How to move a null to the end of array in java using FOR loop?

Question

Assume there is only one null in an array. I am trying to move it right to the end using a for loop. this is what i tried.

    String example[] = new String[5];
    example[0] = "a";
    example[1] = null; 
    example[2] = "c"; 
    example[3] = "d"; 
    example[4] = "e";

I want the output to be : a,c,d,e,null. I want to able to move the null to the end regardless of its index using a FOR loop.

This is what i tried

    String asd[] = new String[creatureList.length];

    for (int i = 0 ; i < creatureList.length; i++) {
        if (creatureList[i] != null){

        asd[i] = creatureList[i];



        }
        creatureList = asd;

There are several ways to do it. Is your approach not working or are you looking for a better approach (better in terms of?) ? — Praba
– Praba, Commented Sep 11, 2014 at 12:48

peter.petrov · Accepted Answer · 2014-09-11 13:01:07Z

2

Just search for the null element and if found, shift all elements 1 position to the left. This code works even if there are more than 1 null elements in the array.

public class Test10 {

    public static void main(String[] args) {
        String example[] = new String[5];
        example[0] = "a";
        example[1] = null; 
        example[2] = "c"; 
        example[3] = "d"; 
        example[4] = "e";

        for (int j=0; j<example.length; j++){
            if (example[j]==null){
                for (int k=j+1; k<example.length; k++){
                    example[k-1] = example[k];
                }
                example[example.length-1] = null;
                break;
            }
        }

        for (int j=0; j<example.length; j++){
            System.out.println(example[j]);
        }
    }

}

edited Sep 11, 2014 at 13:01

answered Sep 11, 2014 at 12:55

peter.petrov

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Comments

Guilherme Eler · Accepted Answer · 2014-09-11 12:55:24Z

1

You can just use the principle of bubble sort in here. Try this

String example[] = new String[5];
example[0] = "a";
example[1] = null; 
example[2] = "c"; 
example[3] = "d"; 
example[4] = "e"; 

for(int i=0; i < example.length - 1; ++i) {
    if(example[i] == null) {
        example[i] = example[i+1];
        example[i+1] = null;
    }
}

for(String s : example)
    System.out.print(s + " ");

If you only need to print the values that way, try this

for(String s : example)
    if(s != null)
        System.out.print(s + " ");
System.out.print(null + " ");

answered Sep 11, 2014 at 12:55

Guilherme Eler

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3 Comments

jbutler483 Over a year ago

This wouldn't be the most efficient way of doing it, but still should work.

Guilherme Eler Over a year ago

I figured that out, but I almost never find a situation to use bubble sort (or at least its main idea). I just got carried out. =D

jbutler483 Over a year ago

that's because it's known as one of the 'simple' sorts - if you're learning about advanced sorts, you might want to look at the 'advanced sorts'

LionC · Accepted Answer · 2014-09-11 12:53:28Z

0

A possible solution without creating a new Array would be to move the elements. Example, assuming that example is an Array:

boolean found = false;

for(int i = 0; i < example.length; i++) {
  if(found)
    example[i-1] = example[i];
  if(example[i] == null)
    found = true;
  if(i == example.length - 1 && found)
    example[i] = null;
}

I want to add that this would be a lot easier if you would use Collections (for example an ArrayList), I highly recommend using them instead of Arrays if performance is not a huge factor.

answered Sep 11, 2014 at 12:53

LionC

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Comments

Gorbles · Accepted Answer · 2014-09-11 12:55:20Z

0

I'd have a different implementation for ArrayLists (as .remove() is handy), but here we are with a bounded array:

for(int n = 0; n < array.length; n++) {
   if(null == array[n]) {
        // to avoid issues with references, unsure if necessary
        String s = array[n + 1];
        array[n] = array[s];
        array[n + 1] = null;
    }
}

answered Sep 11, 2014 at 12:55

Gorbles

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Comments

ethanfar · Accepted Answer · 2014-09-11 12:57:33Z

0

Here's a solution that is based on your assumption that there's only one null:

for (int i=1; i<example.length; i++) {
    if (example[i-1] == null) {
        example[i-1] = example[i];
        example[i] = null;
    }
}

answered Sep 11, 2014 at 12:57

ethanfar

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Comments

Dan · Accepted Answer · 2021-03-04 00:18:39Z

0

I think the answers before me were not correct, so here is a code that works and puts all the nulls at the end of a array.

public static void moveNullToTheEnd( Object[] arr ) 
{
    
    Object[] temp = new Object[arr.length];
    int counter = 0;
    int nullcounter = arr.length-1;
    for(int i= 0; i < arr.length; i++) {
        if(arr[i] == null){
            temp[nullcounter] = null;
            nullcounter--;
            
        }
        else {
            temp[counter] = arr[i];
            counter++;
            
        }
    }
    arr = temp;
    
    System.out.println(Arrays.toString(arr));
    
}   

    
public static void main (String[] args)
{
      Object [] test = new Object[] {1, 2, null, null, 4};
      moveNullToTheEnd(test);
}

answered Mar 4, 2021 at 0:18

Dan

1

1 Comment

Henry Twist Over a year ago

Why you think the accepted answer is not correct? I have tested it and it seems to work as intended. This is a very old question and probably doesn't need anymore answers unless you have something new to add.

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