confused on a for loop

Question

I'm getting an error when ever my program reaches this for loop and tries to run it

for (int m = 0; m < students; m++){
        allScores = allScores + scores[m];

students was declared as an int allScores was declared as a double as was the scores[] array.

This is the error message:

Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: 2 at Lab4.main(Lab4.java:51)

line 51 is the for for (int m = 0; m < students; m++) line

Edit:

This is my entire code, i may not have done the scores[] array correctly:

import java.util.*;
import java.text.*;

public class Lab4 {
public static void main(String[] args){
    Scanner s= new Scanner(System.in);
    String input;
    int students;
    int d=1;
    double allScores = 0;

    char [] answerKey= { 'B' , 'D' , 'A' , 'A' , 'C' , 'A' , 'B' , 'A' , 'C' , 'D' , 'B' , 'A' };
    char [] userAnswers = new char[answerKey.length];

    DecimalFormat df = new DecimalFormat("#0.0");

    System.out.print("how many students are in the class? ");
    input = s.nextLine();
    students=Integer.parseInt(input);

    String [] name = new String[students];
    double [] scores = new double[students];

    for (int j = 0; j < students; ++j)
    {
        int correctAnswers=0;

        System.out.print("Enter name of student " + (j + 1) + ": ");
        name[j] = s.nextLine();

        System.out.print("Enter quiz score answers :");
        String line = s.nextLine().toUpperCase();
        for (int k = 0; k < answerKey.length; k++) {
        userAnswers[k] = line.charAt(k);
            }


            for (int i = 0; i < userAnswers.length; i++)
        {

                if(userAnswers[i] == answerKey[i])
                {
                    correctAnswers++;
                }



        }
            System.out.println(name[j]);
            System.out.println((df.format((((float)(correctAnswers) / answerKey.length)) * 100)) + "%");
            scores [d] = ((float)((correctAnswers) / answerKey.length) * 100);
            d++;
    }
    System.out.println("the high score is "   + "and the low score is "   );
    for (int m = 0; m < scores.length; m++){
        allScores = allScores + scores[m];
    }
    System.out.println("Average is:" + ((allScores/students)));

}

}

EDIT:

Sorry I read the lines wrong line 51 was

scores [d] = ((float)((correctAnswers) / answerKey.length) * 100);

Edit:

new code no longer giving me error messages, but I can't seem to get the averaging of scores to work.

import java.util.*;
import java.text.*;

public class Lab4 {
public static void main(String[] args){
    Scanner s= new Scanner(System.in);
    String input;
    int students;
    int d=1;
    double allScores = 0;

    char [] answerKey= { 'B' , 'D' , 'A' , 'A' , 'C' , 'A' , 'B' , 'A' , 'C' , 'D' , 'B' , 'A' };
    char [] userAnswers = new char[answerKey.length];

    DecimalFormat df = new DecimalFormat("#0.0");

    System.out.print("how many students are in the class? ");
    input = s.nextLine();
    students=Integer.parseInt(input);

    String [] name = new String[students];
    double [] scores = new double[students + 1];

    for (int j = 0; j < students; ++j)
    {
        int correctAnswers=0;

        System.out.print("Enter name of student " + (j + 1) + ": ");
        name[j] = s.nextLine();

        System.out.print("Enter quiz score answers :");
        String line = s.nextLine().toUpperCase();
        for (int k = 0; k < answerKey.length; k++) {
        userAnswers[k] = line.charAt(k);
            }


            for (int i = 0; i < userAnswers.length; i++)
        {

                if(userAnswers[i] == answerKey[i])
                {
                    correctAnswers++;
                }



        }
            System.out.println(name[j]);
            System.out.println((df.format((((float)(correctAnswers) / answerKey.length)) * 100)) + "%");
            scores [d] = ((float)((correctAnswers) / answerKey.length) * 100);
            d++;
    }
    System.out.println("the high score is "   + "and the low score is "   );
    for (int m = 0; m < scores.length; m++){
        allScores = allScores + scores[m];
    }
    System.out.println("Average is:" + ((allScores/students)));

}

}

Answer 1

The problem is that students is initialized to a value that is greater than scores.length . You can use scores.length instead of students as the upper limit for the loop; alternatively, initialize students to scores.length (or something lower than it).

If you just want to iterate through the entire array, you can use the enhanced for loop syntax:

for (double score : scores) {
    allScores = allScores + score;
}

EDIT I counted lines myself (kind of tedious) and I land on this line:

scores [d] = ((float)((correctAnswers) / answerKey.length) * 100);

What's happening is that d gets too large. That's probably because you initialize d to 1 instead of 0.

Answer 2

Use length property in array for iteration.

for (int m = 0; m < scores.length; m++){
        allScores = allScores + scores[m];

However, you mentioned that the error is happened in line 51, but line 51 is :

scores [d] = ((float)((correctAnswers) / answerKey.length) * 100);

So your d is equals or larger than scores.length.

I'm curious about why you intialize d with an 1?

change it to 0 may solve the problem.

int d=0;

Another problem in your code is:

((float)((correctAnswers) / answerKey.length) * 100);

change it to

(((float)correctAnswers / answerKey.length) * 100);

Otherwise, you change the result, which is already an int to float. That will impact the result.

Answer 3

Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException:  

This is because some where you are accessing array beyond its limit. may be in scores[m];

you can use following:

for (int m = 0; m < scores.length; m++){
        allScores = allScores + scores[m];  

Answer 4

there are some irregularities with your code:

int d=1; // your score[index] starts with this just change to 0;

scores [d] = ((float)((correctAnswers) / answerKey.length) * 100);

//your scores length will now be d + students; use the students instead
for (int m = 0; m < scores.length; m++){  //for (int m=0;m<students;m++)
        allScores = allScores + scores[m];
    }