errors in java - finding maximum values in an ArrayList

Question

Following is the program I wrote as an answer for the question -

"Now use ArrayList and the Integer wrapper class to store the values and initialize the elements by reading input from console using Scanner class.Extend the program to identify the n maximum values in the ArrayList."

import java.util.ArrayList;
import java.util.Scanner;    

public class ArraylistInput {
/**
 * @param args
 */
public static void main(String[] args) {

    ArrayList<Integer> val = new ArrayList<Integer>();
    Scanner in = new Scanner(System.in);
    System.out.println("Enter the length of you Array List ");
    int nos = in.nextInt();
    // Recorrd the input numbers
    for (int i = 0 ; i < nos; i++)
    {
        System.out.println("Enter values for the ArrayList ");
        int Input = in.nextInt();
        val.add(Input);
    }

    // Display the arraylist
    for (int j = 0; j < nos; j++)
    {
        int x = val.get(j);
        System.out.println("Index " + (j+1) + ": " +  x);
    }
    System.out.println("How meny maximmum values do you want? ");
    int max =0; // initial max value
    int nmax = in.nextInt(); // number of maximum values
    int length = val.size(); // size of the arraylist

    // finding the maximum values in ascending order without sorting
    for (int h = 1; h <= nmax ; h++)
    {           
        for (int k=0;k < length; k++)
        {
            if (val.get (k) > max)
            {
                 max = val.get(k);
            }               
        }
        System.out.println ("maximmum = " + max);
        int z = val.indexOf(max); // removing the higest value after printing
        val.remove(z);
    }           
    }
}

Output and Error:

Enter the length of you Array List

3

Enter values for the ArrayList

12

Enter values for the ArrayList

45

Enter values for the ArrayList

8

Index 1: 12 Index 2: 45 Index 3: 8

How meny maximmum values do you want?

2

maximmum = 45

Exception in thread "main" maximmum = 45 java.lang.ArrayIndexOutOfBoundsException: -1 at java.util.ArrayList.elementData(Unknown Source) at java.util.ArrayList.remove(Unknown Source) at ArraylistInput.main(ArraylistInput.java:46)

Edwin Dalorzo · Accepted Answer · 2012-04-21 14:37:41Z

2

I would do the following:

Collections.sort(myList, Collections.reverseOrder());
List<Integer> maxn = myList.subList(0, n);
System.out.printf("The maximum %d values are: %s%n", n, maxn);
maxn.clear(); //This clears the sublist and removes its elements from the source list

That would give you a list with the maximun n elements in your list.

edited Apr 21, 2012 at 14:37

answered Apr 21, 2012 at 14:22

Edwin Dalorzo

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2 Comments

nansen Over a year ago

I would prefer this too. But keep in mind that subList() returns a view. Any changes to myList will reflect in maxn.

Edwin Dalorzo Over a year ago

@nansen Then add the sublist to another list: List<Integer> maxn=new ArrayList<>(myInts.subList(0,n)).

Hunter McMillen · Accepted Answer · 2012-04-21 14:18:23Z

0

You only have a single ArrayList you don't need the nested for loop to find the maximum:

int max = Integer.MIN_VALUE;

for(int i = 0; i < list.size(); i++)
{
   current = list.get(i);
   if(current > max)
      max = current;
}

The nested for loop when you are searching for the maximum tries to access a value in your list that doesn't exist, which is why you are receiving this error.

answered Apr 21, 2012 at 14:18

Hunter McMillen

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2 Comments

Edwin Dalorzo Over a year ago

I believe the question is about getting the n maximun elements, not just one.

Hunter McMillen Over a year ago

pretty simple to adjust this to get the n max

Marko Topolnik · Accepted Answer · 2012-04-21 14:19:13Z

0

Your max is never getting assigned and then you are trying to remove a non-existing element from the arraylist. Set max to some value that cannot occur in the list and then check whether it ever got assigned in the loop.

answered Apr 21, 2012 at 14:19

Marko Topolnik

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Comments

ulmangt · Accepted Answer · 2012-04-21 14:20:00Z

0

When you remove an element from the list with:

val.remove(z);

You change the size of the list but you don't update your length variable. This causes you to try to access indices beyond the size of the array, resulting in a java.lang.ArrayIndexOutOfBoundsException.

Also, consider saving both the index of the max value and the max value itself. Then, when you go to remove the value, you can use ArrayList.remove() directly without searching the whole list again for the max index (which is what ArrayList.indexOf() will do).

answered Apr 21, 2012 at 14:20

ulmangt

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