CS50 Week 1: Introduction to C Programming

Overview

Today we delve into C programming, a traditional and older programming language. Although C might look cryptic initially, it builds on concepts from week zero, such as functions, loops, and conditionals. The focus is on learning problem-solving strategies, using C capabilities.

Transition from Scratch to C

• Last Week: Learned programming basics using Scratch.
• This Week: Focused on learning C programming language.
• Key Goal: Equip students with problem-solving tools week by week.

Source Code, Compilation, and Execution

• Source Code: Code written by humans in a high-level language (C in this case).
• Machine Code: Binary code (zeros and ones) that the computer executes.
• Compiler: A program that translates source to machine code.
• Compilation and Execution Steps:
  1. Writing Code: Using a text editor to write source code files (e.g., hello.c).
  2. Compiling Code: make filename converts source code to an executable.
  3. Running Code: ./filename executes the compiled machine code.

First Program in C: Hello, World!

• Example:

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

• Key Components:
  • #include <stdio.h>: Includes standard input-output library.
  • int main(void) { ... }: Main function that returns an integer.
  • printf(...): Function to print formatted output.
  • \n: Newline character.

Advanced Example: Hello with Variables

• Prompt User for Input:

  #include <cs50.h>
  #include <stdio.h>
  int main(void) {
      string name = get_string("What's your name? ");
      printf("Hello, %s\n", name);
      return 0;
  }
  

• CS50 Library Functions: get_string to get user input.

Data Types in C

• Common Types: int, float, char, string (CS50 specific)
• Examples:
  • Declaring an integer: int x = 5;
  • Declaring a float: float y = 5.5;
  • Declaring a char: char z = 'A';
  • Declaring a string: string s = "hello";

Conditionals in C

• If-Else Statement:

  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");
  }
  

• Key Points: Use of if, else if, else to handle multiple conditions, double equals == for comparison.

Loops in C

• Patterns: Repeating code multiple times using loops.
• Example: While Loop:

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

• For Loop (most common):

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

Functions in C

• Define Custom Function:

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

• Functions with Arguments and Return Values:

  int add(int a, int b) {
      return a + b;
  }
  
  int main(void) {
      int sum = add(2, 3);
      printf("%d\n", sum);  // Outputs 5
      return 0;
  }
  

Command-Line Interface (CLI)

• Common Commands:
  • ls: List files in directory
  • cd: Change directory
  • cp: Copy files
  • mv: Move (rename) files
  • rm: Remove files
• Example Usage:

  ls  // Show directory contents
  cd foldername  // Change to directory
  cp source destination  // Copy a file
  mv oldname newname  // Rename a file
  rm filename  // Delete a file
  

Floating Point Imprecision

• Limitation: Finite bits can't represent all possible decimal numbers.
• Example:

  float x = 1.0 / 10.0;
  printf("%.10f\n", x);  // Might not perfectly print 0.1000000000
  

• Precision Issues: Using larger types like double can mitigate this but not eliminate it entirely.

Real-World Applications and Issues

• Y2K and 2038 Problems: Illustrates how limitations in computing, like integer overflow, can have significant impacts.
• Notable Bugs:
  • Pac-Man: Level 256 bug due to overflow.
  • Donkey Kong: Level 22 bug due to 8-bit limit.
  • Boeing 787: Power loss after 248 days due to overflow.

Summary

• C is foundational for understanding low-level operations and managing computer memory precisely.
• Important concepts include data types, conditionals, loops, functions, and the limitations of memory representation.
• The transition from graphical (Scratch) to text-based (C) programming helps understand computational thinking deeply.

---

Next steps include diving into problem sets that necessitate the application of these concepts, gradually building proficiency in solving complex problems programmatically.