CS50P - Lecture 1 - Conditionals

Introduction

This tutorial provides a comprehensive overview of conditionals in Python, a fundamental concept in programming that allows you to make decisions based on user input or other values. Understanding conditionals is crucial for building interactive programs, as they enable your code to respond differently under varying circumstances.

Chapter 1: Understanding Conditionals

Conditionals, or conditional statements, allow you to execute different lines of code based on whether a condition is true or false. In Python, you can use several comparison operators to create these conditions:

• Greater than: >
• Greater than or equal to: >=
• Less than: <
• Less than or equal to: <=
• Equal to: ==
• Not equal to: !=

These operators help you create boolean expressions, which are statements that evaluate to either true or false.

Practical Tips

• Use == for comparison, not = (which is used for assignment).
• Use indentation to define blocks of code that belong to a conditional statement.

Chapter 2: Basic Conditional Statements

To illustrate the use of conditionals, let’s create a simple program called compare.py that compares two integers input by the user.

1. Create Variables:

   x = int(input("What's x? "))
   y = int(input("What's y? "))
   

2. Implement Conditionals:

   if x < y:
       print("x is less than y")
   elif x > y:
       print("x is greater than y")
   else:
       print("x is equal to y")
   

Key Points

• The if statement checks if a condition is true and executes the code block below it if it is.
• The elif statement allows you to check multiple conditions.
• The else statement provides a default action if none of the previous conditions are met.

Chapter 3: Improving Conditional Logic

Using multiple if statements can lead to redundancy. Instead, you can streamline your code using elif and else to avoid unnecessary checks.

1. Refactor Code: Instead of checking all conditions independently, structure your checks to be mutually exclusive, so once a condition is met, the rest are skipped:

   if x < y:
       print("x is less than y")
   elif x > y:
       print("x is greater than y")
   else:
       print("x is equal to y")
   

Practical Advice

• Aim for efficiency in your condition checks by using elif and else to create a clear flow in your logic.

Chapter 4: Using Logical Operators

You can combine conditions using logical operators such as and, or, and not.

1. Example with Logical Operators:

   if x < 0 or y < 0:
       print("At least one number is negative")
   

Common Pitfalls

• Ensure logical conditions are correctly structured to avoid unexpected results.

Chapter 5: Implementing a Grading System

Let’s apply conditionals in a more practical scenario by creating a grading system based on user input.

1. Get User Input:

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

2. Determine Grade:

   if score >= 90:
       print("A")
   elif score >= 80:
       print("B")
   elif score >= 70:
       print("C")
   elif score >= 60:
       print("D")
   else:
       print("F")
   

Key Takeaways

• You can simplify your conditions by ensuring they are structured to minimize redundancy.
• The grading system effectively demonstrates how conditions can be nested.

Chapter 6: The Modulo Operator

The modulo operator (%) is useful for checking even or odd numbers.

1. Check Even or Odd:

   number = int(input("Enter a number: "))
   if number % 2 == 0:
       print("Even")
   else:
       print("Odd")
   

Practical Application

• Use the modulo operator to determine divisibility or parity in various applications.

Conclusion

In this tutorial, you learned about conditionals in Python, including how to construct basic conditional statements, improve the efficiency of your logic, use logical operators, and apply these concepts in practical scenarios like grading systems and parity checks. Understanding and mastering conditionals will open up many possibilities for creating dynamic and responsive Python programs.

Next Steps

• Explore loops to repeat actions based on conditions.
• Experiment with more complex conditions and nested statements in your projects.