day" if the user enters a number outside the range 1 to 7.

Sample Solution

date = int(input("Enter today's date (1=Mon, 7=Sun): "))
if date == 6:
    print("It's the weekend!")
if date == 7:
    print("It's the weekend!")
if date < 1 or date > 7:
    print("Invalid day")

(Note: This uses or, which is covered in Tutorial 5 – a great preview!)

Summary Checklist

Before moving to Tutorial 2, ensure you can confidently answer:

What does if do when the condition is False?
Why do I need int() around input() when comparing numbers?
What is the difference between = and ==?
Why does indentation matter so much in Python?

Once you've finished the quizzes, exercises, and homework, you're ready for Tutorial 2: The if-else Statement. Happy coding!

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Tutorial 1: The if Statement – Making a Single Decision

Learning Objective

1. The Theory (Deep Dive)

What is a Condition?

The Flowchart of an if Statement

Truthiness (Advanced but Important)

The Role of input() and Type Conversion

2. Syntax

3. Expanded Code Examples

Example 1: Checking Weather

Example 2: Skipping the Block

Example 3: Comparing Strings (Case Sensitivity)

Example 4: Checking if a Number is Even

4. Common Comparison Operators (Cheat Sheet)

5. Common Pitfalls

6. Quizzes (Quick Knowledge Check)

7. Hands-on Practice Exercises

8. Homework Assignment

Part A: Code Predictor (No Computer!)

Part B: Bug Hunter

Part C: Write a Program

Bonus Challenge (Optional)

Summary Checklist

Tutorial 1: The `if` Statement – Making a Single Decision

The Flowchart of an `if` Statement

The Role of `input()` and Type Conversion