Odtnhj

Im trying to evaluate a polynomial P(x):

However i dont think im entering the polynomial itself correctly in my function.
Code for function:

directpoly1 <- function(x, coef, seqcoef = seq(coef) - 1) 
 sum(coef*x^seqcoef)

directpoly <- function(x, coef) 
 seqcoef <- seq(coef) - 1
 sapply(x, directpoly1, coef, seqcoef)

Code for using function:

directpoly(x=seq(-10,10, length=5000000), rep(c(2,-1),20))

Any ideas on how to enter it correctly?

Here are two ways of writing your function.

1. With a sapply loop. Makes the code more readable and some times faster.


2. With a standard for loop.

There is no need to write a separate function outside the function to be called with the x values, I have placed that function in the body of the main functions.

Then both functions are tested, with a smaller input vector. As you can see, the for loop function is faster.

directpoly <- function(x, n = 39, coef = NULL)
 f <- function(y) sum(coef * y^seqcoef)
 if(all(is.null(coef))) coef <- rep(c(2, -1), length.out = n + 1)
 seqcoef <- rev(seq_along(coef) - 1)
 sapply(x, f)


directpoly2 <- function(x, n = 39, coef = NULL)
 f <- function(y) sum(coef * y^seqcoef)
 if(all(is.null(coef))) coef <- rep(c(2, -1), length.out = n + 1)
 seqcoef <- rev(seq_along(coef) - 1)
 y <- numeric(length(x))
 for(i in seq_along(y))
 y[i] <- f(x[i])
 y


library(microbenchmark)
library(ggplot2)

x <- seq(-10, 10, length = 50000)
mb <- microbenchmark(
 Sapply = directpoly(x),
 Forloop = directpoly2(x)
)

autoplot(mb)