Java：使用变量指定数组长度的编码？

Question

我正在开发一个学生分数应用程序，该应用程序接受一个或多个学生的姓氏、名字和分数，并将结果存储在一个数组中。然后它按姓氏的字母顺序打印学生和他们的分数。我们不知道有多少学生，但会少于 100 人。

我们必须在学生信息的末尾显示班级平均分，并在每个成绩低于班级平均分 10 分以上的学生之后显示一条消息。

我的第一个问题是我创建了一个 do/while 循环来询问用户是否想输入另一个，但它不起作用！？！？

其次，我不知道如何在个别学生身上显示“低于 10 分”的消息。

public class Student implements Comparable
{
    String firstName;
    String lastName;
    int score; 

    //stores last name, first name and score for each student
    public Student(String lastName,String firstName,int score)
    {
        this.lastName = lastName;
        this.firstName = firstName;
        this.score = score;    
    }
    //implement the comparable interface so students can be sorted by name
    public int compareTo(Object o)
    {
        Student otherStudent = (Student)o;

        if(otherStudent.lastName.equals(lastName))
            {
            return firstName.compareToIgnoreCase(otherStudent.firstName);
            }
        else
            {
            return lastName.compareToIgnoreCase(otherStudent.lastName);
            }
    }
    public String toString()
    {
        return lastName + ", " + firstName + ": " + score; 
    }
}

import java.util.Scanner;
import java.util.Arrays;

public class StudentApp
{
    static Scanner sc = new Scanner(System.in);

    public static void main(String [] args)
    {
        Student [] studentArray;
        String lastName;
        String firstName;
        int score = 0;
        double average = 0;


        System.out.println("Welcome to the Student Scores Application.");
        System.out.println();

        do{

            //code that uses variable to specify the array length
        int nStudent = 100;  //array size not set unit run time
        studentArray = new Student[nStudent];

            for (int i=0; i<nStudent; i++)
            {
            System.out.println();

            lastName = Validator.getRequiredString(sc, 
                           "Student " + (i+1) +  " last name: ");
            firstName = Validator.getRequiredString(sc, 
                           "Student " +  " first name: ");               
            score = Validator.getInt(sc, 
                         "Student " + " score: ",
                        -1, 101);

            studentArray[i] = new Student(lastName, firstName, score);

            double sum = 0.0;
            sum += score;
            average = sum/nStudent;
            }
        }while (getAnotherStudent());

        Arrays.sort(studentArray);

        System.out.println();

        for (Student aStudent: studentArray)
        {
            System.out.println(aStudent);
            if (score<= (average-10))
            {
                System.out.println ("Score 10 points under average");
            }
        }
        System.out.println("Student Average:" +average);
    }
    public static boolean getAnotherStudent()
    {
        System.out.print("Another student? (y/n): " );
        String choice = sc.next();
        if (choice.equalsIgnoreCase("Y"))
            return true;
        else
            return false;
    }
}

Answer 1

这里有几个问题：

• 每次执行...同时，您都会重新实例化studentArray 和sum。这意味着当 getAnotherStudent() 为真时，您之前迭代的所有数据都将被核对 - 您希望实例化数组并只求和一次。
• 如果您有超过 100 名学生，您不会停止。您的循环中也需要一个围绕 nStudent 的结束条件。

• 您应该对getAnotherStudent() 进行一些调整，以便您可以阻止数据，并在输入有效数据时等待 - 通过使用循环：

   public static boolean getAnotherStudent() {
       Scanner sc = new Scanner(System.in);
       System.out.print("Another student? (y/n): " );
       if (sc.hasNext()) {  
           String choice = sc.next();
           // blocks here - ignores all input that isn't "y" or "n"
           while(!((choice.equalsIgnoreCase("Y") || choice.equalsIgnoreCase("N")))) {
               if (choice.equalsIgnoreCase("Y")) {
                   return true;
               }
               System.out.print("Another student? (y/n): " );
               choice = sc.next();
           }
        }
        return false; // obligatory
  

Answer 2

您的代码很接近，只是有几个问题。你的 do while 循环不起作用的原因是你里面有一个 for 循环。这意味着您将询问 100 名学生，然后再询问他们是否要添加另一名学生。您的总和正在此循环中创建，因此每次都会重置。

最后，您不知道将添加多少学生，但您的代码假定将有 100 名学生。这意味着您不能使用 for each 循环遍历数组，因为有些可能为空。只需使用常规 for 循环直到您添加的学生的最后一个索引。以下是更改：

    Student[] student = new Student[nStudent]; 
    int studentCount = 0; //declear the counter outside the loop
    double sum = 0.0; //declear the sum outside the loop
    do {
        System.out.println();
        lastName = Validator.getRequiredString(sc, 
                       "Student " + (i+1) +  " last name: ");
        firstName = Validator.getRequiredString(sc, 
                       "Student " +  " first name: ");          
        score = Validator.getInt(sc, 
                     "Student " + " score: ",
                    -1, 101);

        student[studentCount] = new Student(lastName, firstName, score); 

        sum += score; //increase the sum

        studentCount++; //increment the counter

    } while (studentCount < nStudent && getAnotherStudent()); //stop if the user says 'n' or we hit the maximum ammount
    average = sum / studentCount; //work out the average outside the loop

    System.out.println();

    for (int i= 0; i< studentCount; i++ ) {
        System.out.println(aStudent);
        if (score <= (average - 10)) {
            System.out.println("Score 10 points under average");
        }
    }
    System.out.println("Student Average:" + average);
}

Answer 3

您的 getAnotherStudent() 方法应为：

System.out.print("Another student? (y/n): " );
if (sc.hasNext()) {   // blocks until user entered something     
    String choice = sc.next();
            if (choice.equalsIgnoreCase("Y"))
                return true;
            else
                return false;
} else {
    // won't come here
    return false;
}