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CS50

Introduction

This repository is to share my submissions, alternate implementations, and notes for 2023 and 2024 iterations of Harvard's CS50 project. This work is done as part of the Q1, 2024, C study group organised by the R Core contributors team.

Other Repositories from the R-core Bookclub: c study group 2024

  1. Ella Kaye
  2. Jim Gardner
  3. C-study-group official

Lecture 1 Takeaways

Main problem featured Luhn's Algorithm Main learning outcomes were understanding data types, magic numbers, boolean expressions. looping, conditionals.

C language semantics will truncate any double or float assigned/cast to an integer. This built in feature was highlighted in the learning materials and expected be exploited in the problem set. provides a trunc function out of the box. Which avoids some gotchas that relying on language semantics has. Also, trunc and floor, another function provided by "math.h", are almost identical except in the case of negative numbers which can lead to another type of gotcha when the full domain of reals is possible (edited)

Lecture 2 Takeaways

In the Section tutorial the instructor misspoke and asserted that if the initialiser of an array is incomplete then the uninitialised part of the array would be filled with garbage values. In the C99 standard though they would be set to 0:

If there are fewer initializers in a brace-enclosed list than there are elements or members of an aggregate, or fewer characters in a string literal used to initialize an array of known size than there are elements in the array, the remainder of the aggregate shall be initialized implicitly the same as objects that have static storage duration.

Arguments by functions are passed by value except in the case of arrays where they are passed by reference.

Variables can have either local scope or global scope.

Command line arguments and their use.

Topics covered in the lecture and the problem sets were: quantifying readability of texts and we implementated the Coleman-Liao index; and ciphers and the use of key to enchipher text.

Lecture 3 Takeaways

Introduced Big Oh and Big &Omega notation for worst case and best case

Glossary

Compiling

This term is used loosely to encompass four sequential steps:

  1. Preprocessing -- executes thee preprocessor directives
  2. Compiling -- compiles the source code into assembly language
  3. Assembling -- the actual step that produces machine code for each compilation unit
  4. Linking -- links the assembled compilation units together

Arrays

  1. Arrays are declared with a type, identifier, and array size in square brackets. They are stored in row major order in memory.
  2. Since C99 can also use designated initializers for array elements and those not specified are set to zero.

Operators

  1. =: Is either a) an Initialiser when part of a declaration or b) assignment operator when part of an expression.
  2. +=, -=, *=, /=, %=: Compound Assignment Operators are at a very low precedence level. eg. a *= 2 + 3 evaluates to a = a*5

Preprocessor Directive (AKA macro)

  1. #include filename - inserts the contents of filename into this file
  2. #define macroname definition - creates a symbolic constant macroname and replaces each subsequent instance of it with definition
  3. The directives are not supposed to be terminated with ; semicolons.

Typecasting

  1. Typecasting is used to temporarily covert an expression to a value to compute the correct result. eg float a = (float) 5U or long b = (long)(10 + b) where be is a variable of type int.

Functions

  1. Function Prototypes are a type of declaration that allows a function to be used before it is defined.
  2. Functions consist of an identifier a return type and an agrgument list. eg int power(int a) {return a * a;}

Function Parameters

  1. Typically "passed by value"
  2. Except arrays which are "passed by reference"

Struct

CS50 prefers the use of the following style definition of a struct:

typedef
struct
{
    string name; // member of the struct
    int votes;
}
candidate;

It think I still prefer the more verbose: ```c struct candidate { string name; int votes; };