Engineering #9: factorials with loops, and randrange confusion

Lottie Collins, Ta-Ra-Ra-Boom-De-Ay (W. Duke, Sons & Co., Actors and Actresses series to promote Duke Cigarettes, 1891)

Exercise 9 of Launch School’s Loops and Iterating chapter, Introduction to Programming with Python book:

Write a function that computes and returns the factorial of a number by using a for or while loop. The factorial of a positive integer n, signified by n!, is defined as the product of all integers between 1 and n, inclusive. You may assume that the argument is always a positive integer.

n!	Expansion	   Result
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

For the while loop, I wanted to make a counter, of course; it’s like 2% of all the code I know right now. This works:

def factorial(num):
    result = 1
    counter = 1
    while counter <= num:
        result *= counter
        counter += 1
    return result

But the Launch School while loop is probably better:

def factorial(num):
    result = 1
    while num > 0:
        result *= num
        num -= 1
    return result

Subtracting ends up as the cleanest code in this instance. And in the for loop, too:

def factorial(num):
    result = 1
    for number in range(num, 0, -1):
        result *= number

    return result

I’m not yet used to plugging in a range into a function like this. It surprised me, but ends up being super tidy.

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And exercise 10:

The following code uses the randrange function from Python's random library to obtain and print a random integer within a given range. Using a while loop, it keeps running until it finds a random number that matches the last number in the range. Refactor the code so it doesn't require two different invocations of randrange.

import random

highest = 10
number = random.randrange(highest + 1)
print(number)

while number != highest:
    number = random.randrange(highest + 1)
    print(number)

I read this for six minutes. “Finds a random number that matches the last number in the range?” And then what happens? And the + 1 is due to the usual Python stuff?

I thought randrange had a number stowed and was running until it matched that number. No: “last number in the range” means the highest/largest number in the range, in this case 10, which we get randrange to access by adding + 1 to “highest”, since Python range functions don’t include the upper bound.

And this is the refactored solution:

import random

highest = 10
while True:
    number = random.randrange(highest + 1)
    print(number)
    if number == highest:
        break

This also showcases how you can use “while True” to be a state of “everything, until…” There is no competing False condition here. Cool to see.