C Language – Basics

Introduction to C

C is a general-purpose, procedural programming language developed by Dennis Ritchie at Bell Labs in 1972. It is one of the most widely used programming languages and serves as the foundation for many other languages including C++, Java, and Python. C is known for its efficiency, portability, and low-level access to memory.

Why C is Used

• System Programming: Used in operating systems, device drivers, and embedded systems
• Performance: Provides direct access to memory and hardware resources
• Portability: Code can run on different platforms with minimal modifications
• Foundation Language: Understanding C helps learn other languages easily
• Industry Standard: Widely used in software development and system programming

Features of C

• Simple and efficient
• Portable across different platforms
• Rich library functions
• Supports modular programming
• Fast execution speed
• Low-level memory manipulation
• Structured programming approach

Structure of a C Program

#include <stdio.h>   // Preprocessor directive

int main() {        // Main function
    printf("Hello, World!\n");
    return 0;       // Return statement
}

Components:

• Preprocessor Directives: #include, #define
• Main Function: Entry point of the program
• Statements: Instructions to be executed
• Comments: // for single line, /* */ for multiple lines

Variables & Data Types

Basic Data Types:

• int: Integer values (e.g., -10, 0, 100)
• float: Floating-point numbers (e.g., 3.14, -2.5)
• double: Double precision floating-point
• char: Single character (e.g., 'A', 'z', '5')
• void: No value

Variable Declaration:

int age;
float salary;
char grade;
int num1, num2, sum;

Variable Initialization:

int age = 25;
float salary = 50000.50;
char grade = 'A';

Input/Output

Output Function - printf():

printf("Hello, World!\n");
printf("Age: %d\n", age);
printf("Salary: %.2f\n", salary);
printf("Grade: %c\n", grade);

Format Specifiers:

• %d: Integer
• %f: Float
• %c: Character
• %s: String
• %lf: Double

Input Function - scanf():

int num;
scanf("%d", &num);

float value;
scanf("%f", &value);

char ch;
scanf(" %c", &ch);

Operators

Arithmetic Operators:

• + : Addition
• - : Subtraction
• * : Multiplication
• / : Division
• % : Modulus (remainder)

Relational Operators:

• == : Equal to
• != : Not equal to
• < : Less than
• > : Greater than
• <= : Less than or equal to
• >= : Greater than or equal to

Logical Operators:

• && : Logical AND
• || : Logical OR
• ! : Logical NOT

Assignment Operators:

• = : Simple assignment
• += : Add and assign
• -= : Subtract and assign
• *= : Multiply and assign
• /= : Divide and assign

Conditional Statements

if Statement:

if (condition) {
    // statements
}

if-else Statement:

if (condition) {
    // statements if true
} else {
    // statements if false
}

if-else if-else Statement:

if (condition1) {
    // statements
} else if (condition2) {
    // statements
} else {
    // statements
}

Switch Statement:

switch (expression) {
    case value1:
        // statements
        break;
    case value2:
        // statements
        break;
    default:
        // statements
}

Looping Constructs

for Loop:

for (initialization; condition; increment/decrement) {
    // statements
}

// Example:
for (int i = 0; i < 10; i++) {
    printf("%d ", i);
}

while Loop:

while (condition) {
    // statements
}

// Example:
int i = 0;
while (i < 10) {
    printf("%d ", i);
    i++;
}

do-while Loop:

do {
    // statements
} while (condition);

// Example:
int i = 0;
do {
    printf("%d ", i);
    i++;
} while (i < 10);

Arrays

Array Declaration:

int arr[5];           // Declaration
int arr[5] = {1, 2, 3, 4, 5};  // Initialization

Array Access:

arr[0] = 10;        // First element
arr[4] = 50;        // Fifth element
printf("%d", arr[2]);  // Print third element

Multi-dimensional Arrays:

int matrix[3][3];    // 3x3 matrix
matrix[0][0] = 1;   // Access element

Strings

String Declaration:

char str[50];                    // Character array
char str[] = "Hello";           // String literal
char str[10] = {'H', 'e', 'l', 'l', 'o', '\0'};

String Functions:

• strlen(): Calculate string length
• strcpy(): Copy string
• strcat(): Concatenate strings
• strcmp(): Compare strings

#include <string.h>

char str1[20] = "Hello";
char str2[20] = "World";
strcpy(str1, str2);  // Copy
strcat(str1, str2);  // Concatenate
int result = strcmp(str1, str2);  // Compare

Functions

Function Syntax:

return_type function_name(parameters) {
    // function body
    return value;
}

// Example:
int add(int a, int b) {
    return a + b;
}

Function Types:

• Built-in Functions: printf(), scanf(), strlen()
• User-defined Functions: Created by programmer
• Functions with return value
• Functions without return value (void)

Pointers

Pointer Declaration:

int *ptr;        // Pointer to integer
int num = 10;
ptr = #      // Store address of num

printf("%d", *ptr);  // Dereference: prints 10
printf("%p", ptr);   // Prints address

Pointer Operations:

• &: Address of operator
• *: Dereference operator
• Pointer arithmetic (+, -)

Pointer to Array:

int arr[5] = {1, 2, 3, 4, 5};
int *ptr = arr;  // Points to first element
printf("%d", *(ptr + 2));  // Access third element

Structures

Structure Definition:

struct Student {
    int roll_no;
    char name[50];
    float marks;
};

// Structure variable declaration
struct Student s1;
s1.roll_no = 101;
s1.marks = 85.5;
strcpy(s1.name, "John");

Structure with Pointers:

struct Student *ptr;
ptr = &s1;
printf("%d", ptr->roll_no);  // Access using arrow operator
printf("%d", (*ptr).roll_no);  // Alternative method

File Handling

File Operations:

• fopen(): Open a file
• fclose(): Close a file
• fprintf(): Write formatted data to file
• fscanf(): Read formatted data from file
• fread()/fwrite(): Binary file operations

Example:

#include <stdio.h>

FILE *fp;
fp = fopen("file.txt", "w");  // Open for writing
if (fp != NULL) {
    fprintf(fp, "Hello, World!");
    fclose(fp);
}

fp = fopen("file.txt", "r");  // Open for reading
char buffer[100];
if (fp != NULL) {
    fscanf(fp, "%s", buffer);
    printf("%s", buffer);
    fclose(fp);
}

File Modes:

• "r": Read mode
• "w": Write mode (overwrites existing)
• "a": Append mode
• "r+": Read and write
• "rb": Read binary mode
• "wb": Write binary mode