#1163: [with patch, needs work] assume seems to have some undesired side-effects
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Reporter: zimmerma | Owner: gfurnish
Type: defect | Status: assigned
Priority: major | Milestone: sage-4.1.2
Component: calculus | Keywords:
Reviewer: | Author:
Merged: |
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Changes (by kcrisman):
* cc: burcin, robertwb (added)
Comment:
Okay, the following patch (which depends on #7084, I think), resolves
'''most''' of the issues in this ticket. See below for the one it
doesn't, and comments.
{{{
sage: assume(x<=1)
sage: assume(x>=1)
sage: assumptions()
[x <= 1, x >= 1] # so far, so good
sage: b = x!=1
sage: b.__nonzero__()
True # WRONG
sage: bool(b)
True # WRONG
sage: bool(x==1)
True # right!
sage: forget()
sage: var('x,y,z')
(x, y, z)
sage: assume(x>=y,y>=z,z>=x)
sage: assumptions()
[x >= y, y >= z, z >= x]
sage: bool(x==z)
True # right!
sage: forget()
sage: assume(x>0)
sage: bool(x<>1) # NO!
True
sage: bool(x!=1) # NO! And note that Python considers <> and != to be
identical
True
sage: bool(x==1)
False # right!
}}}
I've narrowed the problem down to the fact that __nonzero__ has the
following lines:
{{{
res = relational_to_bool(self._gobj)
if res:
return True
}}}
If you put this code after determining whether you need assumptions,
practically every evaluation of bool() yields an infinite loop. But for
some reason the function relational_to_bool (like all these Ginac
functions, completely unsearchable because they live somewhere outside of
devel/sage) doesn't send (at least) != to the assumptions. Here is the
wrapper code, in c_lib/include/ginac_wrap.h:
{{{
bool relational_to_bool(const ex& e) {
if (ex_to<relational>(e))
return 1;
else
return 0;
}
}}}
Now what? The documentation of ex_to in Ginac, even when I found it on
their website, really only will make sense to someone with good C++
knowledge.
--
Ticket URL: <http://trac.sagemath.org/sage_trac/ticket/1163#comment:12>
Sage <http://www.sagemath.org>
Sage: Creating a Viable Open Source Alternative to Magma, Maple, Mathematica,
and MATLAB
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