Introduction to C Programming

C is a powerful procedural programming language that forms the foundation of modern computing. This guide covers all fundamental concepts you need to master C programming.

1. Basic Structure of a C Program

Every C program follows a standard structure:

#include <stdio.h>

int main() {
    printf("Hello, World!\n");
    return 0;
}

2. Variables and Data Types

C is a statically-typed language. You must declare variables before using them.

Primitive Data Types

Type	Size	Description	Example
int	4 bytes	Integer numbers	int age = 25;
float	4 bytes	Single precision decimal	float pi = 3.14;
double	8 bytes	Double precision decimal	double price = 99.99;
char	1 byte	Single character	char grade = 'A';

Variable Declaration and Initialization

int x;          // Declaration
x = 10;         // Assignment
int y = 20;     // Declaration + Initialization

const int MAX = 100;  // Constant (cannot be changed)

3. Operators

Arithmetic Operators

int a = 10, b = 3;
int sum = a + b;        // Addition: 13
int diff = a - b;       // Subtraction: 7
int product = a * b;    // Multiplication: 30
int quotient = a / b;   // Division: 3
int remainder = a % b;  // Modulus: 1

Comparison Operators

a == b    // Equal to
a != b    // Not equal to
a > b     // Greater than
a < b     // Less than
a >= b    // Greater than or equal to
a <= b    // Less than or equal to

Logical Operators

&&    // AND
||    // OR
!     // NOT

4. Control Structures

If-Else Statements

int score = 85;

if (score >= 90) {
    printf("Grade: A\n");
} else if (score >= 80) {
    printf("Grade: B\n");
} else if (score >= 70) {
    printf("Grade: C\n");
} else {
    printf("Grade: F\n");
}

Switch Statement

char grade = 'B';

switch(grade) {
    case 'A':
        printf("Excellent!\n");
        break;
    case 'B':
        printf("Good job!\n");
        break;
    case 'C':
        printf("Average\n");
        break;
    default:
        printf("Invalid grade\n");
}

5. Loops

For Loop

// Print numbers 1 to 5
for (int i = 1; i <= 5; i++) {
    printf("%d ", i);
}
// Output: 1 2 3 4 5

While Loop

int count = 1;
while (count <= 5) {
    printf("%d ", count);
    count++;
}

Do-While Loop

int num = 1;
do {
    printf("%d ", num);
    num++;
} while (num <= 5);

6. Functions

Functions allow you to organize code into reusable blocks.

Function Declaration and Definition

// Function declaration (prototype)
int add(int a, int b);

// Function definition
int add(int a, int b) {
    return a + b;
}

// Function call
int main() {
    int result = add(5, 3);
    printf("Sum: %d\n", result);
    return 0;
}

Void Functions

void greet(char name[]) {
    printf("Hello, %s!\n", name);
}

int main() {
    greet("Alice");
    return 0;
}

Recursion

// Factorial using recursion
int factorial(int n) {
    if (n == 0 || n == 1) {
        return 1;
    }
    return n * factorial(n - 1);
}

int main() {
    printf("5! = %d\n", factorial(5));  // Output: 120
    return 0;
}

7. Arrays

One-Dimensional Arrays

// Declaration and initialization
int numbers[5] = {10, 20, 30, 40, 50};

// Accessing elements
printf("First element: %d\n", numbers[0]);

// Iterating through array
for (int i = 0; i < 5; i++) {
    printf("%d ", numbers[i]);
}

Two-Dimensional Arrays

int matrix[3][3] = {
    {1, 2, 3},
    {4, 5, 6},
    {7, 8, 9}
};

// Accessing elements
printf("Element at [1][2]: %d\n", matrix[1][2]);  // Output: 6

8. Pointers

Pointers are one of the most powerful features in C. They store memory addresses.

Basic Pointer Usage

int x = 10;
int *ptr = &x;  // ptr stores the address of x

printf("Value of x: %d\n", x);
printf("Address of x: %p\n", &x);
printf("Value stored in ptr: %p\n", ptr);
printf("Value pointed to by ptr: %d\n", *ptr);

Pointer Arithmetic

int arr[5] = {10, 20, 30, 40, 50};
int *p = arr;

printf("%d\n", *p);      // 10
printf("%d\n", *(p+1));  // 20
printf("%d\n", *(p+2));  // 30

Pointers and Functions

// Pass by reference using pointers
void swap(int *a, int *b) {
    int temp = *a;
    *a = *b;
    *b = temp;
}

int main() {
    int x = 5, y = 10;
    swap(&x, &y);
    printf("x = %d, y = %d\n", x, y);  // x = 10, y = 5
    return 0;
}

9. Strings

In C, strings are arrays of characters terminated by a null character '\0'.

#include <string.h>

char name[20] = "Alice";
char greeting[50];

// String functions
strcpy(greeting, "Hello");       // Copy string
strcat(greeting, " World");      // Concatenate
int len = strlen(greeting);      // Get length
int cmp = strcmp("abc", "abc");  // Compare strings (0 if equal)

10. Structures

Structures allow you to group related data together.

struct Student {
    char name[50];
    int age;
    float gpa;
};

int main() {
    struct Student s1;
    
    strcpy(s1.name, "John");
    s1.age = 20;
    s1.gpa = 3.8;
    
    printf("Name: %s\n", s1.name);
    printf("Age: %d\n", s1.age);
    printf("GPA: %.2f\n", s1.gpa);
    
    return 0;
}

11. Dynamic Memory Allocation

#include <stdlib.h>

int *arr;
int n = 5;

// Allocate memory
arr = (int*)malloc(n * sizeof(int));

if (arr == NULL) {
    printf("Memory allocation failed!\n");
    return 1;
}

// Use the array
for (int i = 0; i < n; i++) {
    arr[i] = i * 10;
}

// Free memory when done
free(arr);

12. File I/O

Writing to a File

FILE *file = fopen("output.txt", "w");

if (file == NULL) {
    printf("Error opening file!\n");
    return 1;
}

fprintf(file, "Hello, File!\n");
fprintf(file, "Line 2\n");

fclose(file);

Reading from a File

FILE *file = fopen("input.txt", "r");
char buffer[100];

if (file == NULL) {
    printf("Error opening file!\n");
    return 1;
}

while (fgets(buffer, 100, file) != NULL) {
    printf("%s", buffer);
}

fclose(file);

Key Takeaways

• C is a procedural language focused on functions and structured programming
• Memory management is manual - you control allocation and deallocation
• Pointers provide powerful low-level memory access
• Always check return values and handle errors
• Use meaningful variable names and comments
• Practice defensive programming - validate inputs and check bounds