r - Update subset of data.table based on join -

i have 2 data tables, dt1 , dt2:

set.seed(1) dt1<-data.table(id1=rep(1:3,2),id2=sample(letters,6), v1=rnorm(6), key="id2") dt1 ##    id1 id2         v1 ## 1:   2   e  0.7383247 ## 2:   1   g  1.5952808 ## 3:   2   j  0.3295078 ## 4:   3   n -0.8204684 ## 5:   3   s  0.5757814 ## 6:   1   u  0.4874291  dt2<-data.table(id2=c("n","u"), v1=0, key="id2") dt2 ##    id2 v1 ## 1:   n  0 ## 2:   u  0 

i update dt1 based on join dt2, subset of dt1. example, dt1[id1==3], expect value of v1 in row 4 updated in following result:

dt1 ##    id1 id2         v1 ## 1:   2   e  0.7383247 ## 2:   1   g  1.5952808 ## 3:   2   j  0.3295078 ## 4:   3   n          0 ## 5:   3   s  0.5757814 ## 6:   1   u  0.4874291 

i know how update table (using := assignment operator), how join tables (dt1[dt2]), , how subset table (dt1[id1==3]). i'm not sure how 3 @ once.

edit: note original example attempts update 1 column, actual data requires updating many columns. consider additional scenarios in dt1b , dt2b:

set.seed(2) dt1b<-dt1[,v2:=rnorm(6)] # copy dt1 , add new column setkey(dt1b,id2) dt1b ##    id1 id2         v1          v2 ## 1:   2   e  0.7383247 -0.89691455 ## 2:   1   g  1.5952808  0.18484918 ## 3:   2   j  0.3295078  1.58784533 ## 4:   3   n -0.8204684 -1.13037567 ## 5:   3   s  0.5757814 -0.08025176 ## 6:   1   u  0.4874291  0.13242028  dt2b<-rbindlist(list(dt2,data.table(id2="e",v1=0))) # copy dt2 , add new row dt2b[,v2:=-1] # add new column dt2b setkey(dt2b,id2) dt2b ##    id2 v1 v2 ## 1:   e  0 -1 ## 2:   n  0 -1 ## 3:   u  0 -1 

based on helpful answers @nmel , @bluemagister, came solution updated scenario:

dt1b[dt2b[dt1b[id1 %in% c(1,2)],nomatch=0],c("v1","v2"):=list(i.v1,i.v2)] dt1b ##    id1 id2         v1          v2 ## 1:   2   e  0.0000000 -1.00000000 ## 2:   1   g  1.5952808  0.18484918 ## 3:   2   j  0.3295078  1.58784533 ## 4:   3   n -0.8204684 -1.13037567 ## 5:   3   s  0.5757814 -0.08025176 ## 6:   1   u  0.0000000 -1.00000000 

the easiest way can think of key id1 well. eg

setkey(dt1, id2,id1) dt2[, id1 := 3] setkey(dt2, id2, id1)  # use i.v1 reference v1 component dt1[dt2, v1 := i.v1 ]   dt1    id1 id2        v1 1:   2   e 0.7383247 2:   1   g 1.5952808 3:   2   j 0.3295078 4:   3   n 0.0000000 5:   3   s 0.5757814 6:   1   u 0.4874291