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Question about the task

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I checked on debugging mode that value array have only 4 object on end method but when program went back on main method, after executeDefragmentation(), still printing full array with null.

Let's explore how an array of strings is like computer memory. If we remove some elements, then we get "holes" in the array, that is, some elements are equal to null. The executeDefragmentation(String []) method, which takes an array of strings and performs "defragmentation", that is, eliminates the "holes" (cells storing null) by moving all the string objects to the beginning of the array while maintaining the same order. In this task, you need to implement the executeDefragmentation(String[]) method. You can use the main() method to check the result of the executeDefragmentation(String[]) method.

Requirements:

The Memory class must have a public static void executeDefragmentation(String[]) method.
Implement the executeDefragmentation() method in accordance with the task conditions.

package en.codegym.task.pro.task10.task1012; import java.util.Arrays; import java.util.Objects; /* Memory defragmentation */ public class Memory { public static void main(String[] args) { String[] memory = {"object15", null, null, "object2", null, null, null, "object32", null, "object4"}; executeDefragmentation(memory); System.out.println(Arrays.toString(memory)); } public static void executeDefragmentation(String[] array) { int newStringLength = 0; for (int i = 0; i < array.length; i++) { if (array[i] != null) newStringLength++; } String[] newMemory = new String[newStringLength]; int j = 0; for (int i = 0; i < array.length; i++) { if (array[i] != null) { newMemory[j] = array[i]; j++; } } array = newMemory; } }

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Thomas

Level 41 , Bayreuth, Germany

10 November 2022, 12:20

useful

You can not assign a new reference to the main methods local memory variable inside another method. memory is passed there by value, the array variable is just a copy of the memory variable. If you change arrays value, then memory does not (like primitives). If you modify the object the reference is pointing to, then this is of course working. I understand the task that way that you should reorder the values inside the array array so that the first elements are the strings followed by the nulls (defragment). lines 26 and 27 you keep. Then you check in a while loop if i > 0 (so you can step back in the array and the next part does not throw an out of bounds exception) and if array[i - 1] is null. If that's the case you can move the current element one place to the left (arr[i - 1] = arr[i]; arr[i] = null;) and you decrement i, the while loop checks again if the current element can be moved one step to the left. No clue if that validates though, I do not know that task ;)

matemate123

Level 50 , Kraków, Poland

11 November 2022, 10:45

Ok i did it. You was right, but I use two loops for and this is better and clearer way in my opinion. The key is not overthinking task.

Thomas

Level 41 , Bayreuth, Germany

11 November 2022, 11:39

useful

There are many ways to solve coding problems and there's no correct or wrong way (if the code works). It's a matter of your style and preferences. I'd prefer it the way I described ;) but yours may have a better performance (if you copy the objects into a new array and then the new arrays content back to the original one)... depending on how many nulls need to be removed and in what order they are

public static void executeDefragmentation(String[] array) {
    for (int i = 0; i < array.length; i++) {
        if (array[i] != null) {
            while (i > 0 && array[i - 1] == null) {
                array[i - 1] = array[i];
                array[i--] = null;
            }
        }
    }
}

Enjoy coding

matemate123

Level 50 , Kraków, Poland

11 November 2022, 14:01

I did it that way: (before that I had few hours thinking how to do it in while loops)

public static void executeDefragmentation(String[] array) {
    for (int i = 0; i < array.length; i++) {
        if (array[i] == null){
            for (int j = i + 1; j < array.length; j++) {
                if (array[j] != null){
                    array[i] = array[j];
                    array[j] = null;
                    break;
                }
            }
        }

    }
    memory = array;
}

matemate123

Level 50 , Kraków, Poland

11 November 2022, 14:07

Tell me one think. In your code:

array[i - 1] = array[i];
                array[i--] = null;

It's worked? It's not like you set values from array[i] to array[i - 1] and next line you set value null to array[i--] (it's not the same like array[i - 1] ? Or maybe my brain is so hot now that I don't understand something :D

Thomas

Level 41 , Bayreuth, Germany

11 November 2022, 16:29

Admitted, that's not the best coding style as it may be confusing and/ or lead to unexpected behavior. The post-decrement operator first uses the variables value and then decrements it. Means in code that is comparable to

array[i - 1] = array[i];
array[i] = null;
i--; // or i-= 1; // or i = i - 1;

matemate123

Level 50 , Kraków, Poland

11 November 2022, 17:09

aaa yeah that's right. Prefix and postfix rule.