CODEWITHSUNDEEP
pythonsympysymbolic-math
extraymond
extraymond

Reputation: 626

Preserve the order of symbols in sympy equation

I would like to preserve the structure of equation in the following order.

*Note that this is in sympy.

Eq=A*X+B*U

where X,U is assigned to other equations.

Sympy expand my symbols with variables assigned elsewhere. So it ends up being something like: Bx+Bx**2+3*A*x What I want to do is preserver the order of A X B U.

Update for clarification: Hi! Thx for your reply.

  1. After trying different options, I've found that by using Matrixsymbols, it is possible to not operate the elements in the matrix. However I still can't make it to show the desired order.

  2. The reason for this is so that I can save this in a jupyter notebook, where the order might help explaining or taking notes for specific procedure.

Upvotes: 1

Views: 1756

Answers (1)

smichr
smichr

Reputation: 19077

When you create an expression with variables, the value of those variables will be used during creation, e.g.

>>> x = cos(3)
>>> eq = x + 2
>>> eq
cos(3) + 2

So if you want eq to look like x + 2 you have to make sure that x is defined as Symbol x

>>> x = Symbol('x')
>>> eq = x + 2
>>> eq
x + 2

If you have multiple symbols and you want to control the ordering, use unevaluated expressions:

>>> A,B,X,U = symbols('A B X U')
>>> ax = Mul(A, X, evaluate=False)
>>> bu = Mul(B, U, evaluate=False)
>>> eq = Add(ax, bu, evaluate=False)
>>> eq
A*X + B*U

You can substitute in the values that you desire as

>>> eq.subs({U:x+x**2, X:3*x})
3*A*x + B*(x**2 + x)
>>> expand(_)
3*A*x + B*x**2 + B*x

Does that help with what you are trying to do?

Upvotes: 1

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