How to get constant of expression?

[Kevin Hunter]

Hullo Sympy Group,

I have a series of relations that I am receiving from an outside object and code base.  These equations are in no particular format, and I want to convert them to one.  At the moment, the end format I need is

format:  lhs_expression   [op]  constant

example: 3x + 4y <= 7

I have code that currently looks like this:

-----

lhs, rhs = rel.lhs, rel.rhs

lhs -= rhs

rhs = 0

possible_constant = lhs.as_ordered_terms()[ -1 ]

if possible_constant.is_real:

lhs -= possible_constant

rhs -= possible_constant

new_relation = type(rel)(lhs, rhs)

-----

Given that Sympy has already parsed the lhs expression, is there a more efficient/cleaner method of getting its constant?  Something perhaps like:

-----

lhs, rhs = rel.lhs, rel.rhs

lhs -= rhs

rhs = -lhs.constant_term

lhs -= rhs

new_relation = type(rel)(lhs, rhs)

-----

Thanks,

Kevin

[Chris Smith]

The as_coeff_Add method is probably what you want:

>>> (x+y).as_coeff_Add()

(0, x + y)

>>> (x+y+3).as_coeff_Add()

(3, x + y)

>>> (3*x).as_coeff_Add()

(0, 3*x)

>>> (x+y+3.0).as_coeff_Add()

(3.00000000000000, x + y)

/c

[Chris Smith]

On Mon, Jan 9, 2012 at 1:33 PM, Kevin Hunter <hun...@gmail.com> wrote:

Hullo Sympy Group,

I have a series of relations that I am receiving from an outside object and code base.  These equations are in no particular format, and I want to convert them to one.  At the moment, the end format I need is

In case there is any nesting of your expressions, you might want to apply `expand_mul(expand_multinomial(eq.lhs - eq.rhs))` to make sure the expression is flat:

>>> eq

3*x*(x + y) == -x + 3

>>> expand_mul(expand_multinomial(eq.lhs - eq.rhs)).as_coeff_Add()

(-3, 3*x**2 + 3*x*y + x)

>>> Eq(_[1], -_[0])

3*x**2 + 3*x*y + x == 3

[Kevin Hunter]

Thank you.