Split 2D array into array of multiple smaller arrays?

Question

I have a 9x9 2D array that I want to split into an array of 9 3x3 2D arrays.

Here is what I have so far:

int board[][] = new int[9][9];

// Fill board with numbers...

int[][] nw, n, ne, w, c, e, sw, s, se = new int[3][3];
int[][] sections = { { nw, n, ne }, { w, c, e }, { sw, s, se } };

Afterwards:

nw[][] would consist of board[0][0] thru board[3][3].
n[][] consists of board[4][0] thru board[6][3]
etc.

What's the best way to do this without manually adding every single element to the correct section?

Do you really need separate arrays for each section, or do you just want a way to point to those areas within the original array? If the latter, you could probably create a class where each instance represents each particular section and as you call it you retrieve the actual value from board[][] but the abstraction of the class would help you easily see it as just a section. Just an idea...Hope that's not too confusing. — Scott Shipp
– Scott Shipp, Commented May 30, 2013 at 17:58
Smells like sudoku...I suggest that you use a for loop since you know the first and last rows and the first and last columns for the "subboards" in board. — Code-Apprentice
– Code-Apprentice, Commented May 30, 2013 at 18:01
You'll need a = new int[3][3] for each of those. Currently you're just initializing se. — Bernhard Barker
– Bernhard Barker, Commented May 30, 2013 at 18:04
Your code doesn't compile: you need to add another dimension to sections — Arend
– Arend, Commented May 30, 2013 at 20:58

Arthur Dent · Accepted Answer · 2013-05-30 18:14:15Z

2

java.util.Arrays.copyOfRange() can get you part of the way.

answered May 30, 2013 at 18:14

Arthur Dent

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2 Comments

tkroman Over a year ago

Or System.arraycopy as rather fast implementation.

Petefic Over a year ago

How do I use copyOfRange() with 2D arrays? Every example I can find of how to use it shows only 1D arrays.

Oghli · Accepted Answer · 2015-07-29 13:10:07Z

sounds like Sudoko!

I solved like this problem last time you can divide 9X9 array into (9) 3X3 arrays

you can do that in this way :

void getSection(int board[9][9],int result[3][3],int x, int y) 
{
    for (int i=0; i<3; i++) {
        for (int j=0; j<3; j++) {
            result[i][j] = board[3*x+i][3*y+j];
        }
    }
}

after that call getSection for every section in 9X9 array :

 int s[9][9];
 int s1[3][3];

     for(i=0;i<3;i++)
        {
            for(j=0;j<3;j++)
            {
              getSection(s,s1,i,j);
            }
        }

or you can do it manually :

section 0 :

  getSection(s,nw,0,0);

section 1 :

 getSection(s,n,0,1);

section 2 :

 getSection(s,ne,0,2);

section 3 :

 getSection(s,w,1,0);

etc.

note that the solution of your question in c++ but the main idea is same in java and c++.

Has QUIT--Anony-Mousse · Accepted Answer · 2013-05-30 20:14:17Z

0

Java does not allow subindexing of arrays.

What you are referring to is trivially possible in C, but in Java you will need to either:

copy the data to new arrays
use a custom class abstracting from storage.

In java, there is no way that foo[0] permanently refers to the element bar[3] of another array.

If you want to work with int[][], you'll have to copy the arrays. Arrays.copyOfRange and System.arraycopy will be the most efficient choices, but at Sudoku size it does obviously not make much of a difference.

For the second approach, write a custom Matrix class. For example

class Matrix {
  int[] flatStorage;
  int[] offsets;

  Matrix(int[] flatStorage, int[] offsets) {
    this.flatStorage = flatStorage;
    this.offsets = offsets;
  }

  void set(int x, int y, int val) {
    flatStorage[ offsets[x] + y ] = val;
  }

  int get(int x, int y) {
    return flatStorage[ offsets[x] + y ];
  }
}

int[] sharedStorage = new int[27];
Arrays.fill(sharedStorage, -1); // Initialize with -1

int[] allOffsets = new int[]{0,9,18, 27,36,45, 54,63,72};
Matrix nineByNine = new Matrix(sharedStorage, allOffsets);
Matrix northEast = new Matrix(sharedStorage, new int[]{6,15,24});
Matrix southEast = new Matrix(sharedStorage, new int[]{60,69,78});

nineByNine.set(1,7, 2); // Write to middle of upper right quadrant
System.err.println(northEast.get(1, 1)); // Read - should be 2!

Add size information and similar things yourself.

edited May 30, 2013 at 20:14

answered May 30, 2013 at 20:06

Has QUIT--Anony-Mousse

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1 Comment

user1181445 Over a year ago

Haha, I remember when I was starting I tried to sublist an array like it would have been done in python. Years later, a few buddies and I are adding this in to a language similar to Java haha. I like the OOP approach to this method. Very clean code

Arend · Accepted Answer · 2013-05-30 21:03:09Z

0

Would you consider the following solution "manual"? Declare

int[][] getSection(int x, int y) {
    int[][] result = new int[3][3];
    for (int i=0; i<3; i++) {
        for (int j=0; j<3; j++) {
            result[i][j] = board[3+x+1][3*y+j];
        }
    }
    return result;
}

then call

nw = getSection(0,0);
n = getSection(1,0);

etc.

answered May 30, 2013 at 21:03

Arend

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