The error message you're encountering in MATLAB indicates that the fnval function is not able to operate on the input provided because it's expecting a different data type than 'struct'. fnval is a function that evaluates a spline at specified points, and it typically expects a spline object as created by MATLAB's Curve Fitting Toolbox or the Spline Toolbox.
Here are some steps you can take to troubleshoot this error:
- Check the Input Type: Verify the type of the variable I that you are passing to fnval.
- Check the Spline Representation: If I is supposed to be a spline representation, make sure it was created correctly using the appropriate spline functions (e.g., spapi, spap2, csapi, cscvn, etc.).
- Examine the Spline Creation: If you cannot modify the file where I is created, examine the process by which I is being generated to ensure it's producing a valid spline object. There may be a step in the process where the spline is being converted to a struct inadvertently.
- MATLAB Version Compatibility: If the code was written for an older version of MATLAB and you're running it on a newer version, there might be compatibility issues. Check the MATLAB documentation for the version you're using to see if there have been any changes to the spline functions.
- Alternative Evaluation Function: If I contains the necessary information in a different format, you might need to use a different function for evaluation. For example, if I contains coefficients and knots of a B-spline, you might need to reconstruct the spline using these before evaluation.
