在方法中没有行和列索引的情况下递归搜索2D数组？

Question

I'm currently working on a homework assignment and I'm really stuck on this idea of recursively searching an array without a row and column to index. I believe I can use helper methods but I'm new to recursion and find it a little confusing. This is the method that I'm not allowed to change (for the purpose of the assignment)

public Couple search(int[][]array, int element){

}

I'm also provided an inner class from the instructor. We haven't learned anything about inner classes, however, the appear to be nothing special. It is basic and I doesn't do anything special so I won't include the code unless there is something that is needed that I don't know. (I don't want to be cheating and I want to more-or-less figure it out too).

private class Couple{
  // declaration of (int) row and (int) col

  public Couple(row, col){
  // this.row = row
  // col = col
  }

  public String toString(){
  // returns string
  }

}

EDIT: I also can't use any loops

Answer 1

Edit: removed for-loop, added recursive Example

example for recursive subroutine

public class FindElement2DimArrayExample {
    private class Couple {
        private int row;
        private int col;

        public Couple(int row, int col) {
            this.row = row;
            this.col = col;
        }

        public String toString() {
            return "[" + row + ", " + col + "]";
        }
    }

    public static void main(String[] args) {
        int[][] array = new int[][] { { 1, 2, 3 }, { 4, 5, 6 }, { 7, 8, 9 } };
        System.out.println(new FindElement2DimArrayExample().search(array, 5));
    }

    public Couple search(int[][] array, int element) {
        return searchRecursively(array, element, 0, 0);
    }

    public Couple searchRecursively(int[][] array, int element, int actualRow, int actualCol) {
        if (array.length <= actualRow) {
            return null;
        } else if (array[actualRow].length <= actualCol) {
            return searchRecursively(array, element, actualRow + 1, 0);
        } else if (array[actualRow][actualCol] == element) {
            return new Couple(actualRow, actualCol);
        } else {
            return searchRecursively(array, element, actualRow, actualCol + 1);
        }
    }
}

Answer 2

Recursion is not the best way to search a 2D Array, but if it is your assignment, you may try to realize it via the "Divide and Conquer" approach: Split the array into two parts and recursively call the method again on these two parts until you found the element.

Maybe this is useful for you: how to search elements in a 2d array using recursion

Answer 3

You didn't specify that you cant write some other method... So i'd write another search method with signature:

private Couple search(Couple couple, int[][] array, int element)

Containing the following:

            static private Couple search(Couple couple, int[][] array, int element) {
                try {
                    System.out.println("Checking:" + couple + (array[couple.col][couple.row]));
                } catch (ArrayIndexOutOfBoundsException aioobe) {}
                if (couple.col>=array.length) return search(new Couple(0,couple.row+1),array,element);
                if (couple.row>=array[0].length) return new Couple(-1,-1);
                if (array[couple.row][couple.col] == element) return couple;
                else return search(new Couple(couple.col+1,couple.row),array,element);
            }

and call it from your other method by:

static public Couple search(int[][] array, int element) {
    return search(new Couple(0,0),array,element);
}

That should do the trick. Other then that (if you cant write additional method) then I'd use a stack.

full code:

public class NewClass1 {

static class Couple {
    int col, row;

public Couple(int col, int row) {
    this.col = col;
    this.row = row;
}

@Override
public String toString() {
    return "Couple{" + "col=" + col + ", row=" + row + '}';
}


}    
static int[][] getArr(int nx, int ny) {
    Random rand = new Random();
    int[][] arr = new int[nx][ny];
    for (int i = 0; i < nx; i++) {
        for (int j = 0; j < ny; j++) {
            arr[i][j] = rand.nextInt(90)+10;
        }
    }
    return arr;
}

static void print(int [][] arr) {
    for (int i = 0; i < arr.length; i++) {
        for (int j = 0; j < arr[i].length; j++) {
            System.out.print(arr[i][j] + ";");
        }
        System.out.println("");
    }
}

static public Couple search(int[][] array, int element) {
    return search(new Couple(0,0),array,element);
}

static private Couple search(Couple couple, int[][] array, int element) {
    try {
        System.out.println("Checking:" + couple + (array[couple.col][couple.row]));
    } catch (ArrayIndexOutOfBoundsException aioobe) {}
    if (couple.col>=array.length) return search(new Couple(0,couple.row+1),array,element);
    if (couple.row>=array[0].length) return new Couple(-1,-1);
    if (array[couple.row][couple.col] == element) return couple;
    else return search(new Couple(couple.col+1,couple.row),array,element);
}
public static void main(String[] args) {
    int[][] arr = getArr(10,10);
    print(arr);
    System.out.println(search(arr,11));

}
}