accessing Y columns with duplicated names in j of X[Y, j] merges

Question

Suppose I have data like this: ``` set.seed(1) DT <- data.table(id=rep(1:3,each=3),y=1997L+sample(1:9,9)) DT2<- data.table(id=1:3,y=1997L+sample(1:3,3)) ``` I want to use DT2$y after merging with DT. I see that this column is named `y.1` after the merge ``` setkey(DT,id) names(DT[DT2]) # [1] "id" "y" "y.1" DT[DT2][,y.1] # [1] 1998 1998 1998 2000 2000 2000 1999 1999 1999 ``` However, I cannot use it with that name in `j`: ``` DT[DT2,y.1] # Error in `[.data.table`(DT, DT2, y.1) : object 'y.1' not found ``` What is the secret prefix or postfix that I should be using here?

Accepted Answer

You can use

```
DT[DT2, i.y]
```

and if you find yourself surprised that it's not the same output as `DT[DT2][, y.1]`, see this thread

accessing Y columns with duplicated names in j of X[Y, j] merges

Solution

Problem