Simple frequency tables using data.table

Question

I\'m looking for a way to do simple aggregates / counts via data.table.

Consider the iris data, which has 50 observations per species. To count the observations per

Answer 1

data.table has a couple of symbols that can be used within the j expression. Notably

• .N will give you the number of number of rows in each group.

see ?data.table under the details for by

Advanced: When grouping by by or by i, symbols .SD, .BY and .N may be used in the j expression, defined as follows.

....

.N is an integer, length 1, containing the number of rows in the group.

For example:

dt[, .N ,by = Species]

     Species  N
1:     setosa 50
2: versicolor 50
3:  virginica 50