A few things to remember when writing commands in R, to avoid errors and warnings:
Variable_A
is different from variable_A
When a command is run, the R Console may show you warning or error messages in red text.
A warning means that R has completed your command, but had to take additional steps or produced unusual output that you should be aware of.
An error means that R was not able to complete your command.
Look for clues:
The error/warning message will often include a line number for the problem.
If an object “is unknown” or “not found”, perhaps you spelled it incorrectly, forgot to call a package with library(), or forgot to re-run your script after making changes.
If all else fails, copy the error message into Google along with some key terms - chances are that someone else has worked through this already!