If you have functions depending on x for a, b and f, you are done.
If not, you can try the following:
Use "interp1" to interpolate a, b and f in the x-value where the coefficients are needed by bvp4c.
Or fit functions to a, b and f with x as independent variable before calling bvp4c and use these function to supply a, b and f at the relevant x-positions.
