Factorial Calculation in C Language

C language assignment along with its solution. Let's assume the assignment is about creating a program to calculate the factorial of a given number using a recursive function.

**Assignment: Factorial Calculation**

Write a C program to calculate the factorial of a positive integer using recursion. The factorial of a non-negative integer `n` is the product of all positive integers less than or equal to `n`. It is denoted by `n!`.

The factorial of `n` is defined as follows:
- `0!` is equal to `1`.
- For any positive integer `n`, `n! = n * (n-1)!`.

Your program should perform the following steps:
1. Ask the user to input a positive integer (`n`).
2. Calculate the factorial of `n` using a recursive function.
3. Display the result.

**Solution:**

```c
#include <stdio.h>

// Function to calculate factorial using recursion
int factorial(int n) {
    if (n == 0) {
        return 1;
    } else {
        return n * factorial(n - 1);
    }
}

int main() {
    int number;
    
    // Ask the user to input a positive integer
    printf("Enter a positive integer: ");
    scanf("%d", &number);

    if (number < 0) {
        printf("Error: Please enter a positive integer.\n");
    } else {
        // Calculate the factorial using the recursive function
        int result = factorial(number);

        // Display the result
        printf("The factorial of %d is %d.\n", number, result);
    }

    return 0;
}
```

**Explanation:**

1. We start by including the necessary header file `stdio.h`, which allows us to use input/output functions like `printf` and `scanf`.

2. We declare the recursive function `factorial` which takes an integer argument `n` and returns an integer value. The function calculates the factorial of `n` based on the conditions provided in the assignment.

3. In the `main` function, we declare an integer variable `number` to store the user input.

4. We ask the user to enter a positive integer and store it in the `number` variable using `scanf`.

5. We then check if the entered number is less than 0. If it is, we display an error message, indicating that the number should be positive.

6. If the entered number is non-negative (positive or zero), we call the `factorial` function with the entered number as an argument, and the result is stored in the variable `result`.

7. Finally, we display the calculated factorial using `printf`.

That's it! This program will calculate the factorial of a given positive integer using recursion.