Split, apply, and combine in R using PLYR

November 4, 2009

(This article was first published on Getting Genetics Done, and kindly contributed to R-bloggers)

While flirting around with previously mentioned ggplot2 I came across an incredibly useful set of functions in the plyr package, made by Hadley Wickham, the same guy behind ggplot2.  If you’ve ever used MySQL before, think of “GROUP BY”, but here you can arbitrarily apply any R function to splits of the data, or write one yourself.

Imagine you have two SNPs, and 2 different variables you want

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