We wish to select a subset of the columns of a data frame. We wish to identify the columns that we wish to select by specifying their name or their position. In particular:
Selection by Name where we identify the columns we wish to select by specifying their names (as per the columns Index). This is by far the most common column selection scenario.
Selection by Position where we identify the columns we wish to select by specifying their position (as an integer). The positions start at 0 for the left most column.