Python can't divide??!?!

Dan Bishop danb_83 at yahoo.com
Fri Feb 6 04:05:05 CET 2004


"Dan Williams" <dan at ithium.net> wrote in message news:<mailman.1252.1076003166.12720.python-list at python.org>...
> Ummmm...
> 
> This is weird. Sorry if it's known about (how can it NOT be, I wonder?) but
> I haven't seen any reference to it anywhere.

Then you haven't been looking hard enough.
 
> I'm running the latest Python (2.3.3) on Windows XP Pro, i386 architecture.
> 
> Fire up python or whatever.
> 
> Do this:     Result:
> 
> 3 / 5        0                       Fair enough, int / int = int

This is for bug-compatibility with old versions of Python.  It is
strongly recommended that all new Python scripts use "from __future__
import division".

> 3 / 5.0      0.59999999999999998     eh?
> 3.0 / 3      0.59999999999999998     ummmm...
> 3.0 / 5.0    0.59999999999999998     how did I guess...

It makes perfect sense when you remember that computers do math in
binary.  In binary, 3/5 is equal to 0.1 0011 0011 0011 0011 0011
0011..., which is rounded to (C99 notation) 0x1.33333333333333333p-1. 
The exact decimal equivalent of this is
0.59999999999999997779553950749686919152736663818359375.  In the
default 17 significant digits format, this rounds to
0.59999999999999998.

> That's just an example. Python cannot divide. Period.

It's off by only 37 parts per quintillion.

> If you try 1.0 / 3 you get 0.33333333333333331,

More precisely, you get 0x1.5555555555555p-2, or
0.333333333333333314829616256247390992939472198486328125.

> and from this, the above example, and a few tests,
> I speculate that *something* is subtracting 2 from the last digit *after*
> the division is done.

*Nothing* is subtracting from the last digit because your floating
part hardware has no concept of decimal digits, only bits.
 
> Now, is this just me, or can someone else duplicate it?

Everyone in the world who has binary floating part hardware can
duplicate it, whether they're using Python or not.



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