Update subset of data.table based on join

Question

I have two data tables, DT1 and DT2:

set.seed(1)
DT1<-data.table(id1=rep(1:3,2),id2=sample(letters,6), v1=rnorm(6), key=\"id2\")
DT1
##    id1 id2                 


        

Answer 1

This is similar to mnel's solution but uses ifelse instead of a second key.

DT1[DT2, v1  := ifelse(id1==3, i.v1, v1),nomatch=0]