20.Write a program in c to calculate the factorial of a given number using a for loop.

Mathematical Notation
In mathematical notation, the factorial is marked with an exclamation mark "!" .
The formula for the factorial calculation n is expressed as follows:
n! = n x (n-1) x (n-2) x (n-3) x ... x 1

Factorial Program in C using For Loop
This method shows how to calculate the factorial of a number using a for loop in C.
Factorials are important mathematical operations, and this method provides a practical way to calculate it.
Program/Source Code
Here is the source code of a C program to print the factorial of a given number using a loop (iterative solver). The C program has been compiled and executed successfully. The output of the program is shown below.

#include <stdio.h>
int main()
{
 // variable fact to store factorial of number N
    int fact = 1, n;
    // Accept input
    printf("Enter the number: \n");
    scanf("%d", &n);
    // validating number  N
    if (n <= 0)
       printf(“\n Please Enter positive Number”);
    // loop or iterate N times and multiply all positive numbers
    else
    {
        for (int i=1;i<=n;i++)
        {
            fact=fact*i;
        }
    }
    // Print the factorial fact.
    printf("\nFactorial of %d = %5d\n", n, fact);

 return(0);
}

Output
Please Enter positive Number
5
Factorial of 5 =120
Program Explanation
1. input any number and store it in "n".
2. Check if the number is valid or not.
3. If the number is invalid, print a message.
4. Otherwise execute a for loop from 1 to N.
5. Take the variable "fact" and store the total product of N numbers in it.
6. Display the factorial value.
Example:
Let's look at an example where "n" is 7.
First we check if "n" is less than or equal to 0, but it is not. We then loop from 1 to 7, multiplying the numbers along the way and storing the result in variable "fact".
The final result is a factorial of 7 equal to 5040.