This is a really simple (i.e. probably even the most basic possible) example of using the built in aggregate() function in R, basically a sumif in R.
Given a set of data where column a is the values and column b contains the data by which you wish to group (in this example categorical data with values of a or b):
> data a b 1 0 a 2 0 a 3 1 b 4 2 a 5 2 b 6 2 a 7 3 b 8 4 b 9 7 b
The aggregate function as I have used it below takes the following arguments — data$a is the column we are applying a function to; by = refers to a list of names and references to aggregate things by (in this case just one column, so the list has a single element); finally the function we are using here is the built in sum (but this could be a function of your own making, or a mean, or whatever):
> aggregate(data$a, by = list(Category = data$b), FUN = sum) Category x 1 a 4 2 b 17
That’s it. This replaces the most basic Excel sumif functionality using R’s aggregate function.