Efficient way to trim 2d array in Java

Question

I have a following 2 dimensional array:

int[][] array = new int[][]{
        {0, 0, 0, 0, 0, 0, 0, 0, 0, 0},
        {0, 0, 0, 1, 1, 1, 0, 0, 0, 0},
        {0, 0, 0, 1, 1, 1, 1, 0, 0, 0},
        {0, 0, 1, 1, 0, 1, 1, 0, 0, 0},
        {0, 0, 1, 1, 0, 1, 0, 0, 0, 0},
        {0, 0, 0, 1, 1, 1, 1, 0, 0, 0},
        {0, 0, 0, 1, 1, 1, 1, 0, 0, 0},
        {0, 0, 0, 0, 0, 1, 0, 0, 0, 0}
};

and I would like to trim all the surrounding zeroes, so my output would be like this (removing "zeros" outside and preserving the zeroes that are surrounded by "ones"):

        {0, 1, 1, 1, 0},
        {0, 1, 1, 1, 1},
        {1, 1, 0, 1, 1},
        {1, 1, 0, 1, 0},
        {0, 1, 1, 1, 1},
        {0, 1, 1, 1, 1},
        {0, 0, 0, 1, 0}

I'm looking for an efficient way of doing this.

Rather than looking at surrounding zeroes or ones, it seems that your algo consists in removing all the lines and columns where the sum of all items is 0. — assylias, Jun 19 '18 at 12:18
1. [Find the coordinates of the inner 2D array](https://stackoverflow.com/questions/30697790/2d-array-trim-with-specified-value), 2. [Copy it in a new variable](https://stackoverflow.com/questions/16362872/how-to-get-2d-subarray-from-2d-array-in-java) — Oneiros, Jun 19 '18 at 12:30
@OleV.V. the first and last two columns are supposed to be removed so... maybe it's a `first and last N` trick, or maybe it's the sum :) — Shark, Jun 19 '18 at 12:49
I have voted to close this question because the requirements are unclear, and the question neither shows any search and research effort nor any attempt to solve the problem. — Ole V.V., Jun 19 '18 at 13:41

score 3 · Accepted Answer · answered Jun 19 '18 at 12:49

Possible solution (dunno if it is the most efficient way):

public static int[][] trim(int[][] mtx, int rmin, int rmax, int cmin, int cmax) {
   int[][] result = new int[rmax-rmin+1][];
   for (int r = rmin, i = 0; r <= rmax; r++, i++) {
      result[i] = Arrays.copyOfRange(mtx[r], cmin, cmax+1);
   }
   return result;
}

public static int[][] trim(int[][] mtx, int trimmed) {
   int cmin = mtx[0].length;
   int rmin = mtx.length;
   int cmax = -1;
   int rmax = -1;

   for (int r = 0; r < mtx.length; r++)
      for (int c = 0; c < mtx[0].length; c++)
         if (mtx[r][c] != trimmed) {
            if (cmin > c) cmin = c;
            if (cmax < c) cmax = c;
            if (rmin > r) rmin = r;
            if (rmax < r) rmax = r;
         }

   return trim(mtx, rmin, rmax, cmin, cmax);
}

public static void main (String[] args) {
   int[][] array = new int[][]{
      {0, 0, 0, 0, 0, 0, 0, 0, 0, 0},
      {0, 0, 0, 1, 1, 1, 0, 0, 0, 0},
      {0, 0, 0, 1, 1, 1, 1, 0, 0, 0},
      {0, 0, 1, 1, 0, 1, 1, 0, 0, 0},
      {0, 0, 1, 1, 0, 1, 0, 0, 0, 0},
      {0, 0, 0, 1, 1, 1, 1, 0, 0, 0},
      {0, 0, 0, 1, 1, 1, 1, 0, 0, 0},
      {0, 0, 0, 0, 0, 1, 0, 0, 0, 0}
   };
   int[][] trim = trim(array, 0);
   System.out.println(Arrays.deepToString(trim));
}

Efficient way to trim 2d array in Java

1 Answers1