# split {base}

### 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, ...) 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
`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.

### 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`

. 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.

### Values

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.

### 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) 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 3.0.2. License: GPL-2.