How do I shift array elements up one position in java?

Question

I am working on a java assignment where I need to delete an integer element in an array and shift the below elements up on space to keep them in order. The array is currently random integers in descending order. I am not allowed to use array.copy because I will need to collect array usage information as part of the assignment. I have tried a ton of different ways of doing this but cannot seem to get it working.

public static void deletionArray(int anArray[], int positionToDelete) {
    for (int j = anArray[positionToDelete] - 1; j < anArray.length; j++) {
        System.out.println("j is " + j);
        anArray[j] = anArray[j + 1];
    }

    displayArray(anArray);
}

What problem are you facing? – exexzian Feb 02 '13 at 21:20 — exexzian, Feb 02 '13 at 21:20

score 2 · Answer 1 · answered Feb 02 '13 at 21:21

You're iterating until anArray.length (exclusive), but inside the loop, you're accessing anArray[j + 1], which will thus be equal to anArray[anArray.length] at the last iteration, which will cause an ArrayIndexOutOfBoundsException.

Iterate until anArray.length - 1 (exclusive), and decide what should be stored in the last element of the array instead of its previous value.

You're also starting at anArray[positionToDelete] - 1, instead of starting at positionToDelete.

score 1 · Accepted Answer · answered Feb 02 '13 at 21:20

1

You have two bugs there.

Since this is an assignment, I won't give a complete answer - just a hint. Your loop definition is wrong. Think about this: what happens on the first and on the last iteration of the loop? Imagine a 5-element array (numbered 0 to 4, as per Java rules), and work out the values of variables over iterations of the loop when you're erasing element number, say, 2.

answered Feb 02 '13 at 21:20

Seva Alekseyev

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SOLVED! Thanks. My for loop looks like this and it appears to work. `for (int j = positionToDelete; j < anArray.length - 1; j++) { anArray[j] = anArray[j + 1];` – joerdie Feb 02 '13 at 22:12

score 0 · Answer 3 · answered Feb 02 '13 at 21:22

0

Use System.arraycopy faster than a loop:

public static void deletionArray( int anArray[], int positionToDelete) {
    System.arraycopy(anArray, positionToDelete + 1, anArray,
            positionToDelete, anArray.length - positionToDelete - 1);
    //anArray[length-1]=0;        //you may clear the last element 
}

answered Feb 02 '13 at 21:22

MrSmith42

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From the question: *I am not allowed to use arraycopy* – JB Nizet Feb 02 '13 at 21:23
1

@JB Nizet: Sorry I missed that. But `arraycopy` would be the best implementation, in the not *homework* world. – MrSmith42 Feb 02 '13 at 21:25
1

I agree. I have used array.copy in the past to great effect but I cannot here. – joerdie Feb 02 '13 at 21:47

score 0 · Answer 4 · 2013-02-02T21:32:44.820

public static int[] deletionArray(int anArray[], int positionToDelete) {
    if (anArray.length == 0) {
        throw new IlligalArgumentException("Error");
    }
    int[] ret = new int[anArray.length - 1];
    int j = 0;
    for (int i = 0; i < anArray.length; ++i) {
        if (i != positionToDelete) {
            ret[j] = anArray[i];
            ++j;
        }
    }
    return ret;
}

Why do we reserve a new array?

Because if don't, we would use C\C++-style array: an array and a "used length" of it.

public static int deletionArray(int anArray[], int positionToDelete, int n) {
   if (n == 0) {
       throw new IlligalArgumentException("Error");
   }
   for (int i = positionToDelete; i < n - 1; ++i) {
       anArray[i] = anArray[i + 1];
   }
   return n - 1; // the new length
}

score 0 · Answer 5 · edited May 23 '17 at 11:48

0

How's this ? Please note the comment, I don't think you can delete an element in an array, just replace it with something else, this may be useful : Removing an element from an Array (Java)

Updated with 'JB Nizet' comment :

public class Driver {

    public static void main (String args []){

        int intArray[] = { 1,3,5,6,7,8};
        int updatedArray[] = deletionArray(intArray , 3);

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

    }

    public static int[] deletionArray(int anArray[], int positionToDelete) {
        boolean isTrue = false;


           for (int j = positionToDelete; j < anArray.length - 1; j++) {            
                if(j == positionToDelete || isTrue){
                    isTrue = true;
                    anArray[j] = anArray[j + 1];
                }

            }

        anArray[anArray.length-1] = 0; //decide what value this should be or create a new array with just the array elements of length -> anArray.length-2
        return anArray;
    }
}

edited May 23 '17 at 11:48

Community

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answered Feb 02 '13 at 21:25

blue-sky

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This doesn't answer the question. -1 – tckmn Feb 02 '13 at 21:26
ok, but a downvote is used to specify 'this answer is not useful' not that it does not fully answer the question – blue-sky Feb 02 '13 at 21:27
This answer is not useful. It does not do anything that the OP wanted (remove the element from the actual array). – tckmn Feb 02 '13 at 21:28
It doesn't answer the question *at all*. So it's not useful. – JB Nizet Feb 02 '13 at 21:28
It answers the question, but I find your algorithm awful. What's the point in starting at 0? Why not start at position instead? Why not go until length -1 instead of checking if j + 1 i< less than length at each iteration? Look at the OP's "SOLVED!" comment on the accepted answer. Doesn't it look much better? – JB Nizet Feb 02 '13 at 22:13
@JB Nizet yep, makes sense. ive updated the answer with your comment. – blue-sky Feb 02 '13 at 22:18

How do I shift array elements up one position in java?

5 Answers5