Hello Mike,
You have identified a problem with the str2double function. It is currently not supported in the run-time engine. However, it is not appropriately blocked from executing. Consequently, it returns NaN instead of an error code. Our documentation is also incorrect. It should list str2double as not being available in the run-time engine. We will look into making the function behave consistently with the various pieces it should work with (e.g. desktop, built-app, etc.). However, since, as you mention, str2num works in the run-time engine, perhaps str2double can be made to work as well. We will look into implementing this for a future release.
Grant M.
Staff Software Engineer | LabVIEW Math & Signal Processing | National Instruments