filter for value on groups data.table [duplicate]

Question

I'm looking to filter a data.table for a max value on groups.

DT <- data.table(V1 = c(1L, 2L),
                 V2 = LETTERS[1:3],
                 V3 = round(rnorm(4), 4),
                 V4 = 1:12)

 V1 V2     V3     V4
 1:  1  A -0.1346  1
 2:  2  A  0.2309  4
 3:  1  A  0.7067  7
 4:  2  A -1.0082 10
 5:  2  B -1.0082  2
 6:  1  B -0.1346  5
 7:  2  B  0.2309  8
 8:  1  B  0.7067 11
 9:  1  C  0.7067  3
10:  2  C -1.0082  6
11:  1  C -0.1346  9
12:  2  C  0.2309 12

I've tried this but no dice:

DT[,max(V3), by =.(V2)]

   V2     V1
1:  A 1.2281
2:  B 1.2281
3:  C 1.2281

Short of a loop, how would I approach this? I prefer a data.table method.

Answer 1

We can create a row index to subset the dataset

DT[DT[, .I[V3 == max(V3)], by = V2]$V1]

If there is only single max element for each 'V2'

DT[DT[, .I[which.max(V3)], by = V2]$V1]