嵌套循环末尾的OCaml语法错误

Question

我正试图创建一个函数，根据这里的规则计算球队赢得锦标赛的几率。我在这里已经有了python实现，但我想尝试用OCaml来实现它，OCaml是一种对我来说非常新的语言。我遇到了语法错误的问题，我不清楚为什么会出现错误。我也知道，因为我是；翻译"；python的代码，它不是OCaml的最佳选择，所以如果有更好的"；OCaml方式"；关于做我正在努力做的事情，我也想听听这些反馈。

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，我之前问题的答案很有道理，谢谢！我已经更新了代码并清理了：

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

不确定现在的问题是什么，因为我已经检查了let语句，确保开始/结束都关闭了，等等。

Answer 1

我看到的第一件事是，有相当多的let实例没有匹配的in。

在模块的顶层，您可以拥有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(尤其是函数定义内部(，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

它声明了一个值为expr1的局部变量v。这个局部变量的作用域是expr2。in expr2部分不是可选的。

以下是没有匹配in:的let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_ =

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

作为附带注释，在OCaml中，分号用于分隔应按顺序求值的表达式。分号不是语句终止符(在受C影响的语言中(。它是一个分隔符，在很多方面类似于运算符。

更新

作为另一个附带说明，一旦您的代码工作正常，您可能需要寻找使其更惯用的OCaml的方法。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不像其他一些语言那样，它们所称的列表实际上是数组(在恒定时间内随机访问(。OCaml列表实际上是列表(就像在Lisp和其他FP语言中一样(。