On Fri, Apr 3, 2009 at 9:37 AM, Mateusz Paprocki <matt...@gmail.com> wrote:
> Hi,
>
> On Fri, Apr 03, 2009 at 08:41:05AM -0700, Colin Gillespie wrote:
>>
>> Dear All,
>>
>> Quick question. I have a polynomial:
>>
>> p =Poly(2*x*y+y, x, y)
>>
>> and I want to substitute in y = y+5 and x= x+1 How would I do this?
>>
>> I've tried things like:
>>
>> p.subs((y, y+5), (x, x+1))
>>
>> but this doesn't work.
>>
>> Cheers
>>
>> Colin
>>
>
> you should refer to Basic.subs() docstring:
>
> In [1]: p = Poly(2*x*y+y, x, y)
>
> In [2]: p.subs?
> Type: instancemethod
> Base Class: <type 'instancemethod'>
> String Form: <bound method Poly.subs of Poly(2*x*y + y, x, y)>
> Namespace: Interactive
> File: /home/matt/repo/git/sympy/sympy/core/basic.py
> Definition: p.subs(self, *args)
> Docstring:
> Substitutes an expression.
>
> Calls either _subs_old_new, _subs_dict or _subs_list depending
> if you give it two arguments (old, new), a dictionary or a list.
>
> Examples:
>
> >>> from sympy import *
> >>> x,y = symbols('xy')
> >>> (1+x*y).subs(x, pi)
> 1 + pi*y
> >>> (1+x*y).subs({x:pi, y:2})
> 1 + 2*pi
> >>> (1+x*y).subs([(x,pi), (y,2)])
> 1 + 2*pi
>
>
> In [3]: p.subs({y:y+5,x:x+1})
> Out[3]: Poly(2*x*y + 10*x + 3*y + 15, x, y)
>
> In [4]: p.subs([(y, y+5), (x, x+1)])
> Out[4]: Poly(2*x*y + 10*x + 3*y + 15, x, y)
>
> In [5]: p.as_basic().subs({y:y+5,x:x+1})
> Out[5]: 5 + y + 2⋅(1 + x)⋅(5 + y)
>
> In [6]: expand(_) == _4.as_basic()
> Out[6]: True
Right, it actually works directly with Poly, no need to convert to Basic.
Ondrej
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