CS50 Lecture: Week 2 - Memory and Ciphers

Introduction to Memory

• Learning about memory usage with C programs.
• David Malan introduces the lecture with storytime.

Storytelling with Volunteers

• Akshaya reads "One fish, two fish, red fish, blue fish" - identified as a 1st grade level text.
• Ethan reads "Congratulations. Today is your day." - identified as a 3rd grade level text.
• Mike reads an excerpt from 1984 - identified as a 10th-grade level text.
• Characteristics setting reading levels include word length, sentence structure, and punctuation.
• Algorithms can determine reading levels based on text characteristics.

Understanding Compilers

• Compiling code converts source code (*.c) to machine code (0s and 1s).
• make tool simplifies compiling.
• Clang is the compiler being used under the hood.
• Command line arguments -o for output, -l to link libraries.
• Steps in compiling: preprocessing, compiling, assembling, linking.

Debugging with Printf and Debug50

• Use printf for simple debugging; debug50 for more sophisticated debugging.
• Debugging involves setting breakpoints, stepping into functions, and inspecting variables.
• Grace Hopper popularized the term "bug" in computing.

Arrays and Strings

• Arrays store sequences of values of the same type.
• Strings are arrays of characters ending with a null character (\0).
• Arrays in C: Need to manage lengths manually, unlike other languages.
• String Library (string.h) and functions like strlen help manage strings.

Command Line Arguments

• argc: number of command line arguments.
• argv: array of strings representing those arguments.
• Programs can be designed to take input directly from the command line.

Error Handling with Exit Statuses

• Programs return exit statuses: 0 for success, non-zero for errors.
• Uses: Diagnostics, scripting, automate testing.

Introduction to Cryptography

• Plaintext: original readable message.
• Ciphertext: encrypted message, unreadable without decryption key.
• Caesar Cipher: shifts characters in the alphabet by a fixed key.
• ROT13: specific case of Caesar Cipher shifting letters by 13.
• Encryption: transformation process using a key; Decryption: reverting the process using the key.

Practical Examples and Advanced Libraries

• Handling arrays, strings and memory manually in C.
• Utilizing libraries like ctype for character type handling (e.g., toupper).
• Advanced C functions and usage, like strlen.

Conclusion

• Assignments and practical examples using these concepts.
• Distribution of rubber ducks for debugging practices.