[R-pkgs] reshape2: a reboot of the reshape package

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[R-pkgs] reshape2: a reboot of the reshape package

Hadley Wickham-2
Reshape2 is a reboot of the reshape package. It's been over five years
since the first release of the package, and in that time I've learned
a tremendous amount about R programming, and how to work with data in
R. Reshape2 uses that knowledge to make a new package for reshaping
data that is much more focussed and much much faster.

This version improves speed at the cost of functionality, so I have
renamed it to `reshape2` to avoid causing problems for existing users.
 Based on user feedback I may reintroduce some of these features.

What's new in `reshape2`:

 * considerably faster and more memory efficient thanks to a much better
   underlying algorithm that uses the power and speed of subsetting to the
   fullest extent, in most cases only making a single copy of the data.

 * cast is replaced by two functions depending on the output type: `dcast`
   produces data frames, and `acast` produces matrices/arrays.

 * multidimensional margins are now possible: `grand_row` and `grand_col` have
   been dropped: now the name of the margin refers to the variable that has
   its value set to (all).

 * some features have been removed such as the `|` cast operator, and the
   ability to return multiple values from an aggregation function. I'm
   reasonably sure both these operations are better performed by plyr.

 * a new cast syntax which allows you to reshape based on functions
   of variables (based on the same underlying syntax as plyr):

 * better development practices like namespaces and tests.

This work has been generously supported by BD (Becton Dickinson).

Assistant Professor / Dobelman Family Junior Chair
Department of Statistics / Rice University

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