This Blog entry is from the Loading and Shaping section in Learn R.
In these Blog entries, the mutate() function of dplyr has been used to add a vector into a data frame. It is worthy of a brief mention that to remove a vector from a data frame, it is simply a matter of passing NULL to the vector in question:
AAPL$Target <- NULL
These Blog entries do not make mention to the deletion of vectors from a data frame, rather it is mentioned only for completeness.