How to SortBy Last and in Reverse order?

Question

I am looking at the two functions SortBy and ReverseSort.

SortBy[{{1, 2, 3}, {2, 3, 1}, {3, 1, 2}, {2, 2}}, Last]

{{2, 3, 1}, {2, 2}, {3, 1, 2}, {1, 2, 3}}

How do I sort this by the last element, but in the reverse order? I want to get this:

{{1, 2, 3}, {3, 1, 2}, {2, 2}, {2, 3, 1}}

Answer 1

If you don't care about the ordering of {3, 1, 2} and {2, 2}, do this:

SortBy[{{1, 2, 3}, {2, 3, 1}, {3, 1, 2}, {2, 2}}, Minus @* Last]
   {{1, 2, 3}, {2, 2}, {3, 1, 2}, {2, 3, 1}}

Otherwise, add a second sorting criterion; e.g.

SortBy[{{1, 2, 3}, {2, 3, 1}, {3, 1, 2}, {2, 2}}, {Minus @* Last, Minus @* Length}]
   {{1, 2, 3}, {3, 1, 2}, {2, 2}, {2, 3, 1}}

or just wrap the sorting criterion in a list (per Alan):

SortBy[{{1, 2, 3}, {2, 3, 1}, {3, 1, 2}, {2, 2}}, {Minus @* Last}]
   {{1, 2, 3}, {3, 1, 2}, {2, 2}, {2, 3, 1}}

Answer 2

Using:

l = {{2, 3, 1}, {2, 2}, {3, 1, 2}, {1, 2, 3}};

One option, just using Sort:

Sort[l, Last[#2] < Last[#1] &]

Another option, using SortBy and Reverse:

Reverse@SortBy[l, Last]

I'm not completely sure about ReverseSort (my version of Mathematica does not have it), but I imagine something like this would be at least a start (not too sure about "tie" cases, or how concerned you are with them):

ReverseSort[l, Last[#1] < Last[#2] &]

Not an expert on this, so anyone should feel free to correct me if I'm wrong.