Learn C in 10 minutes

7 October 2025 — Programming Language

C is a powerful, general-purpose programming language known for its efficiency and close-to-hardware capabilities. This tutorial covers the fundamentals of C programming, helping you quickly understand the language.

1. Writing Your First C Program

Let’s start with the classic “Hello, World!” program. Create a file named hello.c and enter the following code:

#include <stdio.h>

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

Save the file and compile it using a C compiler like GCC:

gcc hello.c -o hello
./hello

The output will be:

Hello, World!

This simple program demonstrates C’s basic structure:

#include <stdio.h> includes the standard input/output library
int main() is the program’s entry point
printf() displays text output
return 0 indicates successful execution

2. Basic Syntax

C uses a structured syntax with semicolons to terminate statements and curly braces {} to define code blocks.

// This is a single-line comment
/* This is a multi-line comment */

#include <stdio.h>

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

Basic syntax rules in C:

Semicolons: Every statement must end with a semicolon ;
Comments: Single-line comments use //, multi-line comments use /* */
Code Blocks: Defined by curly braces {}
Case Sensitivity: C is case-sensitive (main vs Main)

3. Variables and Data Types

C is a statically typed language, meaning you must declare variable types before use.

Basic variable naming rules:

Variable names can contain letters, numbers, and underscores
Variable names cannot start with a number
Variable names are case-sensitive
C keywords cannot be used as variable names

C’s main data types:

int: Integer numbers (e.g., 42, -10)
float: Floating-point numbers (e.g., 3.14, -2.5)
double: Double-precision floating-point numbers
char: Single characters (e.g., 'A', 'z')
void: No type

int age = 25;
float temperature = 36.5;
double pi = 3.14159265359;
char grade = 'A';

3.1 Integer Types

C provides several integer types with different sizes:

char small_number = 100;        // Usually 1 byte
short medium_number = 32000;    // Usually 2 bytes
int regular_number = 1000000;   // Usually 4 bytes
long large_number = 1000000000; // Usually 4 or 8 bytes

3.2 Floating-Point Types

float single_precision = 3.14f;
double double_precision = 3.14159265359;
long double extended_precision = 3.14159265358979323846L;

3.3 Character Type

Characters are stored as integers using ASCII encoding:

char letter = 'A';
char digit = '7';
char newline = '\n';
char tab = '\t';

4. Constants

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

const int MAX_SIZE = 100;
const float PI = 3.14159;
const char NEWLINE = '\n';

#define MAX_USERS 1000
#define PI 3.14159

5. Input and Output

C uses functions from stdio.h for input and output operations.

5.1 Output with printf()

#include <stdio.h>

int main() {
    int age = 25;
    float height = 1.75;
    char name[] = "John";

    printf("Hello, %s!\n", name);
    printf("You are %d years old\n", age);
    printf("Your height is %.2f meters\n", height);

    return 0;
}

Common format specifiers:

%d - integer
%f - float/double
%c - character
%s - string
%p - pointer

5.2 Input with scanf()

#include <stdio.h>

int main() {
    int age;
    float height;
    char name[50];

    printf("Enter your name: ");
    scanf("%s", name);

    printf("Enter your age: ");
    scanf("%d", &age);

    printf("Enter your height: ");
    scanf("%f", &height);

    printf("Hello %s, you are %d years old and %.2f meters tall\n",
           name, age, height);

    return 0;
}

6. Operators

C provides a rich set of operators for various computations.

