Introduction to C Programming

• These are study notes on the basics of C programming

Introduction to a C Program

• #include <stdio.h> calls the input/output default library, containing basic utilities for program execution
• main() is a required function and MUST be int main(); programs without this function will not run
• return 0; in main() is optional but recommended
• Text or numbers can be printed in the terminal using the printf() function
• printf("text you want to print"); is the syntax for this function
• A semicolon (;) must be at the end of commands
• Text must be within double quotes (")

Variables

• Variables are data stored in the computer's memory
• The value of a variable can be changed
• Constants cannot have their values changed
• Variables are needed to perform calculations and counting

Declaring Variables

• Variables need a type and a name to be declared
• type defines the data type (integer, float, character)
• name is how the variable is called, following certain rules
• Optionally, a variable can be given a value upon declaration

Common Mistakes Naming Variables

• Using a reserved keyword as the variable name
• Starting a variable name with a number
• Using special characters like $, @, +, -, *, /, #, %, &

Printing Variables

• The printf() function is used to print variables to the terminal
• printf("%placeholder", variable_name); is the basic syntax
• Placeholders fill in the value of variables
• Use different placeholders depending on the variable type
  • %d for integers
  • %f for floats
  • %c for characters
  • %s for strings
• Placeholders must be in between double quotes (")
• Multiple variables can be declared at once and will be filled in order into the respective placeholders

Entering Data from Keyboard

• The scanf() function is used to input a number or character from the keyboard
• scanf("%placeholder", &variable_name); is the syntax
• The variable must be declared before inputting it
• The placeholder should match the data type
• The “&” represents the address of the variable in computer memory

Data Types

• Each data type has its meaning and capacity
• Common data types include
  • int for integers
    • Takes up 2 or 4 bytes (16 or 32 bits)
  • float for real numbers
    • Takes up 4 bytes (32 bits)
  • char for characters
    • Takes up 1 byte (8 bits)
  • long long int
    • Takes up 8 bytes (64 bits)
  • double
    • Takes up 8 bytes (64 bits)
  • void, refers to an empty data type and cannot be declared

Operators

• Operators are math functions (+, -, *, /) that act on operands
• Operations are similar to standard math

Basic Operations

• + is addition
• - is subtraction
• * is multiplication
• / is division
• % finds the remainder after division (modulo)
• The modulo operation can determine if a number is odd or even

Compare Operations

• These operations return 1 if true or 0 if false
• == tests for equality
• > is greater than
• < is less than
• != tests for inequality
• >= is greater than or equal to
• <= is less than or equal to
• == compares, while = assigns

Logical Operations

• These operations return 1 if true or 0 if false
• && is the logical AND operator, requiring both operands to be true
• || is the logical OR operator, requiring either operand to be true
• ! is the logical NOT operator, reversing the logical state

Assignments Operations

• These operations reduce redundancy and repetition
• a += b; is equivalent to a = a + b;
• a -= b; is equivalent to a = a - b;
• a *= b; is equivalent to a = a * b;
• a /= b; is equivalent to a = a / b;
• a %= b; is equivalent to a = a % b;

Increments / Decrements

• For increments/decrements of 1, there are special cases
  • a++; is equivalent to a = a + 1; or a += 1;
  • a--; is equivalent to a = a - 1; or a -= 1;

Types of Increment

• There are two types
  • Preincrement: increase first, execute after
  • Postincrement: execute first, increase after

Conditional (If / Else)

• Conditional statements (if / else / else if) execute code lines if a condition is met
• When condition == 1 (true), the dothis; block executes
• When condition == 0 (false), the dothat; block executes
• The condition must be inside parentheses ()

Loops

• Loops repeat a block of code until a condition is met

For Loop

• Counting loops are used when the number of loops is limited and known
• for (declaration; condition; increment) { things_to_do; } is the syntax to define a for loop
  • declaration declares a variable for calculations
  • condition runs if true. Once the condition is false, the loop exits
  • increment/decrement is the action after each loop, affecting when the loop stops executing

While Loop

• Conditional loops are used when it is unknown when the loop will stop
• while (condition) { things_to_do; } is the syntax for a while loop
  • The condition is checked at the beginning of each loop
  • The code runs while true. Once the condition is false, the loop exits
• Code in the while loop should lead to stopping the loop

Which Loop to Use

• If the exact amount of loops is known, use a for loop
• If the stopping point is unknown, use a while loop

Functions

• Functions break down large programs into smaller parts or modularize sections of code
• data_type func_name(parameters) { things_to_do; return; } is the structure
  • data_type is the type the function returns
  • func_name is the name of the function
  • parameters is what we send into the function
• Functions should be defined outside the main() function

Types of Functions

• Mathematical functions will return a result
• Non-returning functions only do specific tasks and must have a data type void
• Recursive functions will call themselves

Array

• An array is a collection of elements of the same type.

Array Declaration and Initialization

• There are three ways
• int arr1[] = {1,2,3}; //Create an array with just big enough to hold the elements
• int arr2[5]; //Create an array with 5 unknown elements
• int arr3[6] = {1,2,3,4,5,6}; //Create an array with 6 known elements

Access Array Values

• To access variables A[i] where A is the array and I am the index for the value
• First entry has index 0
• Second entry has index 1
• Third entry has index 2
• etc.

Passing Arrays

• An array can be passed to a function by defining its parameters with square brackets
• To return array in a function, we use int* (which refers to Pointer):

String

• A string is a collection of letters/characters
• To declare string use char mystring[]

String Access

• Strings can be accessed like arrays with variable A[i], where A is the string and I am the index for the letter/character
• The first character is 0, the second is 1, third is 2 etc

File I/O

• Using files will reduce needing to input things over and over upon each run

Reading files

• Use command FILE *name = fopen("fileName.txt", "r");
• "r" means read
• filename is the name of what you are reading
• After, read command using command fscanf (name, "%d", &var);

Writing files.

• Use command FILE *name = fopen("fileName.txt", "w"); w means write
• Next write command using fprintf (name, "%s", str);:

Strings

• Strings are a collection of letters
• char mystring[] is the command to call astring

Pointers

• a variable containing the address of another variable
  • imagine that you have a map that shows direction to the One Piece treasure. The map is now a pointer to the variable treasure.
  • The syntax for pointer is:
  • int* pointer_name = &variable; or int *pointer_name = &variable;

Pointer Arithmetic

• When declaring a pointer as an array, it points to the first element of that array. (A [0])
• When you increase the pointer above by 1 using p++ (or by n unit using p[n]), it points to the next element: A [1]
• p [3] means p = p +3, or to be more precise, it means *(p+3) or "the value located 3 positions after the memory address pointed to by p".