10 - 4 - 2
= (10 - 4) - 2    // Left to Right
= 6 - 2
= 4

a = b = c = 5;
// Evaluated as: a = (b = (c = 5))
// c = 5 first, then b = 5, then a = 5

factorial(n) = n × factorial(n-1),   for n > 1
factorial(0) = 1                      (base case)
factorial(1) = 1                      (base case)

#include <stdio.h>

// Recursive function to calculate factorial
int factorial(int n) {
    if (n == 0 || n == 1)       // Base condition
        return 1;
    else
        return n * factorial(n - 1);  // Recursive call
}

int main() {
    int num;
    printf("Enter a positive integer: ");
    scanf("%d", &num);

    if (num < 0)
        printf("Factorial is not defined for negative numbers.");
    else
        printf("Factorial of %d = %d\n", num, factorial(num));

    return 0;
}

Enter a positive integer: 5
Factorial of 5 = 120

factorial(5)
  = 5 × factorial(4)
        = 4 × factorial(3)
              = 3 × factorial(2)
                    = 2 × factorial(1)
                          = 1  (base case)
                    = 2 × 1 = 2
              = 3 × 2 = 6
        = 4 × 6 = 24
  = 5 × 24 = 120

int age;          // 'age' is an identifier
float salary;     // 'salary' is an identifier
void display();   // 'display' is an identifier

#include <stdio.h>

int main() {
    char str[100];
    int i;

    printf("Enter a string: ");
    gets(str);   /* read the string */

    /* Convert each character to lowercase */
    for (i = 0; str[i] != '\0'; i++) {
        /* Check if character is uppercase (A=65, Z=90 in ASCII) */
        if (str[i] >= 'A' && str[i] <= 'Z') {
            /* Add 32 to convert uppercase to lowercase */
            str[i] = str[i] + 32;
        }
    }

    printf("Lowercase string: %s\n", str);

    return 0;
}

Enter a string: Hello WORLD
Lowercase string: hello world

Data Types in C
├── Primary (Basic)
│   ├── int
│   ├── char
│   ├── float
│   ├── double
│   └── void
├── Derived
│   ├── Array
│   ├── Pointer
│   └── Function
└── User Defined
    ├── struct
    ├── union
    └── enum

#include <stdio.h>

int main() {
    int n, original, reversed = 0, remainder;

    printf("Enter an integer: ");
    scanf("%d", &n);

    original = n;

    /* Handle negative numbers - negative numbers are not palindromes */
    if (n < 0) {
        printf("%d is NOT a palindrome.\n", n);
        return 0;
    }

    /* Reverse the digits of the number */
    while (n != 0) {
        remainder = n % 10;       /* Extract last digit */
        reversed = reversed * 10 + remainder;  /* Build reversed number */
        n = n / 10;               /* Remove last digit */
    }

    /* Check if original and reversed are equal */
    if (original == reversed) {
        printf("%d is a PALINDROME.\n", original);
    } else {
        printf("%d is NOT a PALINDROME.\n", original);
    }

    return 0;
}

Enter an integer: 121
121 is a PALINDROME.

Enter an integer: 123
123 is NOT a PALINDROME.

/* Documentation Section */
// Program description, author, date

/* Link Section (Header Files) */
#include <stdio.h>
#include <conio.h>

/* Definition Section (Macros/Constants) */
#define PI 3.14159

/* Global Declaration Section */
int globalVar;

/* Main Function */
int main()
{
    /* Local Declaration */
    int a, b;

    /* Executable Statements */
    printf("Hello, World!\n");

    return 0;
}

/* User-Defined Functions */
void myFunction()
{
    // function body
}

data_type array_name[size1][size2]...[sizeN];

int matrix[m][n];  // m rows and n columns

#include <stdio.h>

int main() {
    int m, n, i, j;
    int A[10][10], B[10][10], C[10][10];

    // Input dimensions
    printf("Enter number of rows (m): ");
    scanf("%d", &m);
    printf("Enter number of columns (n): ");
    scanf("%d", &n);

    // Input elements of matrix A
    printf("\nEnter elements of Matrix A (%dx%d):\n", m, n);
    for (i = 0; i < m; i++) {
        for (j = 0; j < n; j++) {
            printf("A[%d][%d]: ", i, j);
            scanf("%d", &A[i][j]);
        }
    }

    // Input elements of matrix B
    printf("\nEnter elements of Matrix B (%dx%d):\n", m, n);
    for (i = 0; i < m; i++) {
        for (j = 0; j < n; j++) {
            printf("B[%d][%d]: ", i, j);
            scanf("%d", &B[i][j]);
        }
    }

    // Calculate sum: C = A + B
    for (i = 0; i < m; i++) {
        for (j = 0; j < n; j++) {
            C[i][j] = A[i][j] + B[i][j];
        }
    }

    // Display the result matrix C
    printf("\nSum Matrix C = A + B:\n");
    for (i = 0; i < m; i++) {
        for (j = 0; j < n; j++) {
            printf("%d\t", C[i][j]);
        }
        printf("\n");
    }

    return 0;
}

Enter number of rows (m): 2
Enter number of columns (n): 2

Enter elements of Matrix A (2x2):
A[0][0]: 1
A[0][1]: 2
A[1][0]: 3
A[1][1]: 4

Enter elements of Matrix B (2x2):
B[0][0]: 5
B[0][1]: 6
B[1][0]: 7
B[1][1]: 8

Sum Matrix C = A + B:
6    8
10   12

1
1 2 1
1 2 3 2 1
1 2 3 4 3 2 1
1 2 3 4 5 4 3 2 1

#include <stdio.h>

int main() {
    int n, i, j;

    printf("Enter the number of rows: ");
    scanf("%d", &n);

    for (i = 1; i <= n; i++) {
        /* Print increasing part: 1 to i */
        for (j = 1; j <= i; j++) {
            printf("%d ", j);
        }

        /* Print decreasing part: i-1 down to 1 */
        for (j = i - 1; j >= 1; j--) {
            printf("%d ", j);
        }

        printf("\n");
    }

    return 0;
}

1
1 2 1
1 2 3 2 1
1 2 3 4 3 2 1
1 2 3 4 5 4 3 2 1

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

// Define the structure
struct Student {
    int roll;
    char name[50];
    char address[100];
    char email[50];
    char phone[15];
};

int main() {
    int i, n;

    printf("Enter number of students: ");
    scanf("%d", &n);

    // Declare an array of structures
    struct Student s[n];

    // Input student information
    for (i = 0; i < n; i++) {
        printf("\n--- Enter details for Student %d ---\n", i + 1);

        printf("Roll Number : ");
        scanf("%d", &s[i].roll);

        printf("Name        : ");
        getchar();  // clear input buffer
        fgets(s[i].name, sizeof(s[i].name), stdin);
        s[i].name[strcspn(s[i].name, "\n")] = '\0';  // remove newline

        printf("Address     : ");
        fgets(s[i].address, sizeof(s[i].address), stdin);
        s[i].address[strcspn(s[i].address, "\n")] = '\0';

        printf("Email       : ");
        fgets(s[i].email, sizeof(s[i].email), stdin);
        s[i].email[strcspn(s[i].email, "\n")] = '\0';

        printf("Phone       : ");
        fgets(s[i].phone, sizeof(s[i].phone), stdin);
        s[i].phone[strcspn(s[i].phone, "\n")] = '\0';
    }

    // Display student information
    printf("\n========== Student Information ==========\n");
    for (i = 0; i < n; i++) {
        printf("\nStudent %d:\n", i + 1);
        printf("  Roll Number : %d\n",   s[i].roll);
        printf("  Name        : %s\n",   s[i].name);
        printf("  Address     : %s\n",   s[i].address);
        printf("  Email       : %s\n",   s[i].email);
        printf("  Phone       : %s\n",   s[i].phone);
    }

    return 0;
}

Enter number of students: 2

--- Enter details for Student 1 ---
Roll Number : 101
Name        : Ram Sharma
Address     : Kathmandu
Email       : [email protected]
Phone       : 9841000001

--- Enter details for Student 2 ---
Roll Number : 102
Name        : Sita Thapa
Address     : Pokhara
Email       : [email protected]
Phone       : 9841000002

========== Student Information ==========

Student 1:
  Roll Number : 101
  Name        : Ram Sharma
  Address     : Kathmandu
  Email       : [email protected]
  Phone       : 9841000001

Student 2:
  Roll Number : 102
  Name        : Sita Thapa
  Address     : Pokhara
  Email       : [email protected]
  Phone       : 9841000002