R cannot be resolved to a variable
There are many possible reason that you will get "R cannot be resolved to a variable" in eclipse. This post I will list some of them that I have met.
1. Your package has been changed, but not complete
Check these position:
Make sure your package name in AndroidManifest.xml, *.java header, must be the same. the name is case sensitive. If the package is not inconsistent each other, you will get "R cannot be resolved to a variable" even if you clear all the project temporary data (Project > Clean).
2. There are some errors with your res directory
Eclipse will not generate R.java if you has something wrong with res files, especially *.xml files. Just open your res and check it again.
Moveover make sure your .xml files are not broken. strings.xml must be checked.
3. Eclipse doe not work again
Project > Clean for your project, and reopen eclipse
4. No permission with your working directory
If you are using linux (eg, ubuntu), make sure you have permision to run/debug the current project.