R: replace NA with item from vector

Question

I am trying to replace some missing values in my data with the average values from a similar group.

My data looks like this:

   X   Y
1  x   y
2  x   y
3  NA  y
4  x   y

And I want it to look like this:

  X   Y
1  x   y
2  x   y
3  y   y
4  x   y

I wrote this, and it worked

for(i in 1:nrow(data.frame){
   if( is.na(data.frame$X[i]) == TRUE){
       data.frame$X[i] <- data.frame$Y[i]
   }
  }

But my data.frame is almost half a million lines long, and the for/if statements are pretty slow. What I want is something like

is.na(data.frame$X) <- data.frame$Y

But this gets a mismatched size error. It seems like there should be a command that does this, but I cannot find it here on SO or on the R help list. Any ideas?

Answer 1

ifelse is your friend.

Using Dirk's dataset

df <- within(df, X <- ifelse(is.na(X), Y, X))