分割二维数组（JAVA）

Question

given an array Array[n][n] (whose size n is a power of two), I want to recursively deal with quadrants of the array. How can I take a segment of the array that encompasses the [0 to (size/2)][0 to (size/2)] elements? I remember in python you could just use indexing splices like [0:size/2], does JAVA have a similar way of handling these elements?

Answer 1

I'm afraid you can't do exactly that.

If all you needed were a unidimensional array you could use ¹ :

Arrays.copyOfRange(T[] original, int from, int to)

Which is verbose but essentially the same as slicing in Python.

But since you need a bidimensional array (which is implemented in Java as an array of arrays ) you need something like this:

class QArray {
  private Integer[][] array;
  private int quadrantSize;

  public QArray(Integer[][] array) {
    this.array = array;
    this.quadrantSize = array.length / 2;
  }

  public QArray getQuadrant(int quadrant) {
    QArray quadrantArray;

    switch(quadrant) {
      case 1:
        quadrantArray = getQuadrant(0, quadrantSize);
        break;
      case 2:
        quadrantArray = getQuadrant(0, 0);
        break;
      case 3:
        quadrantArray = getQuadrant(quadrantSize, 0);
        break;
      case 4:
        quadrantArray = getQuadrant(quadrantSize, quadrantSize);
        break;
      default:
        throw new IllegalArgumentException("Invalid quadrant: " + quadrant);
    }
    return quadrantArray;
  }

  private QArray getQuadrant(int startRow, int startCol) {
    Integer[][] quadrantArray = new Integer[quadrantSize][];
    for (int r = 0; r < quadrantSize; r++) {
      quadrantArray[r] = Arrays.copyOfRange(array[startRow + r], startCol, startCol + quadrantSize);
    }

    return new QArray(quadrantArray);
  }    
}

Where:

System.out.println(qArray);
System.out.println(qArray.getQuadrant(2));
System.out.println(qArray.getQuadrant(1));
System.out.println(qArray.getQuadrant(3));
System.out.println(qArray.getQuadrant(4));

For some qArray would give you:

[ 1,  2,  3,  4]
[ 5,  6,  7,  8]
[ 9, 10, 11, 12]
[13, 14, 15, 16]

[ 1,  2]
[ 5,  6]

[ 3,  4]
[ 7,  8]

[ 9, 10]
[13, 14]

[11, 12]
[15, 16]