If staments

COS102: Problem Solving Using Python - IF Statements & Logic

Learning Outcomes

• Explain decision making in programming.
• Understand the `if`, `elif`, and `else` logic.
• Use relational operators correctly (`==`, `>`, `<=`).
• Identify and fix common syntax & indentation errors.
• Develop programs that respond to user input.

Daily Decision Making & Programming Logic

Just like humans make daily decisions based on questions (e.g., "Am I hungry?"), computers need rules to choose what to do next. This is called decision making in programming.

Real-World Logic Examples:

• Traffic Light:
  • Condition: Is light Green?
  • Action: If Yes → Move. If No → Stop.
• ATM Transaction:
  • Condition: PIN correct?
  • Action: If Yes → Give Cash. If No → Reject.

Binary Choices (True/False)

Decision making is a computer's ability to choose an action based on a Boolean result (True or False). In Python, the if keyword initiates this process.

The "If" Syntax

The basic structure for conditional execution:

if score >= 50:
    print("Pass")

• Keyword: Starts with if.
• The Colon: Must end with `:`.
• Indentation: The "action" (code block) must be spaced in (typically 4 spaces) to indicate it belongs to the `if` statement.

Visualizing the Flow (Flowchart Logic)

When an `if` statement is encountered:

1. Step 1: Ask the question (Condition).
2. Step 2: Check the answer (evaluates to True or False).
3. YES (True): Run the indented code block.
4. NO (False): Skip the indented code entirely.

The "Else" Clause (The Backup)

The else clause provides a backup plan when the `if` condition is False.

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

Think of it: "If the ID is valid, open gate. Else, sound the alarm."

The "Elif" (Multiple Choices)

Use elif (short for "else if") when you have more than two options.

if score >= 70:
    print("Grade: A")
elif score >= 60:
    print("Grade: B")
else:
    print("Grade: C")

Python checks these conditions one by one until it finds a True one. Once a True condition is found, its associated code block is executed, and the rest of the `elif`/`else` chain is skipped.

Relational Operators

These operators are used to compare values and return a Boolean result (True or False).

Code Examples

• ATM Security Check: Comparing `entered_pin` to `stored_pin`.
• Password Comparison: Checking if `pwd` matches "python123".
• Multi-way Logic (Weather): Using `if`, `elif`, `else` to describe weather based on temperature.

Common Mistakes

• Mistake #1: `=` vs. `==`

  • = is for Assignment (giving a value).
  • == is for Comparison (checking if two values are equal).

  WRONG ❌: if age = 18:

  CORRECT ✅: if age == 18:

• Mistake #2: Colon & Indent

  • The Colon: Missing the colon at the end of `if`, `elif`, or `else` statement. (e.g., if score > 50 ← Error! Missing :)
  • Indentation: Not indenting the code block belonging to the conditional statement. Python uses whitespace to define code blocks.

  WRONG:

  if score > 50:
  print("Pass") # Error! Not Indented

  CORRECT:

  if score > 50:
      print("Pass")

Key Takeaways

• Decision making depends on Conditions (True/False).
• Use if, elif, and else for multiple paths.
• Always include the Colon (:) and proper Indentation.
• == compares, = assigns.

What's Next? (Chapter 14: Processing Data)

We will learn about:

• Counting & Summing items
• Swapping variables
• Using Flag variables

Happy Coding!