CODEWITHSUNDEEP
rfunction
Christine Blume
Christine Blume

Reputation: 527

Where does function equal certain value

I have fitted a function to my data:

BCF.plot <- function(x) {
  vv[2] +((vv[3]/(2*(1-vv[4])))*(cos(x-vv[1])-vv[4]+abs(cos(x-vv[1])-vv[4])))
}

It is a baseline (b) cosine wave, i.e. a baseline with a cosine wave on top of it. Now I have a certain value on the Y-axis (dlmo_val) and I want to know which x value corresponds to it. I have tried something like this:

BCF.dlmo <- function(x, dlmo_val = 0) {
  vv[2] +((vv[3]/(2*(1-vv[4])))*(cos(x-vv[1])-vv[4]+abs(cos(x-vv[1])-vv[4])))-b-dlmo_val ## find point where function minus baseline & dlmo_val is 0
}

vv = c(2.3971780, 2.0666526, 11.1775231, 0.7870128)
b = 2.066653
H = 11.17752

dlmo_val = 0.4*H ## dlmo*peak height above baseline, H is result from optimisation
uniroot(BCF.dlmo, c(0.2617994, 6.021386), dlmo_val=dlmo_val)  ## lower & upper are min(x) and max(x)

However, uniroot tells me 

"...values at end points not of opposite sign"

I am not really sure how to go about this. Any recommendations are more than welcome!

Upvotes: 1

Views: 183

Answers (1)

David Robinson
David Robinson

Reputation: 78660

As described in this post, uniroot() is designed for finding only one zero in a function, while you have two zeroes. You could call it on a smaller interval:

uniroot(BCF.dlmo, c(0.2617994, 2.5), dlmo_val = dlmo_val)$root
# [1] 1.886079

As that post describes, you can instead use the unitroot.all function in the rootSolve package to find both zeroes:

library(rootSolve)
uniroot.all(BCF.dlmo, c(0.2617994, 6.021386), dlmo_val = dlmo_val)
# [1] 1.886084 2.908276

Upvotes: 1

Related Questions