split {base} | R Documentation |

## Divide into Groups

### 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.

`f`

is recycled as necessary and if the length of `x`

is not
a multiple of the length of `f`

a warning is printed.
`unsplit`

works only with lists of vectors. 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.

Any missing values in `f`

are dropped together with the
corresponding values of `x`

.

### 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 *used* factor levels given by `f`

. (If `f`

is
longer than `x`

then some of the components will be of zero length.)

The replacement forms return their right hand side. `unsplit`

returns a vector 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`

### Examples

require(stats)
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
z <- unsplit(lapply(split(x, g), scale), g)
tapply(z, g, mean)
# or
z <- x
split(z, g) <- lapply(split(x, g), scale)
tapply(z, g, sd)
## 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 2.5.0

Index]