如何解决嵌套循环末尾的 OCaml 语法错误
我正在尝试创建一个函数,用于根据规则 here 计算球队赢得锦标赛的几率。我已经有了一个 python 实现 here,但我想尝试用 OCaml 来实现,这是一种对我来说很新的语言。我遇到了语法错误的问题,我不清楚为什么我会收到这些错误。我也知道,因为我从 python“翻译”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,我想听听还有反馈。
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,我之前问题的回答有道理,谢谢!我已经更新了代码并进行了清理:
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
现在不确定是什么问题,因为我已经检查过 let 语句,确保 begin/ends 已关闭等。
解决方法
我看到的第一件事是您有相当多的 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中导出的值(粗略地说)。
在其他任何地方(特别是在函数定义中),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 v
的局部变量 expr1
。这个局部变量的作用域是expr2
。 in expr2
部分不是可选的。
以下是您有 let
且没有匹配的 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_ =
所有这些都是语法错误。一般来说,您可以通过添加 in
并删除分号(如果有)来修复它们。
作为旁注,在 OCaml 中,分号用于分隔应按顺序计算的表达式。分号不是声明终止符(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
作为另一边的评论,一旦你的代码可以工作,你可能想要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(可在恒定时间内随机访问)。 OCaml 列表实际上是列表(如 Lisp 和其他 FP 语言)。
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