Question

Iterating over a 2D array

If we want to visit or process every element of a 2D array, we need a nested for loop to do it. Generally, when you iterate over a 