Java removing zeros from array

Question

I am trying to write a program that will, when executed, will go through an array and remove all instances of 0.0 and change the size of the array equal to the number of non zero elements and put those elements in their previous order. That is, if n=10 and the contents of a[j], j = 0 to n - 1 are initially

0.0, 1.2, 0.0, 0.0, 0.0, 2.3, 0.0, 9.7, 5.6, 0.0

then after execution of the code the contents should be

 n=4, a[0]=1.2, a[1]=2.3, a[2]=9.7, and a[3]=5.6.

This is what I have so far:

import java.util.Scanner;
public class hw2
{
   public static void main(String[] args) 
   {
       Scanner scan = new Scanner(System.in);
       final double KEY = 0.0;
       int n = scan.nextInt();
       double[] a = new double[n];
       for(int i=0; i<n; i++)
       {
           a[i] = scan.nextDouble();
       }
       for(int k = 0; k<n; k++)
       {
           if(a[k] == KEY)
           {
               a[k] = a[k+1];
               n--;
           }
           System.out.println(a[k]);
       }
   }
}

Just a little nudge in the right direction would be appreciated.

cough Consider ArrayList cough Although in this case, you could simply not save the value if it's "0", perhaps. (Also don't increment the "used array variable", except when you add, so you know how much contains "non 0" data or treat the first "0" encountered during iteration as "end of useful data" -- arrays are initialized with the default value of the type.) — user166390
– user166390, Commented Nov 23, 2010 at 2:19
Well its based on if there is already an array named a that has double values and you have to run a code snippet that will go through the already filled array and remove all instances of 0.0 while keeping the other numbers in their same order. — Mike
– Mike, Commented Nov 23, 2010 at 2:22

user166390 · Accepted Answer · 2010-11-23 02:25:43Z

Consider using an ArrayList, which will allow you to add items when you want, grow as needed, and maintain an accurate count.

Although in this case, if you wanted/needed to use an array, you could simply not save the value if it's "0", perhaps. (Also don't increment the "used array variable", except when you add, so you know how much contains "non 0" data or treat the first "0" encountered during iteration as "end of useful data" -- arrays are initialized with the default value of the type.)

If you want to go from an array with zeros to an array with no zeros at all, you must use two passes -- one to count the non-zeros, to construct a new array of the appropriate size, and then copy the non-zero values over. This could be done in reverse as well (compact the initial array, and then copy the "filled" portion of it over), but it's slightly more complicated.

If you do continue with your current approach (which the result array will have zeros, but at the end), you need to maintain two index pointers -- one is the primary loop iterator, the second is the next place to put the non-zero value, which is incremented only when a value is copied (or not moved, as when both indexes are the same, which will be the case until the first 0 is encountered). Make sure to "zero" the place you move the non-zero from. The number of moves can be reduced if order does not need to be preserved.

Emil · Accepted Answer · 2010-11-23 05:09:08Z

Your implementation (2nd for loop) is not right, it will fail to simple test case: Input > 5 2.0 2 0.0 3 0.0 Your program will have wrong output: 2.0 2.0 3.0 3.0

but it should be 2.0 2.0 3

Also, you can't use == to compare two double.

The following code is my solution basing on your current code:

    public class hw21 {
    public static void main(String[] args) {
        Scanner scan = new Scanner(System.in);
        final double KEY = 0.0;
        final Double ACCEPTABLE_TOLERANCE = 0.000000000001d;

        int n = scan.nextInt();
        double[] a = new double[n];
        for (int i = 0; i < n; i++) {
            a[i] = scan.nextDouble();
        }
        for (int k = 0, j = 0; k < n; k++) {
            if (Math.abs(a[k] - KEY) < ACCEPTABLE_TOLERANCE) {
                continue;
            }
            a[j] = a[k];
            System.out.println(a[j]);
            j++;
        }
    }
}

Also I prefer to use an ArrayList like below:

public class hw2 {
public static void main(String[] args) {
    Scanner scan = new Scanner(System.in);
    final double KEY = 0.0;
    final Double ACCEPTABLE_TOLERANCE = 0.000000000001d;

    int n = scan.nextInt();
    double[] a = new double[n];
    for (int i = 0; i < n; i++) {
        a[i] = scan.nextDouble();
    }

    List<Double> newList = new ArrayList<Double>();
    for (int k = 0; k < n; k++) {
        if (Math.abs(a[k] - KEY) < ACCEPTABLE_TOLERANCE) {
            continue;
        }
        newList.add(a[k]);
    }

    System.out.println("There are " + newList.size() + " no-zero double:");
        System.out.println(newList);
    }
}

Ragavan · Accepted Answer · 2010-11-23 03:02:22Z

1

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

public class StackOverflow1
{
    public static final double KEY = 0.0;
    private static final Scanner INPUT = new Scanner(System.in);

    public static void main(String[] args) {


       int length = INPUT.nextInt();
       double[] array = new double[length];

       for(int i=0; i<length; i++) {
           array[i] = INPUT.nextDouble();
       }

       int index = 0;
       for(int k = 0; k < length ; k++) {
           if(array[k] == KEY) {
               continue;
           }
           array[index] = array[k]; // bring the non-zeroth element forward
           if (index != k) array[k] = 0; //make the non-zeroth element zero in the actual location
           index++;
       }
       System.out.println("n = " + index + " array = " + Arrays.toString(array));
   }
}

answered Nov 23, 2010 at 3:02

Ragavan

313 bronze badges

3 Comments

Ragavan Over a year ago

I hope this helps. Please leave a comment if you need further assistance. If you want to shrink the array you need to create a new array and copy the elements of this array to the new one. Or you should use a data structure like linked list, in which case, the problem becomes much easier to solve

asela38 Over a year ago

it good if don't put those unwanted zeros to the array, from the begining

Ragavan Over a year ago

I hope you understood what I said. You can't shrink an array. You need to move the elements of it to another array. Use a linked list otherwise

asela38 · Accepted Answer · 2010-11-23 03:30:13Z

0

you can remove those unwanted zeros as follows but in this case it get sorted.

@org.junit.Test
public void test15() throws Exception {
    double[] arr = new double[]{0.0,1.1,0.1,0.0,2.1};
    double[] nonZeroArr = arr;

    Arrays.sort(nonZeroArr);
    int index = -1;
    while((index = Arrays.binarySearch(nonZeroArr, 0.0)) > -1){
        double[] newArr = new double[nonZeroArr.length-index-1];
        System.arraycopy(nonZeroArr, index+1, newArr, 0, newArr.length);
        nonZeroArr = newArr;
    }
    for (double d : arr) {
        System.out.print(d +",");
    }
    System.out.println();
    for (double d : nonZeroArr) {
        System.out.print(d + ",");
    }
}

answered Nov 23, 2010 at 3:30

asela38

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Comments

Glen P · Accepted Answer · 2011-10-10 03:46:12Z

0

IF you want to sound really smart in class point out that this is a one liner in a lot of languages that are more productive than Java :)

answered Oct 10, 2011 at 3:46

Glen P

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