读取 erlang 中特定内容的文件

Question

我按照这种方法读取文件 - 如何在 Erlang 中读取文件的内容？

提到的答案很好用。但是，我的文件内容如下所示：

{0, {data, node, 0}}
{1, {someData, node, 1}}

每一行都有此数据。因此，在读取文件时，我想过滤它应该读取多少行 像这样：

read(Node, FirstIndex, LastIndex, F, Acc)

这里，F 是折叠函数，它接受单个日志条目和当前累加器并返回新的累加器值。

如何将解决方案与我需要的此功能合并？

Answer 1

-module(my).
-compile(export_all).
get_section(Start, End, Fname) when Start>0, End>=Start ->
{ok, Io} = file:open(Fname, read),
ok = advance_file_pointer(Start, Io),
Lines = getNLines(End-Start+1, Io),
file:close(Io),
Lines.
advance_file_pointer(1, _Io) ->
ok;
advance_file_pointer(Start, Io) ->
{ok, _Line} = file:read_line(Io),
advance_file_pointer(Start-1, Io).
getNLines(N, Io) ->
getNLines(N, Io, []).
getNLines(0, _Io, Acc) ->
lists:reverse(Acc);
getNLines(N, Io, Acc) ->
{ok, Line} = file:read_line(Io),
getNLines(N-1, Io, [Line|Acc]).

在外壳中：

~/erlang_programs$ cat data.txt
{1, {data, node, 1}}
{2, {someData, node, 2}}
{3, {otherData, node, 3}}
~/erlang_programs$ erl
Erlang/OTP 20 [erts-9.2] [source] [64-bit] [smp:4:4] [ds:4:4:10] [async-threads:10] [hipe] [kernel-poll:false]
Eshell V9.2  (abort with ^G)
1> c(my).
my.erl:2: Warning: export_all flag enabled - all functions will be exported
{ok,my}
2> my:get_section(1, 1, "data.txt").
["{1, {data, node, 1}}n"]
3> my:get_section(1, 2, "data.txt").
["{1, {data, node, 1}}n","{2, {someData, node, 2}}n"]
4> my:get_section(1, 3, "data.txt").
["{1, {data, node, 1}}n","{2, {someData, node, 2}}n",
"{3, {otherData, node, 3}}n"]
5> my:get_section(2, 3, "data.txt").
["{2, {someData, node, 2}}n","{3, {otherData, node, 3}}n"]
6> my:get_section(2, 2, "data.txt").
["{2, {someData, node, 2}}n"]
7> my:get_section(0, 2, "data.txt").
** exception error: no function clause matching my:get_section(0,2,"data.txt") (my.erl, line 4)
8> my:get_section(1, 4, "data.txt").
** exception error: no match of right hand side value eof
in function  my:getNLines/3 (my.erl, line 20)
in call from my:get_section/3 (my.erl, line 7)
9> 

您可以在getNLines/3的第二个子句中应用折叠函数。