plyr: plyr: the split-apply-combine paradigm for R.

By design, no plyr function will preserve row names - in general it is too hard to know what should be done with them for many of the operations supported by plyr. If you want to preserve row names, use name_rows to convert them into an explicit column in your data frame, perform the plyr operations, and then use name_rows again to convert the column back into row names.

Plyr also provides a set of helper functions for common data analysis problems:

• arrange: re-order the rows of a data frame by specifying the columns to order by

• mutate: add new columns or modifying existing columns, like transform, but new columns can refer to other columns that you just created.

• summarise: like mutate but create a new data frame, not preserving any columns in the old data frame.

• join: an adapation of merge which is more similar to SQL, and has a much faster implementation if you only want to find the first match.

• match_df: a version of join that instead of returning the two tables combined together, only returns the rows in the first table that match the second.

• colwise: make any function work colwise on a dataframe

• rename: easily rename columns in a data frame

• round_any: round a number to any degree of precision

• count: quickly count unique combinations and return return as a data frame.