What's the biggest R-gotcha you've run across?

Question

Is there a certain R-gotcha that had you really surprised one day? I think we'd all gain from sharing these.

Here's mine: in list indexing, my.list[[1]] is not my.list[1]. Learned this in the early days of R.

Answer 1

Removing rows in a dataframe will cause non-uniquely named rows to be added, which then errors out:

> a<-data.frame(c(1,2,3,4),c(4,3,2,1))
> a<-a[-3,]
> a
  c.1..2..3..4. c.4..3..2..1.
1             1             4
2             2             3
4             4             1
> a[4,1]<-1
> a
Error in data.frame(c.1..2..3..4. = c("1", "2", "4", "1"), c.4..3..2..1. = c(" 4",  : 
  duplicate row.names: 4

So what is going on here is:

1. A four row data.frame is created, so the rownames are c(1,2,3,4)

2. The third row is deleted, so the rownames are c(1,2,4)

3. A fourth row is added, and R automatically sets the row name equal to the index i.e. 4, so the row names are c(1,2,4,4). This is illegal because row names should be unique. I don't see why this type of behavior should be allowed by R. It seems to me that R should provide a unique row name.