function call from another r file

If you want to handle a generic case, best thing is to create an R
package.

And better than best is to use devtools to save you a fiddly
edit/build/install cycle. You don't even have to think of it as a
package, its just a folder called R with your .R files in, and a
DESCRIPTION file (metadata is always a good thing), and your functions
get attached in a separate position so they don't clutter ls(), and
load_all() will load the just the ones you've edited. Magic.

 http://rpubs.com/geospacedman/lazydevtools

Easy peasy.

Barry