It's just rounded. Shown value is different form the actual value.

the 2nd is 0.0065 and the 1st is 0.013.

The only way I could come up with something close to 0.2222  - was to round during the intermediate calculation.

This is surely a big no-no.  This is as bad as using floats for currency.
Doesn't anyone else have concerns about this?

I agree... it's not something I want to hassle them about right now.. but I do think it warrants discussion and should be public knowledge.

The logs show it as $17.29000 - and I'm fairly sure I asked for the trade to occur at 17.29!
It is simply not legitimate to show it with this many decimal places if it was rounded off at 2DP.




No. This is the problem with amateur programmers who try to code with currency.
It is *very easy* to get apparently simple things wrong.

e.g 0.1 * 21.55
depending on the precision used could come out as say 2.1550000000000002

now store that in a variable $x 
check if you have the expected value  $x == 2.155   and the program returns FALSE.
In this example, I've used floats - just to demonstrate one pitfall.

Rounding during intermediate calculations is another way to introduce inaccuracies and it looks to me as though this is what mt gox is doing.

When MtGox comes back up, it will have an entirely new backend.  The old backend was built by the previous owner and MagicalTux has beeem working on a rewrite since right after he purchased the site.  Discussing an issue with the previous backend us pointless at this point.  Wait until it is back up and then check.  If it's still an issue, open a ticket and let us know.

That's good to hear.  It was just a point of concern..  and I understand if it's not a priority just now.

+1, Funny!

It's actually very, very easy to get maths wrong on computers, that's why there's fields of study and university courses about it. If it was simple, this would not be needed.

This makes it all the more embarrassing for you.

Rounding is a major concern on just about anything that does a significant amount of calculation.. It wouldn't be surprising at all if mtgox got that bit wrong... compared to some of the other stuff I wouldn't even consider that an issue right now.  OTOH from the initial 1BTC example it looks right (0.0065 rounds to 0.006 using bankers rounding).

btw. you never compare floats for equality.. you compare within a tolerance, or don't use float.  I'm not sure why mtgox would need to be comparing floats anyway.. mostly they're showing the results of simple calculations.

That 0.2222 error.. I've made that kind of mistake myself by rounding too early.. it happens.  It'll probably be fixed by the code rewrite and if it isn't then's the time to point it out, not now.