How come the outcome of my collatz function isn't the same as it is supposed to be when I put large numbers?

Question

So I got this Python code where it should give me the amount of steps for each number when we apply the Collatz algorithm on it.

When I input small integers, it gives the correct amount of steps. But when I input large numbers, it gives me the wrong number of steps.

Here's a code that reproduces the same thing:

import math

number = 931386509544713451 # Number here

def collatz_algorithm(n):
    x = math.floor(n)
    A_N = 0
    while x != 1:
        if x % 2 == 0:
            x = x / 2
        else:
            x = 3*x + 1
        A_N += 1
    print("\nNumber of steps for {}: {}".format(str(n),str(A_N)))
 
collatz_algorithm(number)

When you input small numbers such as 7, it does just fine and it gives the right number of steps. But when you input a large number as 931386509544713451, it gives the wrong number of steps (As shown in the Wiki here: https://en.wikipedia.org/wiki/Collatz_conjecture)

I tried to do many things, such as converting to ulonglong with ctypes but still didn't work, I was thinking it could increase the number limit, but it didn't.

So could anyone help?

score 3 · Accepted Answer · answered Sep 24 '21 at 19:10

3

x = x / 2 is converting your number to a float which is imprecise. Replace it with x = x // 2 so that integer division will be used.

Output (matches Wikipedia):

Number of steps for 931386509544713451: 2283

answered Sep 24 '21 at 19:10

Random Davis

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Damn, thanks, I didn't think float division was imprecise. Thanks for letting me know and thanks for correcting my code! – Elmatador DeAngel Sep 24 '21 at 19:14
@ElmatadorDeAngel happy to help, for more info see: https://stackoverflow.com/questions/588004/is-floating-point-math-broken – Random Davis Sep 24 '21 at 19:46

How come the outcome of my collatz function isn't the same as it is supposed to be when I put large numbers?

1 Answers1