1. sum(3) => 6; sum(1,3) => 6; sum(6,6) => 6;
sum(N) when is_integer(N) -> sum_acc(N,0); sum(_) -> {error, {bad_argument}}. sum_acc(0, Sum) -> Sum; sum_acc(N, Sum) -> sum_acc(N-1, Sum + N). sum(N, M) when is_integer(N), is_integer(M), M >= N -> sum_acc3(N, M, 0); sum(_, _) -> {error, {bad_argument}}. sum_acc3(N, N, Sum) -> Sum + N; sum_acc3(N, M, Sum) -> sum_acc3(N, M-1, Sum+M).
2. creat(3) => [1,2,3];
creat(N) when N > 0 -> creat_acc(N, []); creat(_) -> {error, {bad_argument}}. creat_acc(0, List) -> List; creat_acc(N, List) -> creat_acc(N-1, [N | List]).
3. 打印 1~N 的整数
print(N) when is_integer(N), N > 0 -> print_acc(N,0); print(_) -> {error, {bad_argument}}. print_acc(0,List) -> io:format("Numer:~p~n", [List]); print_acc(N,List) -> print_acc(N-1,[N|List]).
3-5 列表操作
filter(List, N) when is_list(List) -> filter_acc(List, N, []); filter(_, N) -> {error, {bad_argument}}. filter_acc([], N, List) -> onelist(List); filter_acc([H|T], N, List) when H =< N -> filter_acc(T, N, [onelist(List)|H]); filter_acc([H|T], N, List) when H > N -> filter_acc(T, N, List). onelist([]) -> []; onelist([H|T]) when is_list(H) -> H ++ onelist(T); onelist([H|T]) when not is_list(H) -> onelist(H) ++ onelist(T); onelist(H) -> [H].
erlang 中的 ++ 是很耗性能的，这在erlang 编程指南中有明确指出， 而且onlist 对于头部是复杂列表（非结构良好列表）时什么报错比如
[[[[[[[]|1]|2]|3]|5]|6]|7] ++ [8] 就会报错，++ 号两边列表结构不对等, 下面优化下。
filter(List, N) when is_list(List) -> filter_acc(List, N, []); filter(_, N) -> {error, {bad_argument}}. filter_acc([], N, List) -> io:format("List : ~p | ~p~n", [List, concat(List)]), concat(List); filter_acc([H|T], N, List) when H =< N -> filter_acc(T, N, [List|H]); filter_acc([H|T], N, List) when H > N -> filter_acc(T, N, List). concat([H|[]]) -> [H]; concat([[]|T]) -> [T]; concat([H|T]) -> lists:concat([concat_cc(H) , concat_cc(T)]).
concat_cc(N) when is_list(N) -> concat(N); concat_cc(N) -> [N].
