Hadley Wickham has written several R packages to tweak data structures, for example, a major part of his thesis concentrated on how to reshape data frames with his reshape package. Since then, this general aggregation and restructuring package has been renewed to be more efficient with the most commonly used tasks, and it was released with a new version number attached to the name: reshape2 package.

This was a total rewrite of the reshape package, which improves speed at the cost of functionality. Currently, the most important feature of reshape2 is the possibility to convert between the so-called long (narrow) and wide tabular data format. This basically pertains to the columns being stacked below each other, or arranged beside each other.

These features were presented in Hadley's works with the following image on data restructuring, with the related reshape functions and simple use cases:

As the reshape package is not under active development anymore, and...