instead of NA, is it possible to get left_join / right_join to fill rows that were not joined with something else?

Question

> data.frame(A=1:10, B=1:10) |> dplyr::left_join(data.frame(A=1:5, C=1:5, D=1:5))
Joining, by = "A"
    A  B  C  D
1   1  1  1  1
2   2  2  2  2
3   3  3  3  3
4   4  4  4  4
5   5  5  5  5
6   6  6 NA NA
7   7  7 NA NA
8   8  8 NA NA
9   9  9 NA NA
10 10 10 NA NA

Instead of the NA, can I tell left_join to fill the cells that were not joinable with something else?

Answer 1

We could write a function: Learned here: R Left Outer Join with 0 Fill Instead of NA While Preserving Valid NA's in Left Table

library(dplyr)
library(tidyr) 

left_join_999 <- function(x, y, fill = 999, ...){
  z <- left_join(x, y, ...)
  new_cols <- setdiff(names(z), names(x))
  z <- replace_na(z, setNames(as.list(rep(fill, length(new_cols))), new_cols))
  z
}

left_join_999(df1, df2, by="A")

output:

  A  B   C   D
1   1  1   1   1
2   2  2   2   2
3   3  3   3   3
4   4  4   4   4
5   5  5   5   5
6   6  6 999 999
7   7  7 999 999
8   8  8 999 999
9   9  9 999 999
10 10 10 999 999