Hi,
On 27 November 2015 at 19:34, Riccardo Rossi <[email protected]> wrote:
> Dear list,
>
> i am a newby to sympy, and i should say that i liked what i found, so ...
> first of all kudos to the developers.
>
> as of now i can succesfully generate my finite element matrices using sympy,
> which saves me quite a lot of work.
>
> the point is that now i would like to optimize a bit what i did, and i would
> like to collect some common factors between the entries of a matrix.
>
> for example imagine that i have (pseudocode and just an example, no physics
> behind)
>
> a,b = symbols('a b')
>
> A = Matrix(2,1)
> A[0] = a*(exp(a+b)+exp(b^2))
> A[1] = b*(exp(a+b)+exp(b^2))
>
> i would like a way to detect that the term
> (exp(a+b)+exp(b^2))
>
> is common to the different entries and eventually later on do something of
> the type
>
> aux = (exp(a+b)+exp(b^2))
> A[0] = a*aux
> A[1] = b*aux
>
> note that later on for me it would be still interesting to do something
> similar on SOME of the entries of the matrix
>
> for example if i had
>
> A = Matrix(3,1)
> A[0] = a*(exp(a+b)+exp(b^2))
> A[1] = b*(exp(a+b)+exp(b^2))
> A[2] = a+b
>
> i would still love to have
>
>
> aux = (exp(a+b)+exp(b^2))
> A[0] = a*aux
> A[1] = b*aux
> A[2] = a+b
>
you could use cse() (common subexpression elimination) for this, e.g.:
In [1]: from sympy import *
In [2]: var('a,b')
Out[2]: (a, b)
In [3]: aux = exp(a + b) + exp(b**2)
In [4]: Matrix([a*aux, b*aux, a + b])
Out[4]:
Matrix([
[a*(exp(b**2) + exp(a + b))],
[b*(exp(b**2) + exp(a + b))],
[ a + b]])
In [5]: replacements, (M,) = cse(_)
In [6]: M
Out[6]:
Matrix([
[a*x1],
[b*x1],
[ x0]])
In [7]: replacements
Out[7]: [(x0, a + b), (x1, exp(b**2) + exp(x0))]
In [8]: M.subs(list(reversed(replacements)))
Out[8]:
Matrix([
[a*(exp(b**2) + exp(a + b))],
[b*(exp(b**2) + exp(a + b))],
[ a + b]])
However, this may not be exactly what you want, because it eliminates
`a + b` as well.
Mateusz
>
>
> thanks in advance for any suggestion.
>
> cheers
> Riccardo
>
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