R: Apply a Function to Rows of a Matrix or Vector

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 `FUN` argument
`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 `TRUE` to keep result as a matrix even if `simplify` is `TRUE`, in the case of only one stratum

### 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
f.harrell@vanderbilt.edu

### See Also

`asNumericMatrix`, `matrix2dataFrame`, `tapply`, `sapply`, `lapply`, `mapply`, `by`.

### Examples

    require(datasets, TRUE)
    a <- mApply(iris[,-5], iris$Species, mean)

[Package Hmisc version 4.2-0 Index]