Lab 4.1 - Factorial

The factorial function, written as n! in math, is where you mulitply all of the whole numbers from 1 to whatever n is. Thus:

• 1! = 1 = 1
• 2! = 1 * 2 = 2
• 3! = 1 * 2 * 3 = 6
• 4! = 1 * 2 * 3 * 4 = 24
• 5! = 1 * 2 * 3 * 4 * 5 = 120
• etc...

Create an application that asks for a number from the user and displays the factorial of that number. Your program should error-check to make sure that the user hasn't entered a negative value.

You should have:
• A Button that opens an InputBox for the number
• A Label to display the answer.

Here's sample input and answers that you should get:

Input	Answer
8	40320
-3	[useful error message]
0	1

Common mistakes include:
• Trying to write the program on the fly rather than writing down an algorithm in pseudocode in your notebook.
• Not checking the input to avoid taking a negative value.
• Using global scope when you should be using local scope.
• Not leaving enough space to display the full result.
• Wasting real estate or cutting off the title of the program.

HINTS: Use either a Do-Loop-While or a Do-While-Loop (you'll have to figure out which) to run not only the factorial algorithm, but also the error-checking for numbers less than 0. Don't be afraid to add an extra variable if you need one. Use a good sentinel and a variable to keep track of the answer. Factorial numbers get really big, really fast so use a Long instead of an Integer.

Note that your program will crash if you enter an input higher than 20, because the result is too big to fit in a Long variable.