The R Language Definition is handy for answering these types of questions:
R has three basic indexing operators, with syntax displayed by the following examples
x[i] x[i, j] x[[i]] x[[i, j]] x$a x$"a"
For vectors and matrices the
[[forms are rarely used, although they have some slight semantic differences from the
[form (e.g. it drops any names or dimnames attribute, and that partial matching is used for character indices). When indexing multi-dimensional structures with a single index,
x[i]will return the
ith sequential element of
For lists, one generally uses
[[to select any single element, whereas
[returns a list of the selected elements.
[[form allows only a single element to be selected using integer or character indices, whereas
[allows indexing by vectors. Note though that for a list, the index can be a vector and each element of the vector is applied in turn to the list, the selected component, the selected component of that component, and so on. The result is still a single element.