The R-inferno, or the basic R-documentation will explain why using df$* is not the best approach here. From the help page for "[" :
"Indexing by [ is similar to atomic vectors and selects a list of the specified element(s). Both [[ and $ select a single element of the list. The main difference is that $ does not allow computed indices, whereas [[ does. x$name is equivalent to x[["name", exact = FALSE]]. Also, the partial matching behavior of [[ can be controlled using the exact argument. "
I recommend using the [row,col] notation instead. Example:
Rgames: foo
x y z
[1,] 1e+00 1 0
[2,] 2e+00 2 0
[3,] 3e+00 1 0
[4,] 4e+00 2 0
[5,] 5e+00 1 0
[6,] 6e+00 2 0
[7,] 7e+00 1 0
[8,] 8e+00 2 0
[9,] 9e+00 1 0
[10,] 1e+01 2 0
Rgames: foo<-as.data.frame(foo)
Rgames: foo[foo$y==2,3]<-foo[foo$y==2,1]
Rgames: foo
x y z
1 1e+00 1 0e+00
2 2e+00 2 2e+00
3 3e+00 1 0e+00
4 4e+00 2 4e+00
5 5e+00 1 0e+00
6 6e+00 2 6e+00
7 7e+00 1 0e+00
8 8e+00 2 8e+00
9 9e+00 1 0e+00
10 1e+01 2 1e+01