split {base} | R Documentation |
Divide into Groups and Reassemble
Description
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
.
Usage
split(x, f, drop = FALSE, ...)
## Default S3 method:
split(x, f, drop = FALSE, sep = ".", lex.order = FALSE, ...)
split(x, f, drop = FALSE, ...) <- value
unsplit(value, f, drop = FALSE)
Arguments
x |
vector or data frame containing values to be divided into groups. |
f |
a ‘factor’ in the sense that |
drop |
logical indicating if levels that do not occur should be dropped
(if |
value |
a list of vectors or data frames compatible with a
splitting of |
sep |
character string, passed to |
lex.order |
logical, passed to |
... |
further potential arguments passed to methods. |
Details
split
and 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 value
.
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 x
.
The default method calls interaction
when f
is a
list
. If the levels of the factors contain ‘.’
the factors may not be split as expected, unless sep
is set to
string not present in the factor levels
.
Value
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).
The replacement forms return their right hand side. unsplit
returns a vector or data frame for which split(x, f)
equals
value
References
Becker, R. A., Chambers, J. M. and Wilks, A. R. (1988) The New S Language. Wadsworth & Brooks/Cole.
See Also
cut
to categorize numeric values.
strsplit
to split strings.
Examples
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)
## Alternative using a formula
identical(l, split(airquality, ~ Month))
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)