Aggregate – A Powerful Tool for Data Frame in R

By David Kun

This post gives a short review of the aggregate function as used for data.frames and presents some interesting uses: from the trivial but handy to the most complicated problems I have solved with aggregate.

Aggregate (data.frame): Technical Overview

Aggregate is a function in base R which can, as the name suggests, aggregate the inputted data.frame d.f by applying a function specified by the FUN parameter to each column of sub-data.frames defined by the by input parameter.

The by parameter has to be a list. However, since data.frame‘s are handled as (named) lists of columns, one or more columns of a data.frame can also be passed as the by parameter. Interestingly, if these columns are of the same data.frame as the one inputted as x, those columns are not passed on to the FUN function.

The function to apply has to be able to accept a vector (since it will be called with parts of a column of a data.frame as input).

The sub-data.frames defined by the by input parameter can be thought of as logical indexing:

d.f 
and do this for every i between 1 and length(unique(by)). Note that the by variable doesn't have to agree with one (or more) column of the data.frame but could be anything. Hence, one can reproduce the aggregate functionality by a for cycle running the cycle variable over the unique values of the variable passed as by and an sapply applying the function passed as FUN to each column of the data.frame sub.data.frame. Such a workaround however would be very difficult to document, as it would be unclear what (and why) this code is actually doing.
Aggregate always returns a data.frame as a result. This data.frame will contain the (now unique) values from the input parameter by as the first column and then columns containing the results of the call to  ...read more
Source:: r-bloggers.com