R cannot be resolved to a variable

Question

I would like to fix this error

R cannot be resolved to a variable

I looked up many answers, but I could not get the right one;

Answer 1

Nothing mentioned above worked for me. However, I fixed it myself.

Solution:

If you've any other files except activity_main.xml / main.xml inside app_name/res/layout remove it and try clean and build.

Note:

The unwanted files or (rather used later) files inside app_name/res/layout would be *.out.xml. Remove the same.