The order in printing can be controlled with the order flag to the
printer. Using latex(expr, order='rev-lex') will cause polynomials to
print in reverse lexicographic order, which is the order you want.
It's not a good idea to use evaluate=False to try to control printing
behavior, as this can break other things. Instead, you should use the
existing flags in the printer in question, or customize the printer
with a subclass if the built-in behavior doesn't meet your needs.
Aaron Meurer
On Sat, Feb 10, 2024 at 1:58 PM Thomas Ligon <thomassli...@gmail.com> wrote:
>
> I am trying to reverse the order of an expression before printing it for my
> documentation, and I want ascending order, but SymPy always gives me
> descending order. All of the expressions are polynomials, specifically
> partial sums of power series. Since, according to Internet searches, there is
> no easy way to do this, I have tried a number of things, including
> manipulating the expression tree. The original expression is always Add of a
> tuple, and each tuple is a rational number and x**n. When I loop through the
> expression and Add the components, I always get the same order. The elegant
> solution would be to reverse the tuple and Add it all at once, but that is
> deprecated and gives me a tuple instead of a sum.
>
> Here is some sample code, a test program that demonstrates the problem.
>
> from sympy import symbols, Rational, Add, latex, Eq
> x = symbols('x')
> lhs = symbols('X')
> rhs = Rational(3)/Rational(4)*x**3 + Rational(2)/Rational(5)*x**2 +
> Rational(1)/Rational(4)*x
> X1 = rhs.args
> #X2 = X1[::-1] # Why don't I need this? My debugger shows the expression and
> the tuple in reverse order.
> X2 = X1
> # Try the expected order of Add. This produces a sum, but with an extra set
> of parantheses, and not the desired order.
> X3 = X2[0]
> X2R = X2[1:len(X2)]
> for indT in range(0, len(X2R)):
> termT = X2R[indT]
> X3 = Add(X3, termT, evaluate=False)
> # Try the other order of Add. This looks the same as X3.
> X4 = X2[0]
> for indT in range(0, len(X2R)):
> termT = X2R[indT]
> X4 = Add(termT, X4, evaluate=False)
> # Try single step. This is deprecated, gives the correct order, but returns a
> tuple instead of a sum.
> #X2 = X1[::-1] # Why don't I need this?
> X2 = X1
> X5 = Add(X2, evaluate=False)
>
> print(latex(Eq(lhs, rhs))) # original order
> print(latex(Eq(lhs, X3))) # still original order
> print(latex(Eq(lhs, X4))) # still original order
> print(latex(Eq(lhs, X5))) # desired order, but tuple instead of sum
>
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