6.1 Arithmetic Operators

int a = 10, b = 3;

printf("Addition: %d\n", a + b);      // 13
printf("Subtraction: %d\n", a - b);   // 7
printf("Multiplication: %d\n", a * b); // 30
printf("Division: %d\n", a / b);      // 3
printf("Modulus: %d\n", a % b);       // 1

6.2 Comparison Operators

int x = 5, y = 10;

printf("Equal: %d\n", x == y);     // 0 (false)
printf("Not equal: %d\n", x != y); // 1 (true)
printf("Greater than: %d\n", x > y);  // 0
printf("Less than: %d\n", x < y);  // 1

6.3 Logical Operators

int a = 1, b = 0;

printf("AND: %d\n", a && b);  // 0
printf("OR: %d\n", a || b);   // 1
printf("NOT: %d\n", !a);      // 0

6.4 Bitwise Operators

unsigned int a = 5;  // 0101 in binary
unsigned int b = 3;  // 0011 in binary

printf("AND: %d\n", a & b);   // 1 (0001)
printf("OR: %d\n", a | b);    // 7 (0111)
printf("XOR: %d\n", a ^ b);   // 6 (0110)
printf("NOT: %d\n", ~a);      // depends on system
printf("Left shift: %d\n", a << 1);  // 10 (1010)
printf("Right shift: %d\n", a >> 1); // 2 (0010)

7. Control Flow

C provides several control flow statements to manage program execution.

7.1 if Statements

int age = 20;

if (age >= 18) {
    printf("Adult\n");
} else if (age >= 13) {
    printf("Teen\n");
} else {
    printf("Child\n");
}

7.2 switch Statements

int day = 3;

switch (day) {
    case 1:
        printf("Monday\n");
        break;
    case 2:
        printf("Tuesday\n");
        break;
    case 3:
        printf("Wednesday\n");
        break;
    default:
        printf("Other day\n");
}

7.3 for Loops

for (int i = 0; i < 5; i++) {
    printf("i = %d\n", i);
}

7.4 while Loops

int count = 0;
while (count < 5) {
    printf("Count: %d\n", count);
    count++;
}

7.5 do-while Loops

int count = 0;
do {
    printf("Count: %d\n", count);
    count++;
} while (count < 5);

7.6 break and continue

for (int i = 0; i < 10; i++) {
    if (i == 5) {
        break;  // Exit loop
    }
    if (i % 2 == 0) {
        continue;  // Skip even numbers
    }
    printf("i = %d\n", i);  // Output: 1, 3
}

8. Arrays

Arrays store multiple values of the same type.

8.1 One-dimensional Arrays

int numbers[5] = {1, 2, 3, 4, 5};

// Accessing elements
printf("First element: %d\n", numbers[0]);
printf("Last element: %d\n", numbers[4]);

// Modifying elements
numbers[0] = 10;

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

8.2 Multi-dimensional Arrays

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

// Accessing elements
printf("matrix[1][2] = %d\n", matrix[1][2]);  // 6

// Looping through 2D array
for (int i = 0; i < 2; i++) {
    for (int j = 0; j < 3; j++) {
        printf("%d ", matrix[i][j]);
    }
    printf("\n");
}

9. Strings

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

char greeting[] = "Hello";  // Automatically includes null terminator
char name[20] = "John";

// String functions from string.h
#include <string.h>

char str1[20] = "Hello";
char str2[20] = "World";

printf("Length: %lu\n", strlen(str1));  // 5
strcpy(str1, str2);  // Copy str2 to str1
printf("After copy: %s\n", str1);  // World

if (strcmp(str1, str2) == 0) {
    printf("Strings are equal\n");
}

10. Functions

Functions are reusable blocks of code that perform specific tasks.

10.1 Function Definition and Calling

#include <stdio.h>

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

int main() {
    int result = add(5, 3);
    printf("5 + 3 = %d\n", result);
    return 0;
}

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

10.2 Function with No Return Value

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

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

10.3 Recursive Functions

int factorial(int n) {
    if (n <= 1) {
        return 1;
    }
    return n * factorial(n - 1);
}

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

11. Pointers

Pointers are variables that store memory addresses.

