Introduction to C Programming Language

1. What is C Language?

C is a general-purpose, procedural programming language developed in the early 1970s by Dennis Ritchie at Bell Labs. It was originally designed for implementing the UNIX operating system. C is one of the most widely used programming languages of all time, and its influence is evident in many modern languages like C++, Java, and C#.

Why Learn C?

• Foundation for other programming languages
• Efficient and fast execution
• Close to hardware (system programming)
• Portable across different platforms
• Structured programming language
• Rich standard library
• Widely used in embedded systems
• Helps understand computer architecture

2. History of C

3. Key Features of C

4. Basic Structure of a C Program

// Preprocessor directives

#include <stdio.h>

#define MAX_VALUE 100


// Global declarations

int global_var = 10;


// Function prototypes

int add_numbers(int a, int b);


// Main function - program entry point

int main() {

    // Local variables

    int a = 5, b = 10;

    int result;


    // Statements

    result = add_numbers(a, b);


    // Output

    printf("The sum is: %d\n", result);


    return 0;

}


// User-defined functions

int add_numbers(int a, int b) {

    return a + b;

}
                
            

Components of a C Program:

• Preprocessor Directives: Instructions for the compiler that begin with #
• Global Declarations: Variables and functions accessible throughout the program
• Main Function: The entry point of every C program
• Local Variables: Variables declared inside functions
• Statements: Instructions that perform operations
• User-defined Functions: Functions created by the programmer

5. Data Types

Data types define the type of data a variable can hold. C has several built-in data types:

Basic Data Types:

Type Modifiers:

Modifiers can alter the meaning of basic data types:

• signed: Can store both positive and negative values (default)
• unsigned: Can store only positive values (0 and above)
• short: Reduces the size of int (at least 2 bytes)
• long: Increases the size of int (at least 4 bytes)

#include <stdio.h>


int main() {

    char grade = 'A';

    int age = 25;

    float height = 5.9;

    double salary = 50000.50;


    printf("Grade: %c\n", grade);

    printf("Age: %d\n", age);

    printf("Height: %.1f\n", height);

    printf("Salary: %.2f\n", salary);


    return 0;

}
                    
                

6. Variables and Constants

Variables:

Variables are named memory locations that store data which can be changed during program execution.

// Variable declaration

int count;


// Variable initialization

count = 10;


// Declaration and initialization

float price = 99.99;


// Multiple variables

int x = 5, y = 10, z = 15;
                
            

Rules for naming variables:

• Can contain letters, digits, and underscores
• Must begin with a letter or underscore
• Cannot use C keywords as variable names
• Are case-sensitive (age, Age, and AGE are different)

Constants:

Constants are fixed values that cannot be changed during program execution.

// Using const keyword

const float PI = 3.14159;


// Using #define preprocessor

#define MAX_SIZE 100


// Literal constants

int days_in_week = 7;

char first_letter = 'A';
                
            

7. Operators

Operators are symbols that perform operations on variables and values.

Arithmetic Operators:

Relational Operators:

Logical Operators:

#include <stdio.h>


int main() {

    int a = 10, b = 5;


    // Arithmetic operations

    printf("a + b = %d\n", a + b);

    printf("a - b = %d\n", a - b);

    printf("a * b = %d\n", a * b);

    printf("a / b = %d\n", a / b);

    printf("a %% b = %d\n", a % b);


    // Relational operations

    printf("a == b: %d\n", a == b);

    printf("a > b: %d\n", a > b);


    // Logical operations

    printf("(a > 0) && (b > 0): %d\n", (a > 0) && (b > 0));


    return 0;

}
                    
                

8. Control Statements

Control statements determine the flow of execution in a program.

