CS50's Introduction to Programming with Python - Week on Functions and Variables

Overview

• Lecture by David Malan
• Focus on basics of programming and Python.

Getting Started with Python

• Use Visual Studio Code (VS Code) for writing code.
• Code is ultimately just text; any text editor can be used to write it.
• Python file names typically end with .py.

Writing Your First Program

• Example: print("hello, world")
  • This program prints "hello, world" to the screen.
  • Running a program is done via command line using python hello.py.

Functions in Programming

• Functions are actions or verbs that allow the programmer to perform tasks.
• Example function: print()
  • Takes arguments which influence its behavior.
  • Arguments are inputs to functions, influencing output.
• Output can be a side effect, like displaying text on the screen.

Common Errors in Programming

• Syntax errors occur when code is not written correctly (e.g., missing parentheses).
• Bugs are mistakes in the program; debugging involves identifying and fixing these issues.

User Input

• Using input() to prompt users for input (e.g., "What's your name?").
• Input is always returned as a string.
• To use the input value further, store it in a variable.

Variables

• Variables are containers for storing values (e.g., name).
• Assignment uses = to store values from right to left.
• Example: name = input("What's your name?")

Comments and Code Documentation

• Comments in code are written using # and are ignored by the interpreter.
• Useful for explaining code and intentions.

Improving the Program

• Making the greeting more dynamic by using user input rather than hard-coding the name into the program.
• Example: print(f"hello, {name}") using f-strings for cleaner output.

String Manipulation

• Built-in string functions/methods allow for cleaning input (e.g., strip(), capitalize(), title()).
• split() can break strings into smaller substrings.
• Example: first, last = name.split() to separate first and last names.

Working with Numbers

• Python supports integers (int) and floating-point numbers (float).
• Arithmetic operations: addition (+), subtraction (-), multiplication (*), division (/).
• The modulo operator (%) gives the remainder of the division.

Interactive Python

• Python supports interactive mode for immediate code execution and feedback.
• Use as a calculator for simple math.

Creating a Simple Calculator

• Building a basic calculator program using user input and arithmetic operations:

  x = float(input("What's x? "))
  y = float(input("What's y? "))
  print(x + y)
  

• Handling potential errors when user inputs non-numeric values.

Defining and Using Functions

• Custom functions can be defined using def.
• Example: def hello(name): to say hello to a specific user.
• Functions can take parameters and return values using return keyword.

Scope of Variables

• Variables defined in functions only exist within that function's scope.
• Passing variables between functions demonstrates scope (e.g., name variable).

Final Notes

• Building programs incrementally using functions helps with organization and readability.
• Use of main function to organize code and manage execution flow.
• Python allows defining default parameter values in functions for flexibility.

Conclusion

• The lecture covered fundamental programming concepts with Python, focusing on functions, variables, and user input.
• Encourages hands-on practice and experimentation with code.