Insert elements or remove element from a specified position in 2d-array in java

Question

I have a 2d-array of variable number of rows and columns. The array is stored as JSON in the database. I have a requiremnt to add given elements after certain element specified by the user or remove a element specified by the user. Each elemet in the array is unique. The sample json value looks like [[4,5],[3,1,6,7],[34,21,55]]. Consider a situation I want to add 2 elements 12,13 after the element 7, the resultant array looks like [[4,5],[3,1,6,7,12,13],[34,21,55]]. And to remove if I given 1 as input the result should be [[4,5],[3,6,7,12,13],[34,21,55]].Using Gson I have parsed the json value stored to an array. How can I achieve it in Java with less time complexity.

My code to parse json data from db looks like

Gson gson = new GsonBuilder().create();
if (myTemplate.getQuestionOrder() != null) {
    long[][] questionOrder = gson.fromJson(myTemplate.getQuestionOrder(), long[][].class);
}

Kirill Simonov · Accepted Answer · 2018-02-08 15:51:54Z

3

Try the following method.

private static void insertAfter(long[][] array, int value, long[] insertion) {
    boolean found = false;
    for (int i = 0; i < array.length; i++) {
        long[] sub = array[i];
        for (int j = 0; j < sub.length; j++) {
            if (sub[j] == value) {
                long[] newSub = new long[sub.length + insertion.length];
                System.arraycopy(sub, 0, newSub, 0, j + 1);
                System.arraycopy(insertion, 0, newSub, j + 1, insertion.length);
                System.arraycopy(sub, j + 1, newSub, j + 1 + insertion.length, sub.length - j - 1);
                array[i] = newSub;
                found = true;
                break;
            }
        }
        if (found) break;
    }
}

Example usage:

insertAfter(questionOrder, 7, new long[]{12, 13});
System.out.println(gson.toJson(questionOrder));

This will print [[4,5],[3,1,6,7,12,13],[34,21,55]]

To remove an element you can use similar, but slightly modified, logic:

private static long[][] remove(long[][] array, int value) {
    boolean found = false;
    int emptyIndex = -1;
    for (int i = 0; i < array.length; i++) {
        long[] sub = array[i];
        for (int j = 0; j < sub.length; j++) {
            if (sub[j] == value) {
                long[] newSub = new long[sub.length - 1];
                System.arraycopy(sub, 0, newSub, 0, j);
                System.arraycopy(sub, j + 1, newSub, j, sub.length - j - 1);
                array[i] = newSub;
                if (array[i].length == 0) emptyIndex = i;
                found = true;
                break;
            }
        }
        if (found) break;
    }
    if (emptyIndex >= 0) {
        long[][] newArray = new long[array.length - 1][];
        System.arraycopy(array, 0, newArray, 0, emptyIndex);
        System.arraycopy(array, emptyIndex + 1, newArray, emptyIndex, array.length - emptyIndex - 1);
        array = newArray;
    }
    return array.length == 0 ? null : array;
}

This method will remove the given item from the inner array and, if the inner array becomes empty, removes it from the outer array. It returns the modified array or null if it's empty.

Example usage:

questionOrder = remove(questionOrder, 4);

edited Feb 8, 2018 at 15:51

answered Feb 6, 2018 at 5:04

Kirill Simonov

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6 Comments

Senchu Thomas Over a year ago

can you mention what we need to remove a particular element

Kirill Simonov Over a year ago

@SenchuThomas check my udpdate

Senchu Thomas Over a year ago

Thank you for the help. I find a small difference for example I have an array of [[5]] when I request to remove 5 now the resultant array is [[]]. But actually it wants to be null as it doesnot contain any elements

Kirill Simonov Over a year ago

@SenchuThomas You're welcome. To return null just replace array[i] = newSub; with array[i] = newSub.length > 0 ? newSub : null;

Kirill Simonov Over a year ago

@SenchuThomas If you want an empty inner array to be removed from the outer array, you need to remember its index and make a couple of System.arraycopy after you exit the loop. Also in this case you have to make the function return a new array - because we can't change the given array's length in place.

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