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
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. 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.
>>> 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?
--
http://mail.python.org/mailman/listinfo/python-list
