Array not holding the previous value in recursion(JAVA)

Question

I want to hold the previous value after returning from a recursion. It worked for COUNT, but Array is not holding the previous value that I want.

The result is:

before recursion[0, 0, 0, 0]count: 0

before recursion[0, 1, 0, 0]count: 1

before recursion[0, 1, 2, 0]count: 2

before recursion[0, 1, 2, 3]count: 3

After recursion[0, 1, 2, 3]count: 3

After recursion[0, 1, 2, 3]count: 2

After recursion[0, 1, 2, 3]count: 1

After recursion[0, 1, 2, 3]count: 0

But the result i want is:

before recursion[0, 0, 0, 0]count: 0

before recursion[0, 1, 0, 0]count: 1

before recursion[0, 1, 2, 0]count: 2

before recursion[0, 1, 2, 3]count: 3

After recursion[0, 1, 2, 3]count: 3

After recursion[0, 1, 2, 0]count: 2

After recursion[0, 1, 0, 0]count: 1

After recursion[0, 0, 0, 0]count: 0

 import java.util.Arrays;

 import  java.util.Scanner;

 public class main {

     public static void boarder(int board[],int count)
     {

         if(count==4)
         {
             return;
         }
         board[count]=count;
         int temp=count+1;

         System.out.println("before recursion"+Arrays.toString(board)+"count: "+(count));
         boarder(board,temp);
         System.out.println("After recursion"+Arrays.toString(board)+"count: "+(count));

     }

     public static void main(String[] args)
     {
         int count=0;
         int board[]={0,0,0,0};
         //state tic=new state(board);
         boarder(board,0);
     }

 }

This is a typical referential problem. Remember that the board array is a single object being passed around by reference and manipulated in your regression call stack. If you want to print the state of the array when returning from your regression calls, you'll have to save a copy of the array in a local variable to print it out. — rafaelbattesti
– rafaelbattesti, Commented May 10, 2018 at 4:02
By copying the array, I mean creating a new object (cloning), therefore saving the state of the array before the regression call. Something like this stackoverflow.com/q/14149733/2727717 — rafaelbattesti
– rafaelbattesti, Commented May 10, 2018 at 4:17

user4910279 · Accepted Answer · 2018-05-10 04:39:15Z

1

You should recover previous value like this.

public static void boarder(int board[], int count) {
    if (count == 4) {
        return;
    }
    int previous = board[count];
    board[count] = count;
    int temp = count + 1;
    System.out.println("before recursion" + Arrays.toString(board) + "count: " + (count));
    boarder(board, temp);
    System.out.println("After recursion" + Arrays.toString(board) + "count: " + (count));
    board[count] = previous;
}

result

before recursion[0, 0, 0, 0]count: 0
before recursion[0, 1, 0, 0]count: 1
before recursion[0, 1, 2, 0]count: 2
before recursion[0, 1, 2, 3]count: 3
After recursion[0, 1, 2, 3]count: 3
After recursion[0, 1, 2, 0]count: 2
After recursion[0, 1, 0, 0]count: 1
After recursion[0, 0, 0, 0]count: 0

answered May 10, 2018 at 4:39

user4910279

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Comments

Mạnh Quyết Nguyễn · Accepted Answer · 2018-05-10 03:26:06Z

0

Your code work as expected.

The final board[] is [0, 1, 2, 3]

You see the after strings are same because it's executed after the recursion complete, when your board[] = [0, 1, 2, 3] so they all print the same string

answered May 10, 2018 at 3:26

Mạnh Quyết Nguyễn

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Comments

Imal · Accepted Answer · 2018-05-10 04:00:37Z

0

board is an array which is an object but count is a primitive variable. For primitives values are passed by value while for objects values are passed by reference. So the same object(same memory location) is passed through the methods for 'board'. So when you do a change to 'board', that change is reflected in all the places 'board' is referred. Primitive variables ('count' in this case) do not behave like this. There's a new memory location created each time they are passed through a method. When you do a change to the passed variable it does not change the original variable.

answered May 10, 2018 at 4:00

Imal

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Anh N. Nguyen · Accepted Answer · 2018-05-10 04:24:10Z

0

You need to read this

Change your boarder funtion to

board[count]=count;
int temp=count+1;
int[] tempArr = Arrays.copyOf(board, board.length);
System.out.println("before recursion"+Arrays.toString(board)+"count: "+(count));
boarder(tempArr,temp);
System.out.println("After recursion"+Arrays.toString(board)+"count: "+(count));

answered May 10, 2018 at 4:24

Anh N. Nguyen

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1 Comment

rafaelbattesti Over a year ago

If he does that, he'll be manipulating different copies of the array, and not the original board array itself. I would suggest printing the copy tempArr but not passing it into the regression call stack.

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