On Sat, Jan 25, 2014 at 1:05 PM, Björn Dahlgren <bjo...@gmail.com> wrote:
> On Saturday, 25 January 2014 19:55:58 UTC+1, Aaron Meurer wrote:
>>
>> Well in theory it's easy. You just need to edit the exclude part of
>> the iterable definition in sympy/core/compatiblity.py to include
>> IndexedBase. But this is actually not so easy because there's no way
>> to import IndexedBase for use in the function, since it is used in the
>> import process before IndexedBase is defined.
>
>
> Yeah I ran into that problem, but iterable takes `exclude` kwarg so I tried:
> iterable(expr, exclude=(string_types, dict, IndexedBase))
Oh sure, you can exclude it in that once place. But we really should
make this work globally.
>
> based on what you said about IndexedBase needing to be added to the list
>
> then there is no infinite loop anymore but the output is not what one would
> expect:
>
> ([(x0, y), (x1, i), (x2, x1 + 1), (x3, x), (x4, (-x3[x1] + x3[x2])**(-1))],
> [x4*(-x0[x1] + x0[x2]), x4])
>
> so it is actually looking for cse's in indices which is maybe a bit
> overkill..
Well, that's a separate issue. All cse() does is look at the .args of
the expressions. Technically i + 1 is a common subexpression (and I
suppose one could in theory have a more complicated common
subexpression in the index).
>
> But apart from that it acutally works!
>
>
>>
>>
>> So probably what should be done is some kind of _not_iterable property
>> put on IndexedBase and iterable() changed to look for it.
>>
>> By the way, I think there are other objects that do the same sort of
>> thing, like MatrixSymbol, which also need to be disabled, since they
>> iterate forever. I seem to remember a few places in the code just
>> check for them explicitly, but we should standardize on iterable(),
>> and make iterable() know what does and doesn't work.
>
>
> Makes sense, I don't nearly have the overview to know what should be
> excluded
> (and if there are exceptions for certain applications). So maybe passing
> modified
> exclude kwarg is a good workaround for now? Making it not look into the
> indices would
> be nice though.
I think IndexedBase and MatrixExpr are the only objects that do this.
Maybe something in the tensor module too. It should be a simple fix.
Just add some property, like
_iterable = False
to the class, and then modify iterable() to look for that property.
I suppose you could also use a mixin, like
class NotIterable(object):
pass
and add that so the subclasses of IndexedBase and MatrixExpr, and to
default the exclude tuple.
Aaron Meurer
>
>
>>
>>
>> Aaron Meurer
>>
>>
>> On Sat, Jan 25, 2014 at 12:45 PM, Björn Dahlgren <bjo...@gmail.com> wrote:
>> > Ok, I poked around a little didn't get it to work on my first try.
>> > Do you agree that it would be the preferred behaviour that cse should
>> > work
>> > with Indexed instances out of the box?
>> > Is it trivial fix for someone - if not I could try to get this to work
>> > but
>> > it will take me quite a bit of work I think.
>> >
>> > Thanks!
>> > /Björn
>> >
>> >
>> > On Saturday, 25 January 2014 19:26:47 UTC+1, Aaron Meurer wrote:
>> >>
>> >> The problem is that the iterable() function in compatibility thinks
>> >> that y is iterable, so it's trying to iterate through y. The fix is to
>> >> add IndexedBase to the set of ignored iterables in the function.
>> >>
>> >> Aaron Meurer
>> >>
>> >> On Sat, Jan 25, 2014 at 5:04 AM, Björn Dahlgren <bjo...@gmail.com>
>> >> wrote:
>> >> > Hi!
>> >> >
>> >> > while using the Indexed class from sympy.tensor I have run into the
>> >> > following problem:
>> >> >
>> >> > cse(...) fails for expressions having "Indexed" instances. They are
>> >> > not
>> >> > handled correctly in opt_cse() in cse_main.py
>> >> > A possible work-around is to substitute the Indexed instances with
>> >> > dummies,
>> >> > perform the CSE elimination and then
>> >> > resubstitute the dummies for the indexed symbols. But this is not
>> >> > pretty
>> >> > and
>> >> > I think that cse(...) should handle this
>> >> > on its own.
>> >> >
>> >> > Example:
>> >> >
>> >> > from sympy import *
>> >> >
>> >> > len_y = 5
>> >> > y = IndexedBase('y', shape=(len_y,))
>> >> > x = IndexedBase('x', shape=(len_y,))
>> >> > Dy = IndexedBase('Dy', shape=(len_y-1,))
>> >> > i = Idx('i', len_y-1)
>> >> >
>> >> > expr1 = (y[i+1]-y[i])/(x[i+1]-x[i])
>> >> > expr2 = 1/(x[i+1]-x[i])
>> >> > print(cse([expr1, expr2]))
>> >> >
>> >> > this will cause an infinite recursive call stack to _find_opts
>> >> > raising:
>> >> > RuntimeError: maximum recursion depth exceeded in
>> >> > __instancecheck__
>> >> >
>> >> >
>> >> > Does anybody know what the best approach here would be? Add something
>> >> > to
>> >> > the
>> >> > Indexed class or modify
>> >> > opt_cse?
>> >> >
>> >> > Best regards
>> >> > /Björn
>> >> >
>> >> > --
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