CS50 Week 1 Lecture Notes

Introduction to C

• Notable shift from Scratch (used in week 0) to C.
• C is older and more traditional programming language.
• Key concepts from Scratch: functions, loops, and conditionals.
• Goals: Problem-solving skills and building a toolkit.
• Later: understand limitations of computers.

First C Program

• Transition from Scratch's 'Hello, world' block to C's equivalent:

  #include <stdio.h>
  int main(void) {
      printf("Hello, world\n");
      return 0;
  }
  

• Key parts:
  • Boilerplate code: #include <stdio.h>, int main(void), {}
  • printf as function equivalent to Scratch's 'say' block.
  • Importance of \n for new line and ; to signal end of statement.

Compilation & Execution

• Source code: what we write.
• Machine code: what the computer understands (binary).
• Compiler: converts source code to machine code (e.g., make command).
• Running programs: ./program_name (equivalent to clicking an icon).

Tools and Environment

• Using cs50.dev (cloud-based environment with VS Code).
• Components of VS Code interface:
  • Editor: writing code.
  • Terminal: typing commands.
  • Explorer: navigating files/folders.
• CS50 specific tools: cs50.h for simplified functions (e.g., get_string).

Functions with Input and Output

• Transition from single-output programs to interactive ones:

  #include <cs50.h>
  #include <stdio.h>
  int main(void) {
      string name = get_string("What's your name?\n");
      printf("Hello, %s\n", name);
  }
  

• Explained the necessity for correct variable types and function signatures.
• Concept of return values and arguments in functions.

Conditionals in C

• Simple conditions:

  if (x < y) {
      printf("x is less than y\n");
  }
  

• Multi-way conditions:

  if (x < y) {
      printf("x is less than y\n");
  } else if (x > y) {
      printf("x is greater than y\n");
  } else {
      printf("x is equal to y\n");
  }
  

• Importance of correct logical order and nesting to ensure efficiency and correctness.

Variables and Data Types

• Introduction to different data types (int, float, char, string).
• Operations on variables:

  int counter = 0;
  counter++;
  counter--;
  

• Using constants to avoid “magic numbers”:

  const int N = 3;
  

Loops in C

• While Loop:

  int i = 3;
  while (i > 0) {
      printf("meow\n");
      i--;
  }
  

• For Loop:

  for (int i = 0; i < 3; i++) {
      printf("meow\n");
  }
  

• Introduction to do-while Loop:

  int n;
  do {
      n = get_int("Size: ");
  } while (n < 1);
  

Command Line Interface

• Introduction to basic CLI commands (e.g., ls, mv, rm, mkdir, rmdir).
• Navigation and file manipulation using CLI.

Error Handling and Design

• Examples of edge cases and handling improper inputs using loops (e.g., only accepting positive integers).
• Encouragement to write clear and maintainable code (use of comments, avoiding redundancy).
• Concepts of scope and variable lifetime.

Limitations in Computing

• Integer Overflow: when values exceed maximum storage bounds:

  Overflow example: int x = 2147483647; // Max int
  x++; // Results in overflow to -2147483648
  

• Floating Point Imprecision:

  float y = 1.0 / 3.0;
  printf("%.20f\n", y); // Reveals imprecision in representation
  

• Real-world problems illustrated with historical bugs (e.g., Y2K, 2038 problem).

Real-world Programming Considerations

• Examples of integer overflow and precision issues in gaming (Pac-Man, Donkey Kong) and larger systems (Boeing 787).
• Takeaways: Awareness of hardware limitations, importance of good design, and anticipation of edge cases.

Conclusion

• Encouragement that making mistakes is part of the learning process.
• Preview of upcoming topics and problem sets.

Welcome to CS50! 🎉