CS50's Introduction to Programming with Python

Lecture: Debugging

Lecturer: David Malan

Key Topics:

• Types of Bugs

  • Syntax Errors: Code doesn't run due to mistakes in the code syntax.
  • Logical Bugs: Code runs but doesn't produce the intended output.

• Initial Example: Mario Pyramid

  • Goal: Print a textual pyramid using hashes (#).
  • Initial Code: def main(): height = int(input("Height: ")) pyramid(height) def pyramid(n): for i in range(n): print("#" * i) if __name__ == "__main__": main()
  • Expected Outcome: Pyramid of given height (e.g., height of 3).
  • Initial Issue: Produces: # ##
  • Cause: For loop starts at 0, leading to an unintended blank first line.

Debugging Techniques

1. Print Statements:

   • Purpose: Quick way to check values of variables.
   • Example in Code: for i in range(n): print(i, end=" ") print("#" * i)
   • Output with Debug Print: 0 1 # 2 ##
   • Realization: The issue is the loop starting at 0.

2. Breakpoints and Debuggers:

   • Definition: A breakpoint pauses the execution of the program at a specific line for inspection.
   • Using VS Code Debugger:
     • Set Breakpoint: Click to the left of the line number in the editor.
     • Step Into: Goes inside function calls.
     • Step Over: Executes the line, but doesn't dive into functions.
     • Continue: Runs the program until the next breakpoint or the end.
   • Example Use:
     • Setting breakpoint at the start of the main() function.
     • Stepping through to identify the value of i.
     • Confirming variable values via the debug panel.

3. Making Corrections and Re-running: Correcting the Logic

   • Fix: Adjust the print statement to add 1 to i.
   • Corrected Code: def pyramid(n): for i in range(n): print("#" * (i + 1))
   • Result: Desired outcome of: # ## ###

Summary

• Importance of Debugging: In the absence of a teacher or peer, tools like print statements and debuggers are essential to diagnosing issues in code.
• When to Use Each Tool:
  • Print Statements: Quick and simple checks in small code sections.
  • Debugger: Stepping through complex or large codebases, inspecting variables, and understanding control flow.