mApply {Hmisc} | R Documentation |
Apply a Function to Rows of a Matrix or Vector
Description
mApply
is like tapply
except that the first argument can
be a matrix or a vector, and the output is cleaned up if simplify=TRUE
.
It uses code adapted from Tony Plate (tplate@blackmesacapital.com) to
operate on grouped submatrices.
As mApply
can be much faster than using by
, it is often
worth the trouble of converting a data frame to a numeric matrix for
processing by mApply
. asNumericMatrix
will do this, and
matrix2dataFrame
will convert a numeric matrix back into a data
frame.
Usage
mApply(X, INDEX, FUN, ..., simplify=TRUE, keepmatrix=FALSE)
Arguments
X |
a vector or matrix capable of being operated on by the
function specified as the |
INDEX |
list of factors, each of same number of rows as 'X' has. |
FUN |
the function to be applied. In the case of functions like '+', ' |
... |
optional arguments to 'FUN'. |
simplify |
set to 'FALSE' to suppress simplification of the result in to an array, matrix, etc. |
keepmatrix |
set to |
Value
For mApply
, the returned value is a vector, matrix, or list.
If FUN
returns more than one number, the result is an array if
simplify=TRUE
and is a list otherwise. If a matrix is returned,
its rows correspond to unique combinations of INDEX
. If
INDEX
is a list with more than one vector, FUN
returns
more than one number, and simplify=FALSE
, the returned value is a
list that is an array with the first dimension corresponding to the last
vector in INDEX
, the second dimension corresponding to the next
to last vector in INDEX
, etc., and the elements of the list-array
correspond to the values computed by FUN
. In this situation the
returned value is a regular array if simplify=TRUE
. The order
of dimensions is as previously but the additional (last) dimension
corresponds to values computed by FUN
.
Author(s)
Frank Harrell
Department of Biostatistics
Vanderbilt University
fh@fharrell.com
See Also
asNumericMatrix
, matrix2dataFrame
, tapply
,
sapply
, lapply
, mapply
, by
.
Examples
require(datasets, TRUE)
a <- mApply(iris[,-5], iris$Species, mean)