[R] How to use tapply with more than one variables grouped

[noobmin]

I'm studying alone the R language for data preparation. I found a course at
MIT for data preparation that uses python but I'm using R to learning. The
first exercise is the preparation of data from a database that shows the
contributions made to candidates for U.S. president. The database is
described in FORMART
ftp://ftp.fec.gov/FEC/Presidential_Map/2012/DATA_DICTIONARIES/CONTRIBUTOR_FORMAT.txt
link. I wonder how to print the table showing how many states are President
Obama the top candidate (by full amount of donations received) with R
language?

I try using tapply method but, i dont understand how to working with more
than one variable grouped. Could anyone help me in advance of the studies?



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