# This tutorial will introduce simple implementations of while loops
# in Python.

# Here's a while loop that calculates the factorial of n.  Start with n = 4
# to make sure things are working properly.  The loop executes until n > 1.
# That is, the the last iteration of the loop will occur when n = 2. (You
# could actually let the while loop run until n = 1 since factorial*1 =
# factorial).
n = 4
print('n =', n)
factorial = 1
while n > 1:
    factorial = factorial*n
    n = n - 1
print('n! =', factorial)

n = 4
n! = 24

# Here's a more interesting factorial
n = 50
print('n =', n)
factorial = 1
while n > 1:
    factorial = factorial*n
    n = n - 1
print('n! =', factorial)

n = 50
n! = 30414093201713378043612608166064768844377641568960512000000000000

# We can force the number to be written in scientific notation while keeping
# only some of the digits after the decimal point using:
print('n! =', float(factorial))

n! = 3.0414093201713376e+64

# Here's a a crazy big number...
n = 170
print('n =', n)
factorial = 1
while n > 1:
    factorial = factorial*n
    n = n - 1
print('n! =', float(factorial))

n = 170
n! = 7.257415615307999e+306

# Here's the factorial of a user-entered value.
n = int(input("Enter an integer: ")) 
print('n =', n)
factorial = 1
while n > 1:
    factorial = factorial*n
    n = n - 1
print('n! =', float(factorial))

Enter an integer: 17
n = 17
n! = 355687428096000.0

# Maybe you want to the user to be able to enter only integer numbers with
# appropriate error messages. 

# See the nested control structure tutorial for an implementation of this
# check.  That tutorial will also show you how to calculate the factorials
# of a bunch of n values and then plot the results vs n.