11.1 Basic Pointer Usage

int number = 42;
int *ptr = &number;  // ptr stores address of number

printf("Value: %d\n", number);     // 42
printf("Address: %p\n", &number);  // Memory address
printf("Pointer value: %d\n", *ptr); // 42 (dereferencing)

// Modifying value through pointer
*ptr = 100;
printf("New value: %d\n", number);  // 100

11.2 Pointers and Arrays

int numbers[] = {1, 2, 3, 4, 5};
int *ptr = numbers;  // points to first element

printf("First element: %d\n", *ptr);        // 1
printf("Second element: %d\n", *(ptr + 1)); // 2

// Array name is essentially a pointer to first element
for (int i = 0; i < 5; i++) {
    printf("numbers[%d] = %d\n", i, *(numbers + i));
}

11.3 Pointers and Functions

void swap(int *a, int *b) {
    int temp = *a;
    *a = *b;
    *b = temp;
}

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

12. Structures

Structures allow you to group related variables together.

12.1 Defining and Using Structures

#include <stdio.h>
#include <string.h>

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

int main() {
    // Creating structure variables
    struct Student student1;

    // Assigning values
    strcpy(student1.name, "Alice");
    student1.age = 20;
    student1.gpa = 3.8;

    // Accessing structure members
    printf("Name: %s\n", student1.name);
    printf("Age: %d\n", student1.age);
    printf("GPA: %.2f\n", student1.gpa);

    return 0;
}

12.2 Structure with Pointers

struct Point {
    int x;
    int y;
};

int main() {
    struct Point p1 = {10, 20};
    struct Point *ptr = &p1;

    printf("Coordinates: (%d, %d)\n", ptr->x, ptr->y);

    return 0;
}

13. Dynamic Memory Allocation

C provides functions for dynamic memory management.

13.1 malloc, calloc, realloc, free

#include <stdio.h>
#include <stdlib.h>

int main() {
    // Allocate memory for 5 integers
    int *numbers = (int*)malloc(5 * sizeof(int));

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

    // Initialize array
    for (int i = 0; i < 5; i++) {
        numbers[i] = i * 10;
    }

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

    // Free allocated memory
    free(numbers);

    return 0;
}

13.2 Dynamic String Allocation

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

int main() {
    char *name = (char*)malloc(50 * sizeof(char));

    if (name != NULL) {
        strcpy(name, "Dynamic string");
        printf("Name: %s\n", name);
        free(name);
    }

    return 0;
}

14. File Operations

C provides functions for reading from and writing to files.

14.1 Writing to a File

#include <stdio.h>

int main() {
    FILE *file = fopen("example.txt", "w");

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

    fprintf(file, "Hello, File!\n");
    fprintf(file, "This is a test.\n");

    fclose(file);
    printf("File written successfully.\n");

    return 0;
}

14.2 Reading from a File

#include <stdio.h>

int main() {
    FILE *file = fopen("example.txt", "r");

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

    char buffer[100];

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

    fclose(file);

    return 0;
}

15. Preprocessor Directives

Preprocessor directives are processed before compilation.

15.1 #include

#include <stdio.h>    // System header file
#include "myheader.h" // User header file

15.2 #define

#define PI 3.14159
#define MAX(a, b) ((a) > (b) ? (a) : (b))

int main() {
    double area = PI * 5 * 5;
    int larger = MAX(10, 20);

    printf("Area: %.2f\n", area);
    printf("Larger number: %d\n", larger);

    return 0;
}

15.3 Conditional Compilation

#define DEBUG 1

int main() {
    #ifdef DEBUG
        printf("Debug mode enabled\n");
    #endif

    #if DEBUG == 1
        printf("Debug level 1\n");
    #elif DEBUG == 2
        printf("Debug level 2\n");
    #else
        printf("No debug\n");
    #endif

    return 0;
}

16. Error Handling

C doesn’t have built-in exception handling, so we use return values and error codes.

#include <stdio.h>
#include <errno.h>
#include <string.h>

int main() {
    FILE *file = fopen("nonexistent.txt", "r");

    if (file == NULL) {
        printf("Error opening file: %s\n", strerror(errno));
        return 1;
    }

    fclose(file);
    return 0;
}

This comprehensive C tutorial covers the essential concepts you need to start programming in C. Practice these examples and explore more advanced topics like linked lists, function pointers, and multi-file programs as you become more comfortable with the language.