R Left Outer Join with 0 Fill Instead of NA While Preserving Valid NA's in Left Table

Question

What is the easiest way to do a left outer join on two data tables (dt1, dt2) with the fill value being 0 (or some other value) instead of NA (default) without overwriting valid

Answer 1

Could you use column indices to refer only to the new columns, as with left_join they'll all be on the right of the resulting data.frame? Here it would be in dplyr:

dt1 <- data.frame(x = c('a', 'b', 'c', 'd', 'e'),
                  y = c(NA, 'w', NA, 'y', 'z'),
                  stringsAsFactors = FALSE)
dt2 <- data.frame(x = c('a', 'b', 'c'),
                  new_col = c(1,2,3),
                  stringsAsFactors = FALSE)

merged <- left_join(dt1, dt2)
index_new_col <- (ncol(dt1) + 1):ncol(merged)
merged[, index_new_col][is.na(merged[, index_new_col])] <- 0

> merged
  x    y new_col
1 a        1
2 b    w       2
3 c        3
4 d    y       0
5 e    z       0