CS50 Lecture - Week 4

Introduction

• Instructor: David J. Malan
• Topics: Deeper look into how images and more are implemented using binary (0s and 1s) and how to manipulate memory using C.

Image Representation & Pixels

• Week 0 Recap: Images are represented as a grid of pixels, each pixel defined by a bit pattern for its color.
• Hollywood Myth: Enhancing images has limits due to pixelation.
• Bitmap: A type of image, represented by a grid of bits where each bit can represent a color (e.g., 1 for white, 0 for black).
• Bitmap Example: Smiley face representation by a pattern of bits.

RGB Representation

• RGB System: Red, Green, Blue values combined to create any color.
• Photoshop Example: RGB (0,0,0) for black, RGB (255,255,255) for white, and corresponding high values for primary colors like red, green, and blue.

Hexadecimal System

• Basics: A base-16 system using digits 0-9 and letters A-F.
• Counting in Hex: Demonstration of counting from 0 to 255 in hex (00 to FF).
• Hex in Memory: Simplifies expressing binary values and memory addresses (e.g., 0x prefix for hex values).

Memory and Pointers

• Introduction to Pointers: Variables that contain the address of another variable.
• Operators: & (address of) and * (dereference/goto) operators.
• Pointer Example: Demonstrating how int *p = &n; and *p = 50; works.

Strings and Memory

• Understanding Strings: Strings in C are char*, pointers to the first character in a sequence of characters terminated by a NULL character (\0).
• Use of &operator and *operator: How to access and modify string characters using pointer arithmetic.

Pointer Arithmetic

• Demonstration: Use of s as a pointer and *s to access the value. Pointer arithmetic examples using strings.
• Syntactic Sugar: Using the array notation (s[i]) vs. pointer arithmetic.
• Importance of Null-Terminator: Implications of missing null-terminator.

String Comparison in C

• STR Compare: Using strcmp to compare strings instead of == due to addressing issue.
• Demo: Comparison example and debugging using print and printf.

Memory Allocation in C

• Dynamic Memory Allocation: Using malloc for dynamic allocation and free for deallocation.
• Examples: Correctly copying strings using malloc and strcpy.
• Best Practices: Always checking for NULL and freeing allocated memory.

Common Bugs in C

• Common Errors: Segmentation faults, buffer overflows, and memory leaks.
• Tools: valgrind for detecting memory errors and leaks.
• Example Fixes: Correctly indexing arrays, freeing memory, and ensuring proper memory allocations.

Functions and Scope

• Swap Function: Example of passing by value vs. passing by reference using pointers.
• Memory Segments: Explanation of stack, heap, and where variables and functions live in memory.
• Examples: Correct implementation of the swap function and troubleshooting common related errors.

Conclusion

• Summary: Review of today's key concepts: images, memory, pointers, dynamic memory allocation, and string manipulation.
• Next Steps: Apply these concepts in problem sets and specific applications like manipulating image pixels for filters.