Converting a positive integer to have a negative value

Question

So today in class we had to make a program that asked to input values between or equal to -25 and 25, and to quit the program when a number outside those values is entered. When I try to input any number higher than 25 or a negative value, the program crashes and an error report pops up.

So here's my question, how to make these negative values work. I included the program below to help anyone you is willing to help figure this problem out.

import java.util.Scanner;
public class Program2 
{

    public static void main(String[] args) 
    {
        Scanner scan = new Scanner (System.in);
        int occurrences[] = new int [51];

        System.out.println ("Enter integers in the range if -25 through 25.");
        System.out.print ("Signal end with a");
        System.out.println ("number outside the range.");

        int entered = scan.nextInt();       
        while (entered >= -25 && entered <= 25)
        {
           occurrences [entered] ++;
           entered = scan.nextInt();
        }

        System.out.println ("Number\tTimes");
        for (int check = -(25); check <= 25; check++)
            if (occurrences [check] >= 1)
                System.out.println (check + "\t" + occurrences [check]);

    }
}

Answer 1

The problem is on line

occurrences [entered] ++;

Negative numbers cannot be used as indices to an array. In order to fix this, you can use a separate variable to track the number of scanned values, such as count and use this to access the array.

Answer 2

The problem is that you can't use negative numbers to index into a Java array.

You could shift array indices like so:

    occurrences [entered + 25] ++;

This will remap the numbers from -25 to 25 as 0 to 50, allowing them to be used as array indices.

(You'll need to change the rest of the program accordingly; I leave this as an exercise for the reader.)

Answer 3

Array indices cannot be negative. So a dirty solution (but working) would be to add 25 to value entered before using it as array index. This will work for negative numbers but not for nurber > 25. To use such values, you need to use a larger array or use a different method of storing the values.