Factorial using a While Loop: Code-Along

Start by creating a new .cs file/project. Then, follow the instructions below to create a factorial while loop application.

Factorial is the product of an integer and all the integers below it; e.g. factorial four (4!) is equal to 4 * 3 * 2 * 1 = 24

Part 1 - User Input

First, ask the user for an integer to find the factorial.

Input

1. Use Console.WriteLine to ask the user to enter an integer.
2. Use Console.ReadLine to receive the input from the user.
3. Use Convert.ToInt32 to convert the user's input from a string to an int.
4. Make sure to store the result in a variable.

Code

Console.WriteLine("Enter a whole number: ");
int num = Convert.ToInt32(Console.ReadLine());

Part 2 - While Loop

Second, we will create the while loop. To start, we will initialize the counter and the result variable.

Counter & Sum

We want to use a counter that starts at the integer the user provided and the result starts at 1.

1. Since we will be counting backwards, we want to start the counter at the number the user provides.
2. Also, we are going to be multiplying the result by the counter, so we don't want to start at 0 or the result will always be 0!

Code

int i = num;
int result = 1;

While Loop Condition

We are going to be counting down and want to stop when we are out of integers to multiply, so we want to go until the counter is equal to 0 but not if the counter is 0.

We will accomplish this by using the boolean operator >.

Code

while (i > 0)

While Loop Body

Now, we need to do the math function! Start with brackets. Then, set the result variable to be itself multiplied by the current value of i.

We also need to decrement the counter to keep the loop going. We do this with i--;.

Code

while (i > 0)
{
    result = result * i;
    i--;
}

Part 3 - Print the results to screen

Finally, we need to let the user know what the answer is! We will do this with Console.WriteLine() outside of the while loop.

Code

Console.WriteLine("Factorial of " + num + " is " + result);

Final Code

using System;

public class Program
{
    public static void Main()
    {
        Console.WriteLine("Enter a whole number");
        int num = Convert.ToInt32(Console.ReadLine());

        int i = num;
        int result = 1;

        while (i > 0)
        {
            result = result * i;
            i--;
        }

        Console.WriteLine("Factorial of " + num + " is " + result);
    }
}