Conditional Statements:

if statement:

if (condition) {

    // code to execute if condition is true

}
                
            

if-else statement:

if (condition) {

    // code to execute if condition is true

} else {

    // code to execute if condition is false

}
                
            

switch statement:

switch (expression) {

    case constant1:

        // code block

        break;

    case constant2:

        // code block

        break;

    default:

        // default code block

}
                
            

Looping Statements:

for loop:

for (initialization; condition; increment) {

    // code to be executed

}
                
            

while loop:

while (condition) {

    // code to be executed

}
                
            

do-while loop:

do {

    // code to be executed

} while (condition);
                
            

#include <stdio.h>


int main() {

    int i;


    // if-else example

    int num = 10;

    if (num > 0) {

        printf("%d is positive\n", num);

    } else {

        printf("%d is not positive\n", num);

    }


    // for loop example

    printf("Counting from 1 to 5: ");

    for (i = 1; i <= 5; i++) {

        printf("%d ", i);

    }

    printf("\n");


    // while loop example

    int count = 5;

    printf("Countdown: ");

    while (count > 0) {

        printf("%d ", count);

        count--;

    }

    printf("\n");


    return 0;

}
                    
                

9. Functions

Functions are self-contained blocks of code that perform a specific task.

Function Definition:

return_type function_name(parameter_list) {

    // function body

    return value; // if return_type is not void

}
                
            

Types of Functions:

• Library Functions: Predefined functions (e.g., printf(), scanf())
• User-defined Functions: Functions created by the programmer

#include <stdio.h>


// Function declaration (prototype)

int add(int a, int b);


int main() {

    int num1 = 5, num2 = 10;

    int sum;


    // Function call

    sum = add(num1, num2);


    printf("Sum of %d and %d is %d\n", num1, num2, sum);


    return 0;

}


// Function definition

int add(int a, int b) {

    return a + b;

}
                    
                

10. Input/Output Operations

Output Functions:

printf():

printf("Hello, World!\n");

printf("Number: %d, Float: %.2f, Character: %c\n", 10, 3.14, 'A');
                
            

putchar():

putchar('A');  // Outputs a single character
                
            

puts():

puts("Hello, World!");  // Outputs a string and adds a newline
                
            

Input Functions:

scanf():

int age;

float height;

char name[50];


printf("Enter your age: ");

scanf("%d", &age);


printf("Enter your height: ");

scanf("%f", &height);


printf("Enter your name: ");

scanf("%s", name);
                
            

getchar():

char ch;

printf("Enter a character: ");

ch = getchar();  // Reads a single character
                
            

gets() and fgets():

char str[100];


// Unsafe - avoid using gets()

// gets(str);


// Safe alternative

printf("Enter a string: ");

fgets(str, sizeof(str), stdin);
                
            

11. Your First C Program

#include <stdio.h>


int main() {

    printf("Hello, World!\n");

    return 0;

}
                    
                

#include <stdio.h>


int main() {

    char operator;

    double num1, num2, result;


    printf("Enter an operator (+, -, *, /): ");

    scanf("%c", &operator);


    printf("Enter two numbers: ");

    scanf("%lf %lf", &num1, &num2);


    switch(operator) {

        case '+':

            result = num1 + num2;

            break;

        case '-':

            result = num1 - num2;

            break;

        case '*':

            result = num1 * num2;

            break;

        case '/':

            if (num2 != 0) {

                result = num1 / num2;

            } else {

                printf("Error: Division by zero\n");

                return 1;

            }

            break;

        default:

            printf("Error: Invalid operator\n");

            return 1;

    }


    printf("Result: %.2lf %c %.2lf = %.2lf\n", num1, operator, num2, result);


    return 0;

}
                    
                

12. Compilation Process

The C compilation process converts human-readable source code into machine-executable code.

Steps in Compilation:

1. Preprocessing: Handles preprocessor directives (#include, #define, etc.)
2. Compilation: Converts preprocessed code to assembly language
3. Assembly: Converts assembly code to machine code (object code)
4. Linking: Combines object files with libraries to create an executable

Compiling with GCC:

# Compile and link in one step

gcc program.c -o program


# Step-by-step compilation

gcc -E program.c -o program.i  # Preprocessing

gcc -S program.i -o program.s  # Compilation

gcc -c program.s -o program.o  # Assembly

gcc program.o -o program       # Linking


# Run the program

./program
                
            

13. Best Practices