Solving Equation Generated by pchipinterp method

Hi,

I have an array of data which I fitted a curve to using the fit function and pchipinterp method, generating a cfit variable. I know that the line y = (a specific scalar value) will cross the fitted line/function 4 times. How can I solve for those 4 x-coordinates given a specific y-value? Thanks.

Answer 1

[John D'Errico]

The simplest way to do this is to use my SLM toolbox.

x = sort(rand(1,20));
y = rand(1,20);
spl = pchip(x,y);
fnplt(spl)
yline(0.3);

What are the x locations where this curve crosses 0.3? There should be 7 such points based on what we see in the figure.

xsol = slmsolve(spl,0.3)
xsol =
  Columns 1 through 4
        0.0856060691907258         0.179689798032185         0.287680862853776         0.374467025155671
  Columns 5 through 7
         0.637324302388667         0.644738455928051         0.984815754063193

You can download my SLM toolbox from the File Exchange, here:

https://www.mathworks.com/matlabcentral/fileexchange/24443-slm-shape-language-modeling