我如何找到Cust_map == 1和2的data.table的NumTrans列总和

Question

> SS
   Cust_map Merch_Map_final TXN_MTH        SPND_CATGY NumTrans
1:        1            2412  201310 BUSINESS SERVICES        2
2:        1            2412  201401 BUSINESS SERVICES        3
3:        1            2412  201403 BUSINESS SERVICES        3
4:        1            2412  201406 BUSINESS SERVICES        3
5:        1            2412  201407 BUSINESS SERVICES        2
6:        2            2415  201310  GROCERY                 5

Answer 1

我们可以按'Cust_map'分组并获取'{1}}'NumTrans'

sum