為什么Java掃描程序跳過while語句中嵌套在for循環中的用戶提示之一？

Question

掃描程序不會在第二個while語句中提示用戶輸入，該語句嵌套在for循環中。 為什么？ 有什么辦法嗎？

   import java.util.Scanner;

    public class ComputeGpa
    {
        public static void main(String args[])
        { 
            char grade = '\0';
            int credits = 0;
            String aGrade = "";

            Scanner in = new Scanner(System.in);

            Gpa gpas = new Gpa();

            int numCourses = 0;
            while(numCourses <= 0)
            {
                System.out.print("Enter number of courses: ");
                numCourses = in.nextInt();
                if(numCourses <= 0)
                {
                    System.out.println("Invalid number of courses - must be greater than 0!");
                }
            }

            for(int i = 0; i < numCourses; i++)
            {
                while(aGrade.length() != 1)
                {
                    System.out.print("Enter grade (one character): ");
                    aGrade = in.next();
                    grade = aGrade.charAt(0);
                    if(aGrade.length() != 1)
                    {
                        System.out.println("Invalid grade - must be exactly one character");
                    }
                }

                while((credits < 0) || (credits > 9))// this is the while statement I have
                    problems with
                    {
                    System.out.print("Enter credits: ");
                    credits = in.nextInt();
                    if((credits <  0) || (credits > 9))
                    {
                        System.out.println("Invalid credits - must be between 0 and 9");
                    }
                    gpas.addTotals(grade, credits);
                    }
            }

            System.out.printf("GPA: %.2f", gpas.calcGpa()); 

        }        
    }

我要驗證的條件是用戶必須輸入0到9之間的數字（包括0和9）。

然后，我得到一個計算值“ NaN”

Answer 1

將您的while循環更改為do ... while循環，因為您希望循環始終在第一次運行，但是您還希望在循環結束時檢查條件。

Answer 2

更改

while((credits < 0) || (credits < 9))

至

while((credits < 0) || (credits > 9))

旁注：“包含”表示您希望將0和9包括為允許的信用值。 但這不是這里的內容，因為您使用的是<而不是<=和>而不是> =。

Answer 3

將其更改為while（（credits <= 0）||（credits> 9）），因為您已將信用額初始化為'0'。只需添加

 while((credits <= 0) || (credits > 9))// this is the while statement I have
                                              //problems with
      {
        System.out.print("Enter credits: ");
        credits = in.nextInt();
        if((credits <  0) || (credits > 9))
        {
            System.out.println("Invalid credits - must be between 0 and 9");
        }
        if(credits == 0)
            break;

     //   gpas.addTotals(grade, credits);
      }

學分將接受0-9，並且不會再次詢問用戶

Answer 4

我試圖修改last while循環以適合您的需求，希望這段代碼對您有所幫助。

do {
    System.out.print("Enter credits: ");
    credits = in.nextInt();
    if ((credits < 0) || (credits > 9)) {
    System.out.println("Invalid credits - must be between 0 and 9");
    }else{
        gpas.addTotals(grade, credits);
        }
} while ((credits < 0) || (credits > 9));