Calculate factorials: Write a function f actorial to compute the factorial n! for any integer n. The input should be the number n and the output should be n!. You might have to use a for loop or a while loop to do the calculation. See Section 4.3.4 on page 114 for a quick description of these loops. (You can use the built-in function prod to calculate factorials. For example, n! is prod ( 1 : n) . In this exercise, however, do not use this function.)