Loop Prints Else Statement Only

Question

我正在進行各種數學培訓計劃。我試圖實現一個for循環，以便用戶選擇的每個問題類型都會產生三個問題。如果用戶輸入正確的變量類型，這可以很好地工作。但是，如果用戶輸入錯誤的變量/變量類型，會發生奇怪的循環。這很難解釋，但如果你拿這些代碼，並嘗試給它一個錯誤的輸入，你很快就會看到。我不知道這可能是如何造成的，所以我想我會在這裏發佈。我已經註釋過，只有一種操作類型（加法）可用，僅用於調試目的。如果你輸入的東西不是程序期望的東西，你會看到。我很新的循環，並已在多個想法盡我所能，但我很短。謝謝！

// Welcome 
    System.out.println("Hello and welcome to the Math Trainer!\n======================================"); 
    System.out.println("Which math operation would you like to practice?"); 

    // Print options to select from 
    System.out.println(" " + "[A]ddition"); 
    System.out.println(" " +"[S]ubtraction"); 
    System.out.println(" " + "[M]ultiplication"); 
    System.out.println(" " + "[D]ivision"); 
    System.out.println(" " + "[R]emainder"); 

    // Ask for user input on which to choose 
    System.out.print("Enter your choice:" + " "); 
    String userLetter = stdin.nextLine(); 

    // Calculate random values from seed and shift them within range 
     for(int count = 0; count < Config.NUMBER_OF_QUESTIONS; count++){ 
     int ran1 = randGen.nextInt(Config.MAX_VALUE - Config.MIN_VALUE + 1); 
     int ran2 = randGen.nextInt(Config.MAX_VALUE - Config.MIN_VALUE + 1); 
     int ran1Shift = ran1 + Config.MIN_VALUE; 
     int ran2Shift = ran2 + Config.MIN_VALUE; 

     // Initialize different answers per operation 
     double additionAnswer = (double)ran1Shift + ran2Shift; 
     double subtractionAnswer = (double)ran1Shift - ran2Shift; 
     double multiplicationAnswer = (double)ran1Shift * ran2Shift; 
     double divisionAnswer = (double)ran1Shift/ran2Shift; 
     double remainderAnswer = (double)ran1Shift % ran2Shift; 



    // Prompt user with a question based upon random numbers and operation selection 
    // Presentation of addition problems 
    if(userLetter.equalsIgnoreCase("a")) { 
     System.out.print("What is the solution to the problem:" + " " + ran1Shift + " " + "+" + " " + ran2Shift + " = "); 
     if (stdin.hasNextDouble()) { 
      double userNum = stdin.nextDouble(); 
      if (userNum == additionAnswer) { 
       System.out.println("That is correct!"); 
      } else { 
       System.out.println("The correct solution is: " + additionAnswer + "."); 
      } 
     } else { 
      System.out.println("All solutions must be entered as decimal numbers."); 
      System.out.println("The correct solution is " + additionAnswer + "."); 
      } 
     } 

    else{ 
    System.out.println("I'm sorry, I only understand choices of: A, S, M, D, or R!"); 
    } 
} 

    // Program exit 
    System.out.println("======================================"); 
    System.out.println("Thank you for using the Math Trainer!"); 
} 

}

Answer 1

如果我輸入 'Z'，而不是一個，S，M，d，或R，例如，此程序將輸出else語句3次。

假設3次是因爲Config.NUMBER_OF_QUESTIONS是3，那是因爲你執行String userLetter = stdin.nextLine();外循環，所以userLetter值不會改變。

如果您修正代碼的縮進，所以for循環的範圍變得清晰，你會發現你需要移動在循環內以下行：

// Ask for user input on which to choose 
System.out.print("Enter your choice:" + " "); 
String userLetter = stdin.nextLine(); 

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原始答案

hasNextDouble()不會消耗任何東西，所以當您提出問題，並且用戶響應I don't know而不是輸入數字時，文本會保留。

當你然後問下一個問題，從以前的問題（壞的）答案仍然是在緩衝區中，系統將無法在未來hasNextDouble()呼叫過，等等，等等，...

這是人們如何使用Scanner的主要缺陷之一。他們忘記錯誤處理。

在這種情況下，答案是每行給出一個答案，所以每當你閱讀答案時，你應該總是調用nextLine()，在答案後放棄任何額外的文本，或者如果給出錯誤的輸入，則放棄整個行。