split: Divide into Groups and Reassemble
In robertzk/monadicbase: The R Base Package

Description Usage Arguments Details Value References See Also Examples

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.

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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. 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.

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

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.

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)