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

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 2.5.0

Index]