Function by
is an objectoriented wrapper for
tapply
applied to data frames.
1 
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 
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.
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
).
tapply
, simplify2array
.
ave
also applies a function blockwise.
1 2 3 4 5 6 7 8 9 10 11  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
tmp < with(warpbreaks,
by(warpbreaks, tension,
function(x) lm(breaks ~ wool, data = x)))
sapply(tmp, coef)

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