lists:keysort/3

对元组列表进行排序

用法：

keysort(N, TupleList1) -> TupleList2

内部实现：

-spec keysort(N, TupleList1) -> TupleList2 when
      N :: pos_integer(),
      TupleList1 :: [Tuple],
      TupleList2 :: [Tuple],
      Tuple :: tuple().

keysort(I, L) when is_integer(I), I > 0 ->
    case L of
	[] -> L;
	[_] -> L;
	[X, Y | T] ->
	    case {element(I, X), element(I, Y)} of
		{EX, EY} when EX =< EY ->
		    case T of
			[] ->
			    L;
			[Z] ->
			    case element(I, Z) of
				EZ when EY =< EZ ->
				    L;
				EZ when EX =< EZ ->
				    [X, Z, Y];
				_EZ ->
				    [Z, X, Y]
			    end;
			_ when X == Y ->
			    keysort_1(I, Y, EY, T, [X]);
			_ ->
			    keysplit_1(I, X, EX, Y, EY, T, [], [])
		    end;
		{EX, EY} ->
		    case T of
			[] ->
			    [Y, X];
			[Z] ->
			    case element(I, Z) of
				EZ when EX =< EZ ->
				    [Y, X | T];
				EZ when EY =< EZ ->
				    [Y, Z, X];
				_EZ ->
				    [Z, Y, X]
			    end;
			_ ->
			    keysplit_2(I, X, EX, Y, EY, T, [], [])
		    end
	    end
    end.

keysort_1(I, X, EX, [Y | L], R) when X == Y ->
    keysort_1(I, Y, EX, L, [X | R]);
keysort_1(I, X, EX, [Y | L], R) ->
    case element(I, Y) of
	EY when EX =< EY ->
	    keysplit_1(I, X, EX, Y, EY, L, R, []);
	EY ->
	    keysplit_2(I, X, EX, Y, EY, L, R, [])
    end;
keysort_1(_I, X, _EX, [], R) ->
    lists:reverse(R, [X]).

对元组列表 TupleList1 里按元组的第 N 个值进行排序，最后返回排序后的新元组列表 TupleList2

TupleList = [{a, 3}, {b, 4}, {c, 1}, {d, 2}],
lists:keysort(2, TupleList).