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plyr is a set of tools for a common set of problems: you need to split up a big data structure into homogeneous pieces, apply a function to each piece and then combine all the results back together. For example, you might want to:

It's already possible to do this with base R functions (like split and the apply family of functions), but plyr makes it all a bit easier with:

Considerable effort has been put into making plyr fast and memory efficient, and in many cases plyr is as fast as, or faster than, the built-in equivalents.

A detailed introduction to plyr has been published in JSS: "The Split-Apply-Combine Strategy for Data Analysis", You can find out more at, or track development at You can ask questions about plyr (and data manipulation in general) on the plyr mailing list. Sign up at