Use merge() to update a data frame with values from a second data frame

Question

I'm trying to figure out how to use merge() to update a data frame.

Take for example the data frame foo

foo <- data.frame(index=c('a', 'b', 'c', 'd'), value=c(100, 101, NA, NA)) 

Which has the following values

index value 1     a   100 2     b   101 3     c    NA 4     d    NA 

And the data frame bar

bar <- data.frame(index=c('c', 'd'), value=c(200, 201)) 

Which has the following values:

 index value 1     c   200 2     d   201 

When I run the following merge() function to update the values for c and d

merge(foo, bar, by='index', all=T) 

It results in this output:

 index value.x value.y 1     a     100      NA 2     b     101      NA 3     c      NA     200 4     d      NA     201 

I'd like the output of merge() to avoid the creation of, in this specific example, of value.x and value.y but only retain the original column of value Is there a simple way of doing this?

Answer 1

Doesn't merge() always bind columns together? Does replace() work?

foo$value <- replace(foo$value, foo$index %in% bar$index, bar$value) 

or match() so the order matters

foo$value[match(bar$index, foo$index)] <- bar$value