Potentially use orthopolynom for more polynomial expansions

[DavisVaughan]

Low dependencies. Seems like it has been out for awhile and is well tested. Chebyshev polynomial expansions were requested in the DC workshop.

polynomial.values(chebyshev.c.polynomials( 10, normalized=TRUE ), rnorm(100))

[topepo]

This could be an option to step_poly()

[EmilHvitfeldt]

This is going to be hard to do the call2() way because of how orthopolynom is written (Unless I missing some rlang magic). orthopolynom depends on polynom which needs to be attached to give access to the function polynom().

orthopolynom_wrapper <- function(degree) {
  polynomials <- rlang::call2(
    "chebyshev.c.polynomials",
    n = degree,
    normalized = TRUE,
    .ns = "orthopolynom"
  )
  eval(polynomials)
}

orthopolynom_wrapper(4)
#> Error in polynomial(c(1/sqrt(h.0))): could not find function "polynom"

library(orthopolynom)
#> Loading required package: polynom

orthopolynom_wrapper(4)
#> [[1]]
#> 0.1994711 
#> 
#> [[2]]
#> 0.1410474*x 
#> 
#> [[3]]
#> -0.2820948 + 0.1410474*x^2 
#> 
#> [[4]]
#> -0.4231422*x + 0.1410474*x^3 
#> 
#> [[5]]
#> 0.2820948 - 0.5641896*x^2 + 0.1410474*x^4

orthopolynom::chebyshev.c.polynomials
#> function (n, normalized = FALSE) 
#> {
#>     recurrences <- chebyshev.c.recurrences(n, normalized)
#>     if (normalized) {
#>         h.0 <- 8 * pi
#>         p.0 <- polynomial(c(1/sqrt(h.0)))
#>         polynomials <- orthonormal.polynomials(recurrences, p.0)
#>     }
#>     else polynomials <- orthogonal.polynomials(recurrences)
#>     return(polynomials)
#> }
#> <bytecode: 0x7fe628dd18d0>
#> <environment: namespace:orthopolynom>

[topepo]

I'd say that we should close the issue. This was a nice to have but we are not going to take on other dependencies for a niche feature.

[EmilHvitfeldt]

Agree. This feature would be a fairly straightforward for someone to put in an extension package as another step if they really need it.

[github-actions]

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