C programs- MOST USEFUL


1. Hello World
A simple program to print "Hello, World!" on the screen, demonstrating the basic structure of a C program.


```c
#include <stdio.h>

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

2. Factorial of a Number
Calculates the factorial of a given number using a loop.


```c
#include <stdio.h>

int main() {
    int n, i;
    unsigned long long factorial = 1;

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

    if (n < 0)
        printf("Factorial of a negative number doesn't exist.\n");
    else {
        for (i = 1; i <= n; ++i) {
            factorial *= i;
        }
        printf("Factorial of %d = %llu\n", n, factorial);
    }

    return 0;
}
```

 3. Palindrome Checker
Checks whether a given number is a palindrome.


```c
#include <stdio.h>

int main() {
    int number, originalNumber, reversedNumber = 0, remainder;

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

    originalNumber = number;

    while (number != 0) {
        remainder = number % 10;
        reversedNumber = reversedNumber * 10 + remainder;
        number /= 10;
    }

    if (originalNumber == reversedNumber) {
        printf("%d is a palindrome.\n", originalNumber);
    } else {
        printf("%d is not a palindrome.\n", originalNumber);
    }

    return 0;
}
```


4. Prime Number Checker
Checks whether a given number is prime.


```c
#include <stdio.h>
#include <stdbool.h>

bool isPrime(int num) {
    if (num <= 1) return false;
    for (int i = 2; i <= num / 2; i++) {
        if (num % i == 0) return false;
    }
    return true;
}

int main() {
    int number;

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

    if (isPrime(number)) {
        printf("%d is a prime number.\n", number);
    } else {
        printf("%d is not a prime number.\n", number);
    }

    return 0;
}
```

 
5. Fibonacci Series
Prints the Fibonacci series up to a given number of terms.


```c
#include <stdio.h>

int main() {
    int n, t1 = 0, t2 = 1, nextTerm;

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

    printf("Fibonacci Series: ");

    for (int i = 1; i <= n; ++i) {
        printf("%d ", t1);
        nextTerm = t1 + t2;
        t1 = t2;
        t2 = nextTerm;
    }

    printf("\n");

    return 0;
}
```

 6.Sum of Digits
Calculates the sum of the digits of a given number.


```c
#include <stdio.h>

int main() {
    int number, sum = 0, remainder;

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

    while (number != 0) {
        remainder = number % 10;
        sum += remainder;
        number /= 10;
    }

    printf("Sum of digits: %d\n", sum);

    return 0;
}
```

7. Reverse a String
Reverses a given string.


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

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

    printf("Enter a string: ");
    gets(str);

    j = strlen(str) - 1;

    for (i = 0; i < j; i++, j--) {
        temp = str[i];
        str[i] = str[j];
        str[j] = temp;
    }

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

    return 0;
}
```


 8. Simple Calculator
A simple calculator that performs basic arithmetic operations.


```c
#include <stdio.h>

int main() {
    char operator;
    double firstNumber, secondNumber;

    printf("Enter an operator (+, -, *, /): ");
    scanf("%c", &operator);
    printf("Enter two operands: ");
    scanf("%lf %lf", &firstNumber, &secondNumber);

    switch (operator) {
        case '+':
            printf("%.2lf + %.2lf = %.2lf\n", firstNumber, secondNumber, firstNumber + secondNumber);
            break;
        case '-':
            printf("%.2lf - %.2lf = %.2lf\n", firstNumber, secondNumber, firstNumber - secondNumber);
            break;
        case '*':
            printf("%.2lf * %.2lf = %.2lf\n", firstNumber, secondNumber, firstNumber * secondNumber);
            break;
        case '/':
            if (secondNumber != 0)
                printf("%.2lf / %.2lf = %.2lf\n", firstNumber, secondNumber, firstNumber / secondNumber);
            else
                printf("Error! Division by zero.\n");
            break;
        default:
            printf("Error! Operator is not correct.\n");
    }

    return 0;
}
```

9. Bubble Sort
Sorts an array of integers using the Bubble Sort algorithm.


```c
#include <stdio.h>

void bubbleSort(int array[], int size) {
    for (int step = 0; step < size - 1; ++step) {
        for (int i = 0; i < size - step - 1; ++i) {
            if (array[i] > array[i + 1]) {
                int temp = array[i];
                array[i] = array[i + 1];
                array[i + 1] = temp;
            }
        }
    }
}

int main() {
    int array[100], n;

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

    printf("Enter %d integers: ", n);
    for (int i = 0; i < n; i++) {
        scanf("%d", &array[i]);
    }

    bubbleSort(array, n);

    printf("Sorted array: ");
    for (int i = 0; i < n; i++) {
        printf("%d ", array[i]);
    }
    printf("\n");

    return 0;
}
```

10. Matrix Multiplication
Multiplies two matrices and displays the result.


```c
#include <stdio.h>

int main() {
    int a[10][10], b[10][10], result[10][10];
    int r1, c1, r2, c2, i, j, k;

    printf("Enter rows and columns for first matrix: ");
    scanf("%d %d", &r1, &c1);

    printf("Enter rows and columns for second matrix: ");
    scanf("%d %d", &r2, &c2);

    if (c1 != r2) {
        printf("Error! Column of first matrix not equal to row of second matrix.\n");
        return 0;
    }

    printf("Enter elements of first matrix:\n");
    for (i = 0; i < r1; ++i) {
        for (j = 0; j < c1; ++j) {
            scanf("%d", &a[i][j]);
        }
    }

    printf("Enter elements of second matrix:\n");
    for (i = 0; i < r2; ++i) {
        for (j = 0; j < c2; ++j) {
            scanf("%d", &b[i][j]);
        }
    }

    for (i = 0; i < r1; ++i) {
        for (j = 0; j < c2; ++j) {
            result[i][j] = 0;
            for (k = 0; k < c1; ++k) {
                result[i][j] += a[i][k] * b[k][j];
            }
        }
    }

    printf("Resultant matrix:\n");
    for (i = 0; i < r1; ++i) {
        for (j = 0; j < c2; ++j) {
            printf("%d ", result[i][j]);
        }
        printf("\n");
    }

    return 0;
}
```

These programs cover a wide range of topics and concepts, providing a solid foundation for understanding C programming.

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