R: Divide into Groups and Reassemble

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, ...)
    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.

### 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)
    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)

[Package base version 3.1.2 Index]