On 10/25/2010 5:44 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
The binary float resulting from the conversion of .1 is slightly greater
than .1, so this increases i by slightly more than .1
z = i * 10.0
so z is increased be lightly more that 1
print
print z
print int(z)
float.__int__ truncates toward 0.
-499.0
-499
-498.0
-498
-497.0
-496
And here the extra increase shows up.
-496.0
-495
-495.0
-494
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.
No it is not. To see this, print more digits (which themselves are
approximations of the actual binary value). With 3.1.2 on x86 system:
i = -50.0
form = '{:24.18f}'.format
print(form(.1))
for x in range(15):
i += 0.1
z = i * 10.0
print()
print(form(z))
print(int(z))
>>>
0.100000000000000006
-499.000000000000000000
-499
-498.000000000000000000
-498
-496.999999999999943157
-496
-495.999999999999943157
-495
-494.999999999999943157
-494
To completely understand, you would have to look at the binary bit
pattern and know the exact behavior of floating point arithmetic on a
system and the exact decimal to binary to decimal conversion algorithm.
--
Terry Jan Reedy
--
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