data.table Subsetting rows by group Selecting groups by condition
Example
# example data
DT = data.table(Titanic)
Suppose we want to see each class only if a majority survived:
DT[, if (sum(N[Survived=="Yes"]) > sum(N[Survived=="No"]) ) .SD, by=Class]
# Class Sex Age Survived N
# 1: 1st Male Child No 0
# 2: 1st Female Child No 0
# 3: 1st Male Adult No 118
# 4: 1st Female Adult No 4
# 5: 1st Male Child Yes 5
# 6: 1st Female Child Yes 1
# 7: 1st Male Adult Yes 57
# 8: 1st Female Adult Yes 140
Here, we return the subset of data .SD only if our condition is met. An alternative is
DT[, .SD[ sum(N[Survived=="Yes"]) > sum(N[Survived=="No"]) ) ], by=Class]
but this has sometimes proven slower.
