Help with infinite while loop

I need to create a simple script that take a number, checks if it odd or even. If it's even: divide by two, if it's odd: multiply by 3 add 1. The script is supposed to repeat this check and calculation until the output reaches a value of one. I've tried the following while loop which calculates the first value but then infinity spits out this same value. Thanks in advance!

clear 
disp('Please input student number')
ID = input('SI#: ');
if mod(ID,2) == 0 
     out = ID/2
else out = ID.*3+1

end
while out > 1
    if mod(ID,2) == 0 
     out = ID/2
else out = ID.*3+1
    end
end

[Matt J]

It doesn't seem easy to prove that this will eventually converge to 1. Is it a well-known result from somewhere?

[Matt Fig]

@Matt J - AFAIK, it is not proven. But it has been extensively tested and never found to fail. Look up Collatz conjecture.

[Azzi Abdelmalek]

Tested in a loop for k=1:10^6 with randi, never fail.

[Walter Roberson]

For example, my BOINC client just finished testing 8E12 numbers starting from 2377489401671993502056 without finding any counter-example.

[Matt J]

OK. Glad I didn't spend my day trying to prove it :)

[Jan]

It is not proved. See http://en.wikipedia.org/wiki/Collatz_conjecture. You can earn 500$ and the Fields medal when you prove it, but this would conflict with the term " algorithmically undecidable".

Answer 1

[Matt Fig]

The problem is that ID never changes in your loop so you are just doing the same thing over and over. Change all 'out' variables to 'ID' and your code should work fine.

Answer 2

[Devan]

thanks!

Answer 3

[Jan]

It is a good idea to ask Google, in this case for "3N+1 Wiki". You find explanations, theorems, algorithms and visualizations, ... Enough material to solve the homework.