How do I aggregate on over the subset and return one value?

How do I aggregate on over the subset and return one value?

References

Reference NO 1

http://stackoverflow.com/questions/15204441/how-do-i-aggregate-on-over-the-subset-and-return-one-value

Reference NO 2

http://stat.ethz.ch/R-manual/R-devel/library/stats/html/aggregate.html

Reference NO 3

https://science.nature.nps.gov/im/datamgmt/statistics/r/fundamentals/manipulation.cfm

Reference NO 4

http://cran.r-project.org/web/packages/reshape/reshape.pdf

Reference NO 5

http://nsaunders.wordpress.com/2010/08/20/a-brief-introduction-to-apply-in-r/

Reference NO 6

http://www.r-statistics.com/2012/01/aggregation-and-restructuring-data-from-r-in-action/

Reference NO 7

http://www.slideshare.net/jeffreybreen/grouping-summarizing-data-in-r

Reference NO 8

http://www.statmethods.net/management/aggregate.html