Many data analysis tasks involve first splitting the data into subsets, applying some operation on each subset, and then combining the results suitably. A common wrinkle in applying this happens to be the numerous possible combinations of input and output object types. The
plyr package provides simple functions to apply this pattern while simplifying the specification of the object types through systematic naming of the functions.
plyr function name has three parts:
plyr function names,
d represents a data frame,
l represents a list, and
a represents ...