Comment #7 on issue 2107 by [email protected]: Real(0.1, 10) - Real(0.1,
10) -> 3.637923296e-13
http://code.google.com/p/sympy/issues/detail?id=2107
This issue is really peculiar. Currently in master I get:
In [1]: a = Float(0.1, 10)
In [2]: b = Float("0.1", 10) # <- note the quotes
In [3]: a - a # this is equivalent to Add(a, -a)
Out[3]: 3.637923296e-13 # because Number doesn't implement __sub__
# in infinities branch I added __sub__ and there
it gives 0
In [4]: b - b # but with quotes it works
Out[4]: 0
In [5]: a + (-a) # this uses Float.__add__ directly
Out[5]: 0
In [6]: b + (-b)
Out[6]: 0
In [8]: Add(a, -a)
Out[8]: 3.637923296e-13
In [9]: Add(b, -b)
Out[9]: 0
In [10]: S.Zero + a + (-a) # this is the interesting part
Out[10]: 3.637923296e-13 # adding S.Zero makes a difference
In [11]: a + (-a)
Out[11]: 0
In [12]: S.Zero + b + (-b) # but not with quotes
Out[12]: 0
I think that the problem is with precision tracking (but then why version
with quotes work -> does this use different implementation?).
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