I have a dataframe 'df' that looks like this:

And another dataframe 'df2' that looks like this:

If want to match values in ComName and CommonName and if they match fill in LatName with LatinName only if LatName is empty to begin with. If LatName isn't empty, then I want that entry left alone, so that the end result of df looks like this:

I left the blank row in there on purpose because some of my rows have nothing in them.

Thank you!

CodePudding user response：

We could do a join and then coalesce

library(dplyr)
left_join(df1, df2, by = c("ComName" = "CommonName")) %>%
   mutate(LatName = coalesce(LatName, LatinName), LatinName = NULL)

-output

    LatName      ComName
1   Todd Smith  Becky Jones
2 Brad Robbins  Becky Jones
3 Brad Robbins  Becky Jones
4 Steve Reisen Rachel Adams

data

df1 <- structure(list(LatName = c("Todd Smith", NA, NA, NA),
 ComName = c("Becky Jones", 
"Becky Jones", "Becky Jones", "Rachel Adams")), class = "data.frame", 
row.names = c(NA, 
-4L))

df2 <- structure(list(LatinName = c("Brad Robbins", "Steve Reisen", 
"Connor McDougal"), CommonName = c("Becky Jones", "Rachel Adams", 
"Charlie Williams")), class = "data.frame", row.names = c(NA, 
-3L))