extract() simply uses a for loop to extract the nth element
from each vector in a list. However, it can also operate on a data frame.
This is equivalent to taking the first element of each vector and making a
those elements the first vector in a new list, and it continues on so until
it reaches the last element.
extract(lst, num = min(lengths(lst)))lst is smaller than
num then an error will be thrown (subscript out of bounds).extract() to extract the elements of lst. Default
value is the length of the shortest vector in the list.