Wrong example in Lesson 18.

[khangquangtran]

In Lesson 18, in the second code section, "this little example":

> d <- data.frame(x=c(1,0,3),y=c(0,0,13))
> d
x y
1 1 0
2 0 0
> which(d[,2] == 0)
[1] 1 2 # ah yes; the 0 elements in column 2 are at indices 1 and 2

The data frame d should have 3 rows and 2 columns and should be as follows (sorry, I can't add whitespaces in front of "x y"):
x y
1 1 0
2 0 0
3 3 13