base (version 3.3.2)

split: 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, ...)
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.

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`

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.

References

Becker, R. A., Chambers, J. M. and Wilks, A. R. (1988) The New S Language. Wadsworth & Brooks/Cole.

`cut` to categorize numeric values. `strsplit` to split strings.

Examples

Run this code
``````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)