"""Collatz Sequence, by Al Sweigart al@inventwithpython.com
Generates numbers for the Collatz sequence, given a starting number.
More info at: https://en.wikipedia.org/wiki/Collatz_conjecture
This code is available at https://nostarch.com/big-book-small-python-programming
Tags: tiny, beginner, math"""
import sys, time
print('''Collatz Sequence, or, the 3n + 1 Problem
By Al Sweigart al@inventwithpython.com
The Collatz sequence is a sequence of numbers produced from a starting
number n, following three rules:
1) If n is even, the next number n is n / 2.
2) If n is odd, the next number n is n * 3 + 1.
3) If n is 1, stop. Otherwise, repeat.
It is generally thought, but so far not mathematically proven, that
every starting number eventually terminates at 1.
''')
print('Enter a starting number (greater than 0) or QUIT:')
response = input('> ')
if not response.isdecimal() or response == '0':
print('You must enter an integer greater than 0.')
sys.exit()
n = int(response)
print(n, end='', flush=True)
while n != 1:
if n % 2 == 0: # If n is even...
n = n // 2
else: # Otherwise, n is odd...
n = 3 * n + 1
print(', ' + str(n), end='', flush=True)
time.sleep(0.1)
print()