MATLAB: Defining equations with interpolant cfit model (ex. linearinterp, pchipinterp)

curve fittingequationMATLABsolve

Hi,
does anyone know how to define equations to be used in solve or vpasolve using interpolant cfit model?
I have pchipinterp (pchip_int_fn) and linearinterp (lin_int_fn) cfit models from my raw data and I want to define an equation like the following:
syms x y z
eqn = x*pchip_int_fn(x)*lin_int_fn(x) + y*pchip_int_fn(y)*lin_int_fn(y) + x*pchip_int_fn(z)*lin_int_fn(z) == 0
solve(eqn)
Matlab gives me an error message saying
"Error using cfit/subsref>iParenthesesReference (line 46)
Cannot evaluate CFIT model for some reason.
Error in cfit/subsref (line 16)
        out = iParenthesesReference( obj, currsubs );"
Does anyone know how to resolve this issue?
Thank you so much for your help!

Best Answer

cfit objects do not work with inputs as symbolic variables. To work around this, you could re-write the cfit model equation using symbolic variables and then use it in 'eqn', instead of calling the cfit object.
The other option you could explore to find results for x,y,z is using the Optimization Toolbox, and minimizing the square of the value of 'eqn'. This would be a numerical optimization, and symbolic variables would not be used.
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