Use data.table to count and aggregate / summarize a column

Question

I want to count and aggregate(sum) a column in a data.table, and couldn\'t find the most efficient way to do this. This seems to be close to what I want R summariz

Answer 1

The post you are referring to gives a method on how to apply one aggregation method to several columns. If you want to apply different aggregation methods to different columns, you can do:

dat[, .(count = .N, var = sum(VAR)), by = MNTH]

this results in:

     MNTH count var
1: 201501     4   2
2: 201502     3   0
3: 201503     5   2
4: 201504     4   2

You can also add these values to your existing dataset by updating your dataset by reference:

dat[, `:=` (count = .N, var = sum(VAR)), by = MNTH]

this results in:

> dat
      MNTH VAR count var
 1: 201501   1     4   2
 2: 201501   1     4   2
 3: 201501   0     4   2
 4: 201501   0     4   2
 5: 201502   0     3   0
 6: 201502   0     3   0
 7: 201502   0     3   0
 8: 201503   0     5   2
 9: 201503   0     5   2
10: 201503   1     5   2
11: 201503   1     5   2
12: 201503   0     5   2
13: 201504   1     4   2
14: 201504   0     4   2
15: 201504   1     4   2
16: 201504   0     4   2

For further reading about how to use data.table syntax, see the Getting started guides on the GitHub wiki.