R cannot be resolved to a variable in Android

First you may clean the project, then run the project. If this does not work then follow the following links:

• Here is the best way to solve this problem: Android Development- Where is my R.Java file?
• R cannot be resolved - Android error
• R cannot be resolved to a variable
• R cannot be resolved to a variable -- mailing list entry
• Fixed: R cannot be resolved to a variable

Source

Make sure that anything the R. links to is not broken. Fix all errors in your XML files. If anything in the ADKs are broken, R will not regenerate.

If you somehow hit something and created import android.R in your activity, remove it.

Run Project -> Clean. This will delete and regenerate R and BuildConfig.

Make sure Project -> Build Automatically is ticked. If not, build it manually via Menu -> Project -> Build Project .

Wait a few seconds for the errors to disappear.

If it doesn't work, delete everything inside the /gen/ folder

If it still doesn't work, try right-clicking your project -> Android Tools -> Fix Project Properties.

Check your *.properties files (in the root folder of your app folder) and make sure that the links in there are not broken.

Right-click your project > properties > Android. Look at the Project Build Target and Library sections on the right side of the page. Your Build Target should match the target in your AndroidManifest.xml. So if it's set to target 17 in AndroidManifest, make sure that the Target Name is Android 4.2. If your Library has an X under the reference, remove and re-add the library until there's a green tick. This might happen if you've moved a few files and folders around -  Source