split(x, f, drop = FALSE, ...) split(x, f, drop = FALSE, ...) <- value unsplit(value, f, drop = FALSE)
- vector or data frame containing values to be divided into groups.
- a ‘factor’ in the sense that
as.factor(f)defines the grouping, or a list of such factors in which case their interaction is used for the grouping.
- logical indicating if levels that do not occur should be dropped (if
factoror a list).
- a list of vectors or data frames compatible with a splitting of
x. Recycling applies if the lengths do not match.
- further potential arguments passed to methods.
split<- are generic functions with default and
data.frame methods. The data frame method can also be used to split a matrix into a list of matrices, and the replacement form likewise, provided they are invoked explicitly.
unsplit works with lists of vectors or data frames (assumed to have compatible structure, as if created by
split). It puts elements or rows back in the positions given by
f. In the data frame case, row names are obtained by unsplitting the row name vectors from the elements of
f is recycled as necessary and if the length of
x is not a multiple of the length of
f a warning is printed.
Any missing values in
f are dropped together with the corresponding values of
The default method calls
interaction. If the levels of the factors contain . they may not be split as expected, so the method has argument
sep which is use to join the levels.
The value returned from
split is a list of vectors containing the values for the groups. The components of the list are named by the levels of
f (after converting to a factor, or if already a factor and
drop = TRUE, dropping unused levels).
Becker, R. A., Chambers, J. M. and Wilks, A. R. (1988) The New S Language. Wadsworth & Brooks/Cole.
require(stats); require(graphics) n <- 10; nn <- 100 g <- factor(round(n * runif(n * nn))) x <- rnorm(n * nn) + sqrt(as.numeric(g)) xg <- split(x, g) boxplot(xg, col = "lavender", notch = TRUE, varwidth = TRUE) sapply(xg, length) sapply(xg, mean) ### Calculate 'z-scores' by group (standardize to mean zero, variance one) z <- unsplit(lapply(split(x, g), scale), g) # or zz <- x split(zz, g) <- lapply(split(x, g), scale) # and check that the within-group std dev is indeed one tapply(z, g, sd) tapply(zz, g, sd) ### data frame variation ## Notice that assignment form is not used since a variable is being added g <- airquality$Month l <- split(airquality, g) l <- lapply(l, transform, Oz.Z = scale(Ozone)) aq2 <- unsplit(l, g) head(aq2) with(aq2, tapply(Oz.Z, Month, sd, na.rm = TRUE)) ### Split a matrix into a list by columns ma <- cbind(x = 1:10, y = (-4:5)^2) split(ma, col(ma)) split(1:10, 1:2)
Documentation reproduced from R 3.0.1. License: GPL-2.