Inconsistent results from int(floatNumber)

Dave Angel davea at ieee.org
Tue Oct 26 00:21:34 CEST 2010


On 2:59 PM, gershar wrote:
> I had some problems with some Python projects that gave variable
> results that I could not track down. Eventually and reluctantly I
> converted them to Java. Later, when I had more time I tried to analyze
> what the Python code was doing and found something strange. The
> following snippet illustrates the problem.
>

>>>> i = -50.0
>>>> for x in xrange(5):
> 	i += 0.1
> 	z = i * 10.0
> 	print
> 	print z
> 	print int(z)
>
> -499.0
> -499
>
> -498.0
> -498
>
> -497.0
> -496
>
> -496.0
> -495
>
> -495.0
> -494
>
>
> The first two iterations look OK but after that the int(z) function
> returns the wrong value. It looks like the value was rounded down.  If
No, the value is truncated upward, towards zero.  Down would be towards 
negative infinity.  And rounding would work, that's not what int() does.
> a just do this:
>>>> int(-497.0)
> -497
> I get the value I expect.
> So what is the problem?
>
> It looks like a rounding problem but on the surface there is nothing
> to round. I am aware that there are rounding limitations with floating
> point arithmetic but the value passed to int() is always correct.
Define "correct."  If you mean that it's exactly an integer, that's 
false.  It's off a little, because 0.1 isn't exact, and multiplying by 
10 doesn't fix it.
>   What
> would cause it to be off by 1 full digit in only some cases? Perhaps
> something behind the scenes in the bowels of the interpreter ?.
>
> I could not get the thing to fail without being inside the for loop;
> does that have something to do with it?
>
> To fix the problem I could use round() or math.floor().  Like this.
>
round() is entirely different than truncating or flooring.
>>>> i = -50.0
>>>> for x in xrange(5):
> 	i += 0.1
> 	z = i * 10.0
> 	print
> 	print z
> 	print(round(z))
>
> -499.0
> -499.0
>
> -498.0
> -498.0
>
> -497.0
> -497.0
>
> -496.0
> -496.0
>
> -495.0
> -495.0
>
> Why should I have to do this?
>
> Is there a general rule of thumb to know when this could be a problem?
>
> Should any float-to-int conversion be suspect?
>
> The above code was run in Python 2.5.4 on WinXP and Python 2.6.2 on
> Linux(Fedora12)
> Can anyone verify if this would be the same on 3.x?
>
This doesn't have anything to do with Python, but everything to do with 
binary floating point.  The thing that's confusing you most is that you 
think that if a value prints as 497.0, that it's actually equal to 
497.0.  False.

When you have an approximate value like 0.1, and you do arithmetic with 
it, sometimes the values aren't exact.

In :Python 3, they avoid this particular symptom usually by printing the 
values differently.  But the problem still exists.


DaveA




More information about the Python-list mailing list