如何摆脱 SML 中的这个无限循环

Question

这是我的代码现在的样子。

我正在尝试制作一个多选题程序

这是一种非常初级的处理方式，但我发现很难以更“专业”的方式构建代码。

fun readlist(filename) =
    let 
        val file = TextIO.openIn filename
        val text = TextIO.inputAll file
        val _ = TextIO.closeIn file
    in
        String.tokens (fn c => c = #"\n") text
    end;
  
(*Get user answer from keybord*)
fun getAnswer() = (
    print "\nPlease enter your answer. Options(case sensitive): \nA-) \tB-) \tC-) \tD-)\n";
    let
        val return = valOf (TextIO.inputLine TextIO.stdIn)
    in
        return
    end
);
    
(*Verify if the choosen line is a new starting question block base on fact that each question block contains 7 line*)
fun getQuestionLine(t) = 
    case t of 
    [] => []
    |t::ts => if t mod 7 = 0 then
        t::getQuestionLine(ts)
     else
        getQuestionLine(ts);

fun toIntList(x:int) = x;
val getblockStarts = getQuestionLine(List.tabulate(List.length(readlist("questionBank.txt")), toIntList));
fun member (x, []) = false
    | member (x, y::yt) = x = y orelse member (x, yt);

fun seed () =
    let
        val aReal = Time.toReal(Time.now()) - 1.0e9
        val floorReal = Real.realFloor(aReal)
        val dec = aReal - floorReal
        val sd1 = Real.floor(dec);
        val sd2 = Real.floor(1000.0 * dec);
    in
        Random.rand(sd1,sd2)
    end;

fun randList (N) =
    let
      fun subfun (N, i) intList =
          if N = 0 then intList
          else
              let
                  val tmp = Random.randRange(1, (List.length(readlist("questionBank.txt"))))
                  val randVal = tmp i
              in
                  if member(randVal, getblockStarts) then
                      subfun (N - 1, i) (randVal :: intList)
                  else
                      subfun (N, i) intList
              end;
    in
        subfun (N, seed()) []
    end;

fun ask(questionIndexList) =
    let
      fun loop (n) =
          let
              val question = print(
                  "\n"^
                  List.nth(readlist("questionBank.txt"), List.nth(questionIndexList, n-1))^"\n\t"^
                  List.nth(readlist("questionBank.txt"), List.nth(questionIndexList, n-1)+1)^"\n\t"^
                  List.nth(readlist("questionBank.txt"), List.nth(questionIndexList, n-1)+2)^"\n\t"^
                  List.nth(readlist("questionBank.txt"), List.nth(questionIndexList, n-1)+3)^"\n\t"^
                  List.nth(readlist("questionBank.txt"), List.nth(questionIndexList, n-1)+4)^"\n\t"^
                  List.nth(readlist("questionBank.txt"), List.nth(questionIndexList, n-1)+5)^"\n"
              )
              val userAns = getAnswer()
          in
              if userAns = List.nth(readlist("questionBank.txt"), List.nth(questionIndexList, n-1)+6)^"\n" then
                (
                  print("Your answer is CORRECT.\n");
                  loop (List.length(List.take(questionIndexList, List.length(questionIndexList)-1)))
                )
              else (
                  print(
                      "Your answer is INCORRECT.\nThe correct answer is: "^
                      List.nth(readlist("questionBank.txt"), List.nth(questionIndexList, n-1)+6)
                      ^"\n\n"
                  );
                  loop (List.length(List.take(questionIndexList, List.length(questionIndexList)-1)))
                )
          end
    in
        loop (List.length(questionIndexList))
    end;

ask(randList(3));

我从我的问题文件中添加了一个捕获。

所有问题都遵循这种格式

我的代码按我想要的方式运行，但循环的最后一圈是无止境的。控制台为我提供了三个我回答的问题，如果我没有找到它，我有正确的答案，但是第三个问题被一遍又一遍地问到没有结束。

我一直在寻找所有地方，但我不知道我的错误在哪里。我需要帮助才能摆脱这个循环。

Answer 1

我将ask 函数更改为使用要提问的问题数量，而不是要显示的问题块列表。

fun ask(n) =
    let
      fun loop (n) =
          let
              val newQues = randList(1)
              val question = print(
                  "\n"^
                  List.nth(readlist("questionBank.txt"), hd newQues)^"\n\t"^
                  List.nth(readlist("questionBank.txt"), (hd newQues)+1)^"\n\t"^
                  List.nth(readlist("questionBank.txt"), (hd newQues)+2)^"\n\t"^
                  List.nth(readlist("questionBank.txt"), (hd newQues)+3)^"\n\t"^
                  List.nth(readlist("questionBank.txt"), (hd newQues)+4)^"\n\t"^
                  List.nth(readlist("questionBank.txt"), (hd newQues)+5)^"\n"
              )
              val userAns = getAnswer()
          in
              if userAns = List.nth(readlist("questionBank.txt"), (hd newQues)+6)^"\n" then
                (
                  print("Your answer is CORRECT.\n");
                  if n > 1 then loop (n-1) else ()
                )
              else (
                  print(
                      "Your answer is INCORRECT.\nThe correct answer is: "^
                      List.nth(readlist("questionBank.txt"), (hd newQues)+6)
                      ^"\n\n"
                  );
                  if n > 1 then loop (n-1) else ()
                )
          end
    in
        loop (n)
    end;

ask(5);

所以它现在可以按我的意愿工作了。

如果有人有优化的想法，我很乐意阅读。

Answer 2

没有条件带你离开loop；你在两个条件分支中无条件递归。

如果不是因为这个事实，你可以通过检查零来解决这个问题

List.length(List.take(questionIndexList, List.length(questionIndexList)-1))

总是相同的数字。
（即List.length(questionIndexList)-1——如果从列表中取出 K 个元素，则结果的长度为 K。无需创建列表来确定其长度。）

但我会采用不同的结构。

每次需要问题时，您都在阅读该文件。
最好读取一次文件并传递列表。

首先，一个代表问题和答案的类型：

type Options = string * string * string * string;
datatype QA = QA of string * Options * string;

还有几个函数可以从笨重的字符串列表中收集它们：

fun toQAs [] = []
  | toQAs (q::o1::o2::o3::o4::a::qas) = (QA (q, (o1,o2,o3,o4), a)) :: (toQAs qas);

fun readQuestions file = toQAs (readlist file);

还有一个打印问题的实用函数：

fun printQuestion (QA (q, (o1,o2,o3,o4), _)) = print ("\n"^ q^"\n\t"^ o1^"\n\t"^ o2^"\n\t"^ o3^"\n\t"^ o4^"\n");

ask 函数只接受一个问题列表并询问所有问题：

fun ask [] = print "No further questions.\n"
  | ask  (q::qs) =
    let val (QA (_, _, answer)) = q
    in
        printQuestion q;
        if getAnswer() = answer^"\n" then
             print "Your answer is CORRECT.\n"
         else
             print ("Your answer is INCORRECT.\nThe correct answer is: "^ answer ^"\n\n");
        ask qs
    end;

最后，一个函数根据文件创建随机问题列表，并提出问题：

fun go () =
    let
        val QAs = readQuestions "questionBank.txt"
    in
        ask (map (fn i => List.nth(QAs, i)) (randList (length QAs) 3))
    end;