the rules of subsetting

Question

Having df1 and df2 as follows:

df1 <- read.table(text =\" x y z
                          1 1 1
                          1 2 1
         


        

Answer 1

In addition to your nice solution using merge (thanks for that, I always forget merge), this can be achieved in base using ?interaction as follows. There may be other variations of this, but this is the one I am familiar with:

> df1[interaction(df1) %in% interaction(df2), ]

Now to answer your question: First, I think there's a typo (corrected) in:

df1[ df1$z %in% df2$c | df2$b == 9,] # second part should be df2$b == 9

You would get an error, because the first part evaluates to

[1] TRUE TRUE TRUE TRUE TRUE

and the second evaluates to:

[1] FALSE FALSE FALSE FALSE

You do a | operation on unequal lengths getting the error:

longer object length is not a multiple of shorter object length

Edit: If you have multiple columns then you can choose the interaction as such. For example, if you want to get from df1 the rows where the first two columns match with that of df2, then you could simply do:

> df1[interaction(df1[, 1:2]) %in% interaction(df2[, 1:2]), ]