嵌套循环结束时的 OCaml 语法错误

Question

我正在尝试创建一个函数来根据规则 here. I already have a python implementation here 计算一支球队赢得锦标赛的几率，但我想尝试使用 OCaml 这种非常新的语言来实现大部头书。我运行遇到语法错误问题，我不清楚为什么会出现这些错误。我也知道，因为我正在“翻译”来自 python 的代码，它不是 OCaml 的最佳选择，所以如果有更好的“OCaml 方式”来做我想做的事情，我会也想听听反馈。

open Format
open List

let tourney = [1;16;8;9;5;12;4;13;6;11;3;14;7;10;2;15];;

let odds x =
  function y -> if x > y then 1.0-.(x/.x+.y) else y/.(x+.y);;



let replace l pos a  = List.mapi (fun i x -> if i = pos then a else x) l;;

let rec index l a =
  match l with
  | [] -> -1
  | x::xs -> if x != a then 1 + index xs a else 0;;

    
let seed_odds L seed =
  let team_ind = index tourney seed
  and rounds = [2;4;8;16]
  and round_odds = [] ;

  for i = 1 to length rounds do
    let temp = [] in
    for j = 1 to length tourney do
      0.0 :: temp;
    done;
    temp :: round_odds;
  done;
  
  for r = 0 to (length rounds)-1 do
    let groups = (length tourney) / (nth rounds r);

    for i = 0 to groups do
      let teams = slice tourney i*(nth rounds r) (i+1)*(nth rounds r);
      for t = 0 to (length teams) do
        let odds_to_advance = ref 0.0;

        
        let teams_ =
          if t < ((length teams) / 2) then slice teams ((length teams)/2) (length teams)-1 else slice teams 0 ((length teams)/2)-1 ;

        for t_ = 0 to length teams_ do
          if nth teams t != nth teams_ t_ then
            begin
              if (nth rounds r) = 2 then
                begin
                  odds_to_advance := odds_to_advance +. odds (nth teams t) (nth teams_ t_);
                end
              else
                begin
                  odds_to_advance := odds_to_advance +. (odds (nth teams t) (nth teams_ t_)) *. (nth (nth round_odds r-1 ) (index tourney (nth teams_ t_) )) ;
                end
            end
          else ()
        done;
        if nths rounds r > 2 then
          begin
            odds_to_advance := odds_to_advance *. (nth (nth round_odds r-1 ) (index tourney (nth teams t) )) ;
          end
        else ()
        (*replace (nth round_odds r) (i * (nth rounds r) + t) odds_to_advance ;*)
      done;
    done;
  done;

好的，我之前的问题回答的很有道理，谢谢！我更新了代码并清理了一些东西：

open Format
open Array
open List

let tourney = [1.;16.;8.;9.;5.;12.;4.;13.;6.;11.;3.;14.;7.;10.;2.;15.];;

let odds x =
  function y -> if x > y then 1.0-.(x/.x+.y) else y/.(x+.y);;

printf "The odds are %f" (odds 2. 15.);;

let replace l pos a  = List.mapi (fun i x -> if i = pos then a else x) l;;

let rec index l a =
  match l with
  | [] -> -1
  | x::xs -> if x != a then 1 + index xs a else 0;;

let rec fold_until f acc n = function
  | [] -> (acc, [])
  | h :: t as l -> if n = 0 then (acc, l)
              else fold_until f (f acc h) (n - 1) t

let slice list i k =
  let _, list = fold_until (fun _ _ -> []) [] i list in
  let taken, _ = fold_until (fun acc h -> h :: acc) [] (k - i + 1) list in
  List.rev taken;;


let seed_odds l seed =
  let team_ind = index tourney seed
  and rounds = [2;4;8;16]
  
  and round_odds = make_matrix 4 16 0.0 in
  
  for r = 0 to (length rounds)-1 do
    let groups = (length tourney) / (nth rounds r) in

    for i = 0 to groups-1 do
      let teams = slice tourney (i*(nth rounds r)) ((i+1)*(nth rounds r)) in
      for t = 0 to (length teams)-1 do
        let odds_to_advance = ref 0.0 in

        
        let teams_ =
          if t < ((length teams) / 2) then slice teams ((length teams)/2) ((length teams)-1) else slice teams 0 (((length teams)/2)-1) in

        for t_ = 0 to (length teams_)-1 do
          if nth teams t != nth teams_ t_ then
            begin
              if (nth rounds r) = 2 then
                begin
                  let od = odds (nth teams t) (nth teams_ t_) in
                  odds_to_advance := !odds_to_advance +. od
                end
              else
                begin
                  let od = odds (nth teams t) (nth teams_ t_)
                  and prev = round_odds.(r-1).(index tourney (nth teams_ t_) ) in
                  odds_to_advance := !odds_to_advance +. od *. prev
                end
            end
          else ()
        done

        if (nth rounds r) > 2 then
          begin
            let prev = round_odds.(r-1).(index tourney (nth teams t)) in
            odds_to_advance := !odds_to_advance *. prev
          end
        else()
        
        round_odds.(r).((i*(nth rounds r))+t) <- !odds_to_advance

      done
    done
  done
  round_odds.(3).(team_ind);;
 


printf "The odds of two winning right now are %f" (seed_odds tourney 2.);;

我现在得到的唯一错误是：

66 |         if (nth rounds r) > 2 then
             ^^
Error: Syntax error

不确定现在是什么问题，因为我检查了 let 语句，确保 begin/ends 已关闭，等等。

Answer 1

我首先看到的是您有很多 let 实例，但没有匹配的 in。

在模块的顶层可以有 let name = value。这声明了一个要从模块导出的值（粗略地说）。

在其他任何地方（特别是在函数定义内部），每个 let 都必须有一个匹配的 in。 let 表达式如下所示：

let v = expr1 in expr2

它声明了一个局部变量v，值为expr1。这个局部变量的范围是expr2。 in expr2 部分不是可选的。

这是你有 let 而没有匹配 in:

的行

let team_ind = index tourney seed

let groups = (length tourney) / (nth rounds r);

let teams = slice tourney i*(nth rounds r) (i+1)*(nth rounds r);

let odds_to_advance = ref 0.0;

let teams_ =

这些都是语法错误。通常，您可以通过添加 in 并删除分号（如果有的话）来修复它们。

作为旁注，在 OCaml 中，分号用于分隔应按顺序求值的表达式。分号不是语句 terminator（就像在受 C 影响的语言中一样）。它是一个分隔符，在很多方面类似于运算符。

更新

作为另一条评论，一旦你的代码开始工作，你可能想寻找方法让它更符合 OCaml 的习惯。例如，使用 List.nth 几乎总是不好的做法，因为到达列表的第 n 个元素需要线性时间。 OCaml 不像其他一些语言，它们所谓的列表实际上是数组（在恒定时间内随机访问）。 OCaml 列表是真正的列表（如 Lisp 和其他 FP 语言）。

嵌套循环结束时的 OCaml 语法错误

OCaml Syntax Error at the end of nested loops

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