Friday, 5 April 2019
r - The difference between bracket [ ] and double bracket [[ ]] for accessing the elements of a list or dataframe
Answer
Answer
R provides two different methods for accessing the elements of a list or data.frame- the [] and [[]] operators.
What is the difference between the two? In what situations should I use one over the other?
Answer
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]]orx[i]will return theith sequential element ofx.
For lists, one generally uses[[to select any single element, whereas[returns a list of the selected elements.
The[[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.
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