What's the difference between [1], [1,], [,1],

Possible Duplicate:
In R, what is the difference between the [] and [[]] notations for accessing the elements of a list?

I'm confused with the difference of [1], [1,], [,1], [[1]] for dataframe type.

As I know, [1,] will fetch the first row of a matrix, [,1] will fetch the first column. [[1]] will fetch the first element of a list.

But I checked the document of data.frame, which says

A data frame is a list of variables of the same number of rows with unique row names

Then I typed in some code to test the usage.

>L3 <- LETTERS[1:3]
>(d <- data.frame(cbind(x=1, y=1:10), fac=sample(L3, 10, replace=TRUE)))
  x  y fac
1  1  1   C
2  1  2   B
3  1  3   C
4  1  4   C
5  1  5   A
6  1  6   B
7  1  7   C
8  1  8   A
9  1  9   A
10 1 10   A
> d[1]
   x
1  1
2  1
3  1
4  1
5  1
6  1
7  1
8  1
9  1
10 1
>d[1,]
  x y fac
1 1 1   C
>d[,1]
 [1] 1 1 1 1 1 1 1 1 1 1
>d[[1]]
 [1] 1 1 1 1 1 1 1 1 1 1

What confused me is: [1,] and [,1] is only used in matrix. [[1]] is only used in list, and [1] is used in vector, but why all of them are available in dataframe?

Could anybody explain the difference of these usage?

In R, operators are not used for one data type only. Operators can be overloaded for whatever data type you like (e.g. also S3/S4 classes).

In fact, that's the case for data.frames.

as data.frames are lists, the [i] and [[i]] (and $) show list-like behaviour.
row, colum indices do have an intuitive meaning for tables, and data.frames look like tables. Probably that is the reason why methods for data.frame [i, j] were defined.

You can even look at the definitions, they are coded in the S3 system (so methodname.class):