If you are using dropbox for keeping your R script files and you are working from multiple computers, setting the working directory every time could be a real pain. The good news is that you can avoid the hassle using rtstudioapi and single line of code in the beginning of your R file.
First install rstudioapi library using packages part of your R Studio environment.
Add the following line of code in the beginning of each of your R script files.
Voila! There is no more need to update the path using setwd every time you are using a different computer.