Data Frame Column Vector
We reference a data frame column with the double square bracket "[]" operator.
For example, to retrieve the ninth column vector of the built-in data set mtcars, we write mtcars[].
We can retrieve the same column vector by its name.
We can also retrieve with the "$" operator in lieu of the double square bracket operator.
Yet another way to retrieve the same column vector is to use the single square bracket "" operator. We prepend the column name with a comma character, which signals a wildcard match for the row position.