R: Give column sums of a matrix or data frame, based on a grouping variable

rowsum {base} | R Documentation

## Give column sums of a matrix or data frame, based on a grouping variable

### Description

Compute column sums across rows of a matrix-like object for each level of a grouping variable. `rowsum` is generic, with a method for data frames and a default method for vectors and matrices.

### Usage

    rowsum(x, group, reorder = TRUE, ...)

    ## S3 method for class 'data.frame':
    rowsum(x, group, reorder = TRUE, na.rm = FALSE, ...)

    ## Default S3 method:
    rowsum(x, group, reorder = TRUE, na.rm = FALSE, ...)

### Arguments

`x` | a matrix, data frame or vector of numeric data. Missing values are allowed. A numeric vector will be treated as a column vector.
`group` | a vector or factor giving the grouping, with one element per row of `x`. Missing values will be treated as another group and a warning will be given.
`reorder` | if `TRUE`, then the result will be in order of `sort(unique(group))`, if `FALSE`, it will be in the order that groups were encountered.
`na.rm` | logical (`TRUE` or `FALSE`). Should `NA` values be discarded?
`...` | other arguments to be passed to or from methods

### Details

The default is to reorder the rows to agree with `tapply` as in the example below. Reordering should not add noticeably to the time except when there are very many distinct values of `group` and `x` has few columns.

The original function was written by Terry Therneau, but this is a new implementation using hashing that is much faster for large matrices.

To sum over all the rows of a matrix (ie, a single `group`) use `colSums`, which should be even faster.

### Value

A matrix or data frame containing the sums. There will be one row per unique value of `group`.

### See Also

`tapply`, `aggregate`, `rowSums`

### Examples

    require(stats)

    x <- matrix(runif(100), ncol=5)
    group <- sample(1:8, 20, TRUE)
    (xsum <- rowsum(x, group))
    ## Slower versions
    tapply(x, list(group[row(x)], col(x)), sum)
    t(sapply(split(as.data.frame(x), group), colSums))
    aggregate(x, list(group), sum)[-1]

[Package base version 2.8.1 Index]