left_join two data frames and overwrite

Question

I'd like to merge two data frames where df2 overwrites any values that are NA or present in df1. Merge data frames and overwrite values provides a data.table option, but I'd like to know if there is a way to do this with dplyr. I've tried all of the _join options but none seem to do this. Is there a way to do this with dplyr?

Here is an example:

df1 <- data.frame(y = c("A", "B", "C", "D"), x1 = c(1,2,NA, 4)) 
df2 <- data.frame(y = c("A", "B", "C"), x1 = c(5, 6, 7))

Desired output:

  y x1
1 A  5
2 B  6
3 C  7
4 D  4

Answer 1

I think what you want is to keep the values of df2 and only add the ones in df1 that are not present in df2 which is what anti_join does:

"anti_join return all rows from x where there are not matching values in y, keeping just columns from x."

My solution:

df3 <- anti_join(df1, df2, by = "y") %>% bind_rows(df2)

Warning messages:
1: In anti_join_impl(x, y, by$x, by$y) :
  joining factors with different levels, coercing to character vector
2: In rbind_all(x, .id) : Unequal factor levels: coercing to character

> df3
Source: local data frame [4 x 2]

      y    x1
  (chr) (dbl)
1     D     4
2     A     5
3     B     6
4     C     7

this line gives the desired output (in a different order) but, you should pay attention to the warning message, when working with your dataset be sure to read y as a character variable.