CS50 Week 2 Lecture Notes

Introduction

• Focus on using memory in programming.
• Storytime activity to discuss reading levels and algorithms.
• Reading levels determined by word complexity, punctuation, sentence length, etc.
• Aim: Implement algorithms to determine reading levels of text.

Applications of Programming Concepts

• Will explore various domains, like cryptography.
• Cryptography: Art and science of secure communication.
• By end of class, decrypt example messages.

Compiling Code

• Make Command: Automates program creation from source code.
• Compiler: Converts source code to machine code (zeros and ones).
• Clang: C language compiler used in CS50.
• Command Line Arguments: Modify program behavior (example: clang -o hello hello.c).

Compilation Process

• Preprocessing: Replace #include directives with corresponding code.
• Compiling: Convert C code to assembly language.
• Assembling: Convert assembly code to machine code.
• Linking: Combine machine code from multiple files into one executable.

Understanding Machine Code

• Reverse engineering possible but difficult.
• Intellectual property concerns with reversing compiled software.

Debugging

• Print Debugging: printf to see variable values.
• Debugger Tool: debug50 to step through code and examine program flow.
• Breakpoints: Pause execution at specific lines.
• Rubber Duck Debugging: Explain your code logic to an inanimate object.

Data Types and Memory

• Overview of data types: int, float, char, etc.
• Memory as a canvas of bytes; variables stored in specific locations.
• Arrays: Sequence of values stored contiguously in memory.
• Strings in C: Array of characters ending with a null terminator (\0).

Arrays and Strings

• Arrays allow storing multiple items of the same type.
• Strings in C are arrays of characters with a null terminator.
• Manipulating strings involves understanding their memory representation.

Libraries and Functions

• String Library: Provides functions like strlen to get string length.
• Ctype Library: Functions like toupper to manipulate character cases.

Command Line Arguments

• argc: Argument count, number of command line inputs.
• argv: Argument vector, array of strings from the command line.
• Example: Modify behavior based on input at the command line.

Exit Status

• Programs return a status code (0 for success, non-zero for errors).
• Used for diagnosing issues and automation in testing.

Cryptography

• Caesar Cipher: Simple encryption technique by shifting alphabet letters.
• Plain Text: Original readable message.
• Cipher Text: Encrypted message.
• Key: Number used to encrypt/decrypt the message.

Conclusion

• Importance of understanding low-level details to build complex applications.
• Debugging, compiling, and encryption are essential skills.