string - 如何用另一个 [ELIXIR/ERLANG] 替换字符串中的第 n 个字符

Question

如何replace_at在 Elixir 或 Erlang 中执行字符串？

例如给定这个固定宽度的文件：

EmployeeFundMappingID EmployeeID  FundID      IsActive EntryDate               ExitDate                ExitTypeID  DateCreated             CreatedByID DateModified            ModifiedByID ConfirmedBy DateConfirmed           GUID                                     IsPooled DatePooled
1                     1118544     1           1        2009-04-20 00:00:00.000 NULL                    NULL        2014-05-17 08:46:48.020 1           2014-10-30 13:34:47.177 NULL         1           2009-04-20 17:48:12.067 NULL                                     NULL     NULL
2                     1027350     1           1        2008-03-03 00:00:00.000 NULL                    NULL        2014-05-17 08:46:48.020 1           2014-10-30 13:34:47.177 NULL         1           2008-05-04 15:13:30.303 NULL                                     NULL     NULL
3                     1024795     1           1        2008-02-29 00:00:00.000 NULL                    NULL        2014-05-17 08:46:48.020 1           2014-10-30 13:34:47.177 NULL         1           2008-05-04 15:13:30.303 NULL                                     NULL     NULL
4                     1116497     1           1        2009-03-24 00:00:00.000 NULL                    NULL        2014-05-17 08:46:48.020 1           2014-10-30 13:34:47.177 NULL         1           2009-03-24 13:00:15.277 NULL                                     NULL     NULL
5                     1116569     1           1        2009-03-24 00:00:00.000 NULL                    NULL        2014-05-17 08:46:48.020 1           2014-10-30 13:34:47.177 NULL         1           2009-03-24 14:43:08.280 NULL                                     NULL     NULL
6                     1116920     1           1        2009-03-27 00:00:00.000 NULL                    NULL        2014-05-17 08:46:48.020 1           2014-10-30 13:34:47.177 NULL         1           2009-03-27 17:16:35.073 NULL                                     NULL     NULL

在 col 位置：

[0, 22, 34, 46, 55, 79, 103, 115, 139, 151, 175, 188, 200, 224, 265, 274]

我们如何在每个 col 位置\s替换为？\t

我正在有效地尝试将Fixed-Width文件转换为csv

标签： stringerlangelixir

Answer 1

我会用一组改变字符串中各个位置的函数来减少原始行。

funs =
  [22, 34, 46, 55, 79, 103, 115, 139, 151, 175, 188, 200, 224, 265, 274]
  |> Enum.map(& &1 - 1)
  |> Enum.map(fn len ->
       fn <<s :: binary-size(len), " ", rest :: binary>> ->
         s <> "\t" <> rest
       end
     end)

input
|> String.trim
|> String.split("\n")
|> Enum.map(fn line ->
     Enum.reduce(funs, line, fn fun, acc -> fun.(acc) end)
   end)

这可以通过使用生成的宏、每个位置一次和递归调用以更优雅的方式完成，但是在这里减少函数列表对我来说看起来更直接。

---

这种方法的优点是它会立即在任何不一致的数据上失败，确保（或多或少）如果它通过了，则转换正确完成，这与所有其他较短的解决方案不同。

它也比任何解决方案都快得多Regex。

---

由于这将应用于 16M 行，因此这可能是性能最高的版本，它一次匹配整行：

input
|> String.trim
|> String.split("\n")
|> Enum.map(
     # [22, 34, 46, 55, 79, 103,
     #  115, 139, 151, 175, 188,
     #  200, 224, 265, 274]
     # note: this assumes the listed positions above are 1-based
     fn <<
        c1 :: binary-size(21),
        " ",
        c2 :: binary-size(11),
        " ",
        c3 :: binary-size(11),
        " ",
        c4 :: binary-size(8),
        " ",
        c5 :: binary-size(23),
        " ",
        c6 :: binary-size(23),
        " ",
        c7 :: binary-size(11),
        " ",
        c8 :: binary-size(23),
        " ",
        c9 :: binary-size(11),
        " ",
        c10 :: binary-size(23),
        " ",
        c11 :: binary-size(12),
        " ",
        c12 :: binary-size(11),
        " ",
        c13 :: binary-size(23),
        " ",
        c14 :: binary-size(40),
        " ",
        c15 :: binary-size(8),
        " ",
        c16 :: binary
        >> ->
     c1 <> "\t" <> 
       c2 <> "\t" <> 
       c3 <> "\t" <> 
       c4 <> "\t" <> 
       c5 <> "\t" <> 
       c6 <> "\t" <> 
       c7 <> "\t" <> 
       c8 <> "\t" <> 
       c9 <> "\t" <> 
       c10 <> "\t" <> 
       c11 <> "\t" <> 
       c12 <> "\t" <> 
       c13 <> "\t" <> 
       c14 <> "\t" <> 
       c15 <> "\t" <> 
       c16
   end)