Partial substitutions of Mul objects do not work for mixed-variable terms

[tbabej]

Consider the following example, which works as expected:

>>> import symengine
>>> x,y,z = symengine.symbols('x y z')
>>> eq = 2*x*y*y + 3*x*x + 4*y*y
>>> eq.subs(y*y, z)
4*z + 2*x*z + 3*x**2

If we try to substitute x*y instead of y*y, which is still contained in the first term, the substitution does not happen:

>>> eq.subs(x*y, z)
2*x*y**2 + 3*x**2 + 4*y**2

This only happens for non-exact matches, exact matches do work:

>>> (2*x*y + 3*y).subs(x*y, z)
3*y + 2*z

[isuruf]

Yes, subs is kind of dumb that way. What happens is it will check whether the expression is equal to 2*x*y**2, then x*y**2 then x then y**2.
This should be fixed though. Original behaviour can stay in xreplace, but subs needs to be fixed.

[tbabej]

@isuruf Can you point me to the place in the code base where this logic is implemented? I can contribute a fix.

[isuruf]

Thanks. It's implemented in XReplaceVisitor at https://github.com/symengine/symengine/blob/master/symengine/subs.h#L74-L108

This method shouldn't be changed, but overriden in the SubsVisitor class which is here, https://github.com/symengine/symengine/blob/master/symengine/subs.h#L270

[eeshan9815]

@tbabej @isuruf Hey, I am new to symengine. If no one is working on it, I would like to start contributing by fixing this issue.

[ShikharJ]

Go for it @eeshan9815

[tbabej]

Indeed, we just settled for a python-based dirty quick workaround in our code. It would be great to support this natively.

…

On Wed, Jan 17, 2018 at 8:26 AM, Shikhar Jaiswal ***@***.***> wrote: Go for it @eeshan9815 <https://github.com/eeshan9815> — You are receiving this because you were mentioned. Reply to this email directly, view it on GitHub <#1359 (comment)>, or mute the thread <https://github.com/notifications/unsubscribe-auth/ACBoXIl0rrJhUT8F_vbjJdw907FbEMahks5tLfUBgaJpZM4QaHIa> .

[eeshan9815]

@isuruf @ShikharJ @tbabej I think I found another issue with subs:
While this works as expected:

>>> (2*x*y + 3*y).subs(x*y, z)
3*y + 2*z

This does not:

>>> (2*x*y).subs(x*y, z)
2*x*y

instead of 2*z
Can someone explain why, and how to fix this issue as well? I have had some progress with the original issue.