java - 使用無效輸入的循環打開

Question

我正在創建一個程序，可根據用戶的輸入計算用戶的成績。除了我需要處理輸入無效條目的用戶之外，所有這一切都很好。

示例：用戶在一行上輸入三個考試分數（例如95 90 87）。如果其中一個分數是負數（95 90 -87），我需要程序不要輸入這三個分數並分配一個字母等級，而是提示用戶正確地重新輸入分數。

這裏是我的代碼：

public class GradeCalculator { 

    public static void main(String[] args) { 
     int classSum = 0;  // variable used to hold sum of entire classes exams 
     int classExams = 0;  // variable used to hold number of exams taken by whole class 

     Scanner s = new Scanner(System.in); 

     System.out.println("Welcome to Gradecalculator!"); 
     System.out.println("Please enter the number of students:"); 
     int students = s.nextInt(); 

     System.out.println("Please enter the number of exams:"); 
     int exams = s.nextInt(); 

     int i = 0; 
     int studentnumber = 1; 
     int sum = 0; 

     while (i < students) { // loop until it matches number of students entered above 
      i++; 
      sum = 0; 

      System.out.println("Enter student " + studentnumber++ + "'s name :"); 
      String studentname = s.next(); 

      System.out.println("Enter exam scores :"); 

      int input = 0; 
      for (; input < exams; input++) { 
       int n = s.nextInt(); 
       sum+=n; 

       if (n < 0) { 
        System.out.println("Invalid exam scores, reenter: "); //if one of the scores entered is  negative, display message 
       } 
      } 

      double average = sum/exams;  // assign letter grade based on average of exams 
      if (average <= 100 && average >= 90) { 
       System.out.println("Letter grade: A"); 
       System.out.println(studentname + " gets 4 stars! ****"); 
      } if (average <= 89 && average >= 80) { 
       System.out.println("Letter grade: B"); 
       System.out.println(studentname + " gets 3 stars! ***"); 
      } if (average <= 79 && average >= 70) { 
       System.out.println("Letter grade: C"); 
       System.out.println(studentname + " gets 2 stars! **"); 
      } if (average <= 69 && average >= 60) { 
       System.out.println("Letter grade: D"); 
       System.out.println(studentname + " gets 1 star! *"); 
      } if (average <= 59) { 
       System.out.println("Letter grade: F"); 
       System.out.println(studentname + " gets 0 stars!"); 
      } 

      classSum += sum;    // add sum of this student's scores to the classSum 
      classExams += exams; // add exams taken by this student to amount of exams taken by whole class 
     } 

     int classAverage = classSum/classExams; // compute class average 
     System.out.println("Class statistics:"); 
     System.out.println("\tAverage: " + classAverage); 
    } 
} 

顯然，我有這樣的代碼在錯誤的地方：

   if (n < 0) { 
        System.out.println("Invalid exam scores, reenter: "); //if one of the scores entered is  negative, display message 
       } 
      } 

因爲這是輸出進入負分，當我得到：

Enter exam scores : 
70 70 -70 
Invalid exam scores, reenter: 
Letter grade: F 
joe gets 0 stars! 

正如你所看到的，它仍然指定一個字母等級和星號，並要求下一個學生的名字。我需要它而不是要求重新輸入這個學生的分數。無法弄清楚如何做到這一點。

Answer 1

您可以減去一個從輸入內部的for循環，使其停留在該循環等方面的投入不被抹消（同時也確保總和不添加這些負輸入）

int input = 0; 
for (; input < exams; input++) { 
    int n = s.nextInt(); 
    if (n>=0) 
     sum+=n; 
    else { 
     input--; 
     System.out.println("Invalid exam score entered, reenter: "); 
    } 

} 

Answer 2

編輯：使用交互式輸入，應該更加用戶友好。

您需要重置循環計數器重新開始循環：

// Delete first output and counter variable initialization before loop 
for (int input = 1; input < exams; input++) { 
    System.out.println(String.format("Enter %d. exam score: ", input + 1)); 

    int n = s.nextInt(); 

    if (n < 0) { 
     System.out.println("Invalid value (must be positive)!"); 
     input--; // Reenter last value 
    } else { 
     sum+=n; // Only use correct scores. 
    } 
}