| rowsum {base} | R Documentation |
Compute sums across rows of a matrix for each level of a grouping variable.
rowsum(x, group, reorder = TRUE)
x |
a matrix or vector of numeric data. Missing values are allowed. |
group |
a vector giving the grouping, with one element per row of
x. Missing values are not allowed. |
reorder |
if TRUE, then the result will be in order of
sort(unique(group)), if FALSE, it will be in the order
that rows were encountered (and may run faster for large matrices).
The default is to reorder the data, so as to agree with
tapply (see example below). |
a matrix containing the sums. There will be one row per unique value
of group.
Terry Therneau
x <- matrix(runif(100), ncol=5) group <- sample(1:8, 20, T) xsum <- rowsum(x, group) ## same result another way, slower, and temp may be much larger than x temp <- model.matrix(~ a - 1, data.frame(a=as.factor(group))) xsum2<- t(temp) %*% x ## same as last one, but really slow xsum3 <- tapply(x, list(group[row(x)], col(x)), sum)