stack Stack or Unstack Vectors from a Data Frame or List

Description

Stacking vectors concatenates multiple vectors into a single vector along with a factor indicating where each observation originated. Unstacking reverses this operation.

Usage

stack(x, ...)
## Default S3 method:
stack(x, drop=FALSE, ...)
## S3 method for class 'data.frame'
stack(x, select, drop=FALSE, ...)

unstack(x, ...)
## Default S3 method:
unstack(x, form, ...)
## S3 method for class 'data.frame'
unstack(x, form, ...)

Arguments

x	a list or data frame to be stacked or unstacked.
select	an expression, indicating which variable(s) to select from a data frame.
form	a two-sided formula whose left side evaluates to the vector to be unstacked and whose right side evaluates to the indicator of the groups to create. Defaults to formula(x) in the data frame method for unstack.
drop	Whether to drop the unused levels from the “ind” column of the return value.
...	further arguments passed to or from other methods.

Details

The stack function is used to transform data available as separate columns in a data frame or list into a single column that can be used in an analysis of variance model or other linear model. The unstack function reverses this operation.

Note that stack applies to vectors (as determined by is.vector): non-vector columns (e.g., factors) will be ignored with a warning. Where vectors of different types are selected they are concatenated by unlist whose help page explains how the type of the result is chosen.

These functions are generic: the supplied methods handle data frames and objects coercible to lists by as.list.

Value

unstack produces a list of columns according to the formula form. If all the columns have the same length, the resulting list is coerced to a data frame.

stack produces a data frame with two columns:

values	the result of concatenating the selected vectors in x.
ind	a factor indicating from which vector in x the observation originated.

Author(s)

Douglas Bates

See Also

lm, reshape

Examples

require(stats)
formula(PlantGrowth)         # check the default formula
pg <- unstack(PlantGrowth)   # unstack according to this formula
pg
stack(pg)                    # now put it back together
stack(pg, select = -ctrl)    # omitting one vector