Some hint

Question about the task

Memory defragmentation

Java Syntax, Level 10, Lesson 5

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Hi, I have a problem with this exercise. Could you give me some hint how to solve this?

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"}; System.out.print("Element to be removed at index: "); executeDefragmentation(memory); System.out.println(); System.out.println(Arrays.toString(memory)); } public static void executeDefragmentation(String[] array) { for (int i = 0; i < array.length; i++) { if (array[i] == null) { array[i] = array[i + 1]; System.out.print(i + ", "); } } } }

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michal9696

Level 18 , Poland, Poland

17 August 2022, 01:36

public static void executeDefragmentation(String[] array) { int i = 0; for (String s : array) { if (s != null) { array[i] = s; i++; } } } output: [object15, object2, object32, object4, null, null, null, object32, null, object4] I don't know why indexes with the same value don't move to earlier indexes.

Thomas

Level 41 , Bayreuth, Germany

17 August 2022, 16:28

solution

you came up with a pretty nice idea and put everything into the right place. You just didn't set the orig value to null. So you have valid values till i (excluding). Afterwards everything should be set to null.

while (i < array.length)
    array[i++] = null;

(not tested... you do that for me ;) ) Better would be to replace the for-each loop with a regular for loop and do everything in that single for loop. I'd do it this way - a loop from 0 to array.length, initialize two counters (shift and i), shift determins how many places you need to shift a value to the left, i is for looping through the array. - if the current index == null, increment shift and continue - if shift == 0 -> nothing to do, continue - set array[i - shift] = array[i]; // and array[i] == null; that's all

Guadalupe Gagnon

Level 37 , Tampa, United States

17 August 2022, 17:50

useful

Thomas gave you good advice. To answer your question of why the values are not moving is because the code you shared copies values but does not move them. Lets say you had a small array of:

array = ["a", "b", "c"]

and you coded this:

array[0] = "c";    /* -OR- */   array[0] = array[2];

all that does is copy "c" into the index that is "a". The result would be:

array = ["c", "b", "c"]

Guadalupe Gagnon

Level 37 , Tampa, United States

17 August 2022, 00:23

useful

The code needs to move all the elements to the beginning. This code does not do that. It will only move an element to the left of a null value 1 space right. Since it only goes through the loop one time this will not solve the task. On top of this if the last element is null then [i + 1] will throw an index out of bounds exception.