Hi Ahmad, welcome to community.rstudio.com!
I hope Joels' reply helps you.
But if it doesn't, and for questions in the future, be sure when you ask a programming question like this to supply a reproducible example. This will help fine tune your question to make it easier to help you. (Not sure what a reproducible example is? FAQ: What's a reproducible example (`reprex`) and how do I do one?)