Question

Why is it that I can’t assign a value to an entire column of a data frame, and then a single element in the same “within” statement? The code:

foo <- data.frame( a=seq(1,10) )

foo <- within(foo, {
  b <- 1  # set all of b to 1
})

foo <- within(foo, {
  c <- 1  # set all of c to 1
  c[2] <- 20  # set one element to 20
  b[2] <- 20
})

foo

Gives:

    a  b  c
1   1  1  1
2   2 20 20
3   3  1  1
4   4  1 20
5   5  1  1
6   6  1 20
7   7  1  1
8   8  1 20
9   9  1  1
10 10  1 20

The value of b is what I expected. The value of c is strange. It seems to do what I expect if the assignment to the entire column (ie b <- 1) is in a different “within” statement than the assignment to a single element (ie b[2] <- 20). But not if they’re in the same “within”.

Is this a bug, or something I just don’t understand about R?

Answer 1

My guess is that the assignments to new columns are done as you “leave” the function. When doing

c <- 1
c[2] <- 20

all you have really created is a vector c <- c(1, 20). When R has to assign this to a new column, the vector is recycled, creating the 1,20,1,20,… pattern you are seeing.