如何有效地从Erlang的STDIN读取数千行？ [英] How to efficiently read thousand of lines from STDIN in Erlang?

问题描述

从STDIN读取数千行时，我偶然发现了一个问题。在我发现针对此问题的一些测试之前，这本来是一个虚构的边缘案例。需要从STDIN读取上千行。起初我以为我的算法不是最佳算法，只是偶然地发现，只有没有任何计算的读取行才能使测试时间浪费一半。

I've stumbled upon an issue when reading thousands of lines from STDIN. This would have been an imaginary edge case until I found out that some tests for this problem require reading thousand of lines from STDIN. At first I thought that my algorithms were not optimal, and only by accident I've found out that only reading lines without any computations could make half of the test time out.

此处的部分代码超时：

process_queries(0, _) -> ok;
process_queries(N, A) -> 
    case io:fread("", "~s~d~d") of
        {ok, _} -> process_queries(N - 1, A)
        %{ok, ["Q", L, R]} -> process_queries(N - 1, apply_q(L, R, A));
        %{ok, ["U", Idx, Val]} -> process_queries(N - 1, apply_u(Idx, Val, A))
    end
.

我故意留下评论以表明所有计算均被禁用。因此，在给出 N = 7984 的情况下，此代码超时。

I deliberately left comments to show that all the computations were disabled. So this code timed out given N=7984.

有没有更好的方法来读取和处理数千行

Is there a better way of reading and processing thousands lines from STDIN in Erlang?

• io：get_line 一次只能得到一行。


• io：get_chars 要求您知道要获得多少个字符。

• io:get_line gets only one line a time.
• io:get_chars requires you to know how many character to get.

推荐答案

我的解决方案有节选。

There is an excerpt from my solution. There are few tricks how to do it really efficient.

read_command(CP) ->
    {ok, Line} = file:read_line(standard_io),
    [C, A, B] = binary:split(Line, CP, [global, trim_all]),
    {case C of <<"Q">> -> 'Q'; <<"U">> -> 'U' end,
     binary_to_integer(A),
     binary_to_integer(B)}.

read_commands(N, CP) ->
    [ read_command(CP) || _ <- lists:seq(1, N) ].

execute(Array, L) ->
    lists:foldl(fun({'Q', F, T}, A) ->
                        {Val, A2} = query(A, F, T),
                        file:write(standard_io, [integer_to_binary(Val), $\n]),
                        A2;
                   ({'U', I, V}, A) ->
                        update(A, I, V)
                end, Array, L).

read_int_line(CP) ->
    {ok, Line} = file:read_line(standard_io),
    [binary_to_integer(X) || X <- binary:split(Line, CP, [global, trim_all])].

main() ->
    ok = io:setopts([binary]),
    CP = binary:compile_pattern([<<" ">>, <<$\n>>]),
    [N] = read_int_line(CP),
    L = read_int_line(CP),
    N = length(L),
    [K] = read_int_line(CP),
    execute(init(L), read_commands(K, CP)).

您必须编写自己的 init / 1 ， update / 3 和 query / 3 当然。

You have to write your own init/1, update/3 and query/3 of course.

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