by {base}  R Documentation 
Apply a Function to a Data Frame Split by Factors
Description
Function by
is an objectoriented wrapper for
tapply
applied to data frames.
Usage
by(data, INDICES, FUN, ..., simplify = TRUE)
Arguments
data 
an R object, normally a data frame, possibly a matrix. 
INDICES 
a factor or a list of factors, each of length

FUN 
a function to be applied to (usually dataframe) subsets of

... 
further arguments to 
simplify 
logical: see 
Details
A data frame is split by row into data frames
subsetted by the values of one or more factors, and function
FUN
is applied to each subset in turn.
For the default method, an object with dimensions (e.g., a matrix) is
coerced to a data frame and the data frame method applied. Other
objects are also coerced to a data frame, but FUN
is applied
separately to (subsets of) each column of the data frame.
Value
An object of class "by"
, giving the results for each subset.
This is always a list if simplify
is false, otherwise a list or
array (see tapply
).
See Also
tapply
, simplify2array
.
array2DF
to convert result to a data
frame. ave
also applies a function blockwise.
Examples
require(stats)
by(warpbreaks[, 1:2], warpbreaks[,"tension"], summary)
by(warpbreaks[, 1], warpbreaks[, 1], summary)
by(warpbreaks, warpbreaks[,"tension"],
function(x) lm(breaks ~ wool, data = x))
## now suppose we want to extract the coefficients by group
tmp1 < with(warpbreaks,
by(warpbreaks, tension,
function(x) lm(breaks ~ wool, data = x)))
sapply(tmp1, coef)
## another way
tmp2 < by(warpbreaks, ~ tension,
with, coef(lm(breaks ~ wool)))
array2DF(tmp2, simplify = TRUE)