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:

更多阅读：

http://www.cnblogs.com/cloudtj/articles/5540913.html

http://www.jianshu.com/p/bfddfe29aa39

http://plyr.had.co.nz/

revalue

library(plyr) # For the revalue() function
birthwt1$smoke

ddply