split                  package: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)

