I am learning OCaml and working my way through the advent of code 2022.

我正在学习OCaml，并在代码2022的到来中努力工作。

But it seems that reading the input.txt file is causing some issue.

但似乎读取input.txt文件会导致一些问题。

A X
B Y
C Z

I am getting this strange error, that comes from the func score. Particularly the expression call scorer hd. Though, when I manually set the lines variable to a list string, everything works.

我收到了这个奇怪的错误，它来自于函数分数。尤其是那个叫得分手HD的表达。不过，当我手动将LINS变量设置为列表字符串时，一切都正常。

The error occurs here inside the func scorer:

在函数记分器中出现以下错误：

    | hd :: tail -> score_helper tail scorer (scorer hd + acc)

let read_lines filename =
  let contents = In_channel.with_open_bin filename In_channel.input_all in
  String.split_on_char '\n' contents

let lines = read_lines "input.txt"

let score input scorer =
  let rec score_helper input scorer acc =
    match input with
    | [] -> acc
    | hd :: tail -> score_helper tail scorer (scorer hd + acc)
  in
  score_helper input scorer 0

(* Gets called on every row, returns the score value *)
let scorer s =
  let theirs =
    match String.sub s 0 1 with
    | "A" -> "X"
    | "B" -> "Y"
    | "C" -> "Z"
    | _ -> assert false
  in
  let ours = String.sub s 2 1 in
  let played =
    match ours with "X" -> 1 | "Y" -> 2 | "Z" -> 3 | _ -> assert false
  in
  let result =
    match (theirs, ours) with
    | "X", "Y" | "Y", "Z" | "Z", "X" -> 6
    | theirs, ours when theirs = ours -> 3
    | _ -> 0
  in
  played + result

let () = Format.sprintf "Part 1: %d" (score lines scorer) |> print_endline

The problem is revealed by inspecting lines:

这个问题是通过检查线路发现的：

# lines;;
- : string list = ["A X"; "B Y"; "C Z"; ""]
#

Note the last element: ""! You most likely want to ignore this empty line.

请注意最后一个元素：“”！您很可能希望忽略这一空行。

A simple way to handle this case is to ignore this blank lines in the score function:

处理这种情况的一种简单方法是忽略Score函数中的空行：

let score input scorer =
  let rec score_helper input scorer acc =
    match input with
    | [] -> acc
    | "" :: tail -> score_helper tail scorer acc
    | hd :: tail -> score_helper tail scorer (scorer hd + acc)
  in
  score_helper input scorer 0

jthuhlu has identified the source of your problem.

Jthorhlu已经确定了您问题的根源。

However, there is another way to approach your problem. Your score function transforms a list into another value. As such, this is a perfect place to use a fold.

然而，还有另一种方法来解决你的问题。score函数将列表转换为另一个值。因此，这是一个完美的地方使用折叠。

let score input scorer =
  List.fold_left 
    (fun i x -> 
       match x with
       | "" -> i
       | _ -> scorer x + i)
    0
    input

Now, if we don't need to look for "" values because we've filtered them out, this can be simpler.

现在，如果我们不需要寻找“”值，因为我们已经过滤掉它们，这可以更简单。

let score input scorer =
  input 
  |> List.fold_left (fun i x -> scorer x + i) 0

We can incorporate filtering into this.

我们可以将过滤纳入其中。

let score input scorer =
  input
  |> List.filter ((<>) "")
  |> List.fold_left (fun i x -> scorer x + i) 0

Now, for large data samples, iterating the whole list and creating a new list with List.filter may be onerous, so we can use sequences instead.

现在，对于大型数据样本，使用List.Filter迭代整个列表并创建新列表可能很麻烦，因此我们可以使用序列。

let score input scorer =
  input
  |> List.to_seq
  |> Seq.filter ((<>) "")
  |> Seq.fold_left (fun i x -> scorer x + i) 0

I figured out the problem.

我发现了问题所在。

My input.txt actually had empty lines, which caused the String.sub in the scorer function to crash the program. I added a fallback case for an empty string to the function:

我的input.txt实际上有空行，这导致记分器函数中的String.Sub使程序崩溃。我为函数添加了一个空字符串的备用用例：

let scorer s =
  let len = String.length s in
  if len < 3 then
    0
  else 
  let theirs =
    match String.sub s 0 1 with
    | "A" -> "X"
    | "B" -> "Y"
    | "C" -> "Z"
    | _ -> assert false
  in
  let ours = String.sub s 2 1 in
  let played =
    match ours with "X" -> 1 | "Y" -> 2 | "Z" -> 3 | _ -> assert false
  in
  let result =
    match (theirs, ours) with
    | "X", "Y" | "Y", "Z" | "Z", "X" -> 6
    | theirs, ours when theirs = ours -> 3
    | _ -> 0
  in
  played + result