Treat a literal variable as a constant with Ryacas (or an alternative)-CodePudding

With Ryacas, one has:

> yac_str("Simplify(2*x^2*4   x^2*5)")
[1] "13*x^2"

Now, instead of 4 and 5, I would like to give two letters treated as constants. That is, Ryacas does:

> yac_str("Simplify(2*x^2*a   x^2*b)")
[1] "2*x^2*a x^2*b"

but I would like that it treats x as an unknown variable and a and b as constants, i.e. I would like to get the result:

(2*a b)*x^2

I spent one hour to try with no luck. Is it possible? Otherwise, with another package?

CodePudding user response：

This seems to be just how yacas wants to simplify that expression. SymPy does it the way you want though, and it has an R wrapper called caracas

library(caracas)

a <- as_sym("a")
b <- as_sym("b")
x <- as_sym("x")

caracas::simplify(2*x^2*a x^2*b)
#> [caracas]:  2        
#>            x *(2*a   b)

I don't like the way it prints x^2, but you can't have everything.

CodePudding user response：

I found the way with caracas.

library(caracas)

def_sym(x, y, z, a, b)

as.character(sympy_func(x^2   a*x^2   2*y   b*y   x*z   a*x*z, "Poly", domain = "RR[a,b]"))
# "Poly((1.0*a   1.0)*x^2   (1.0*a   1.0)*x*z   (1.0*b   2.0)*y, x, y, z, domain='RR[a,b]')"

You can also work with rational numbers:

poly <- 
  sympy_func(x^2   a*x^2   2/3*y   b*y   x*z   a*x*z, "Poly", domain = "QQ[a,b]")
as.character(poly)
# "Poly((a   1)*x^2   (a   1)*x*z   (b   2/3)*y, x, y, z, domain='QQ[a,b]')"

To get the coefficient of a term, e.g. xz (i.e. x^1y^0z^1):

sympy <- get_sympy()
sympy$Poly$nth(poly$pyobj, 1, 0, 1)
# a   1