假设我有以下data.table
> DT
# A B C D E N
# 1: J t X D N 0.07898388
# 2: U z U L A 0.46906049
# 3: H a Z F S 0.50826435
# ---
# 9998: X b R L X 0.49879990
# 9999: Z r U J J 0.63233668
# 10000: C b M K U 0.47796539
现在我需要按一对列进行分组并计算总和N.当您事先知道列名时,这很容易做到:
> DT[, sum(N), by=.(A,B)]
# A B V1
# 1: J t 6.556897
# 2: U z 9.060844 …