连接和分配其中一个为空的data.tables时出错

Question

I ran into this error when joining and assigning columns with an empty data.table . When I try to assign more than one column I get an error. But assigning columns one by one is fine.

> D1 <- data.table(l = "a", x = 0)
> D2 <- data.table(l = character(0), z = numeric(0))
> D1[D2, `:=`(x = x + z, u = x * z), on = "l"] ## Error

Error in `[.data.table`(D1, D2, `:=`(x = x + z, u = x * z), on = "l") : 
RHS of assignment to existing column 'x' is zero length but not NULL. 
If you intend to delete the column use NULL. Otherwise, the RHS must 
have length > 0; e.g., NA_integer_. If you are trying to change the 
column type to be an empty list column then, as with all column type 
changes, provide a full length RHS vector such as 
vector('list',nrow(DT)); i.e., 'plonk' in the new column

> D1[D2, u := x * z, on = "l"] ## Fine
> D1[D2, x := x + z, on = "l"] ## Fine

Not sure why it happens and how to avoid this error. Any thought on that?

EDIT: the version of data.table I use is 1.10.4 and R 3.4.3

Answer 1

Not sure why it's happening (I assume it's the different error handling techniques in single := and grouped := ( ':='(x=x*z,u=x+z) )), but clearly if you change the assignee fields in the line to ones that are not present in D1, it will work. Where NEW replaces x :

D1[D2, ':='(NEW = x + z, u = x * z), on = 'l']

You might also consider something to handle cases:

if (nrow(D1[D2, on = 'l']) == 0) {
  D1[, ':='(x = NA, u = NA)]
} else {
  D1[D2, ':='(x = x + z, u = x * z), on = 'l']
}

Answer 2

I am not exactly sure what you are trying to achieve but i have not get any ERROR as like you

D1 <- data.table(l = "a", x = 0)
D2 <- data.table(l = as.character(0), z = as.numeric(0))
D1[D2, `:=`(x = x + z, u = x * z), on = "l"]