| split {base} | R Documentation |
split divides the data in the vector x into the groups
defined by f. The replacement forms replace values
corresponding to such a division. unsplit reverses the effect of
split.
split(x, f, drop = FALSE, ...) split(x, f, drop = FALSE, ...) <- value unsplit(value, f, drop = FALSE)
x |
vector or data frame containing values to be divided into groups. |
f |
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. |
drop |
logical indicating if levels that do not occur should be dropped
(if f is a factor or a list). |
value |
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 and split<- are generic functions with default and
data.frame methods.
f is recycled as necessary and if the length of x is not
a multiple of the length of f a warning is printed.
unsplit works only with lists of vectors. 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.
Any missing values in f are dropped together with the
corresponding values of x.
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 used factor levels given by f. (If f is
longer than x then some of the components will be of zero length.)
The replacement forms return their right hand side. unsplit
returns a vector for which split(x, f) equals value
Becker, R. A., Chambers, J. M. and Wilks, A. R. (1988) The New S Language. Wadsworth & Brooks/Cole.
require(stats) 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 z <- unsplit(lapply(split(x, g), scale), g) tapply(z, g, mean) # or z <- x split(z, g) <- lapply(split(x, g), scale) tapply(z, g, sd) ## 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)