Java Hackerank left array Rotation

Question

We have a method in which an array and number of rotations are passed as input and we have to return the array after left rotations. Here is my solution.

static int[] rotLeft(int[] a, int d) {
        int i = 0;
        int logicBreak = d;
        int[] copy = new int[a.length];  // SEE HERE GUYS, WHY THIS WORKS
        int[] copy = a;  // AND WHY NOT THIS,

        while(logicBreak < a.length){
            copy[i] = a[logicBreak];
            i++;
            logicBreak++;
        }

        logicBreak = 0;
    
        while(logicBreak < d){
            copy[i] = a[logicBreak];
            i++;
            logicBreak++;
        }

        return copy;
    }

I hope that there is no confusion until now and if you have then, open this page. So my problem is really basic. Why the answer works when I do this int[] copy = new int[a.length]; but does not work when I do this int[] copy = a;. Can you tell me the difference, because we are changing all the values of copy array, so what is the matter we make it new int[] or same as the array a. I have created an android app but I am still not getting this array concept, if there is a difference then please tell me.

For your ease here is the full code.

import java.io.*;
import java.math.*;
import java.security.*;
import java.text.*;
import java.util.*;
import java.util.concurrent.*;
import java.util.regex.*;

public class Solution {

    // Complete the rotLeft function below.
    static int[] rotLeft(int[] a, int d) {
        int i = 0;
        int logicBreak = d;
        int[] copy = new int[a.length];

        while(logicBreak < a.length){
            copy[i] = a[logicBreak];
            i++;
            logicBreak++;
        }

        logicBreak = 0;
    
        while(logicBreak < d){
            copy[i] = a[logicBreak];
            i++;
            logicBreak++;
        }

        return copy;
    }

    private static final Scanner scanner = new Scanner(System.in);

    public static void main(String[] args) throws IOException {
        BufferedWriter bufferedWriter = new BufferedWriter(new FileWriter(System.getenv("OUTPUT_PATH")));

        String[] nd = scanner.nextLine().split(" ");

        int n = Integer.parseInt(nd[0]);

        int d = Integer.parseInt(nd[1]);

        int[] a = new int[n];

        String[] aItems = scanner.nextLine().split(" ");
        scanner.skip("(\r\n|[\n\r\u2028\u2029\u0085])?");

        for (int i = 0; i < n; i++) {
            int aItem = Integer.parseInt(aItems[i]);
            a[i] = aItem;
        }

        int[] result = rotLeft(a, d);

        for (int i = 0; i < result.length; i++) {
            bufferedWriter.write(String.valueOf(result[i]));

            if (i != result.length - 1) {
                bufferedWriter.write(" ");
            }
        }

        bufferedWriter.newLine();

        bufferedWriter.close();

        scanner.close();
    }
}

WJS · Accepted Answer · 2020-09-09 14:51:53Z

2

When you do the following:

int[] copy = a;

You are just assigning the reference of a to copy. They "refer" to the same location in memory.

Here is a demo

int[] a = {1,2,3};
int[] copy  = a;
a[1] = 100;
System.out.println(Arrays.toString(copy));

Prints

[1,100,3]

answered Sep 9, 2020 at 14:51

WJS

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1 Comment

Vijay Over a year ago

oh, I got it now, I solved my doubts in IntelliJ IDE now. Actually it is not copying the array. It is just referencing. If we want to copy then we have to use a loop or a java library. So that's why my answer was wrong in hackerrank because I was copying array an into a and nothing was changing, funny concept I got today. By the way, check the app I developed - PlayStore Link {play.google.com/store/apps/…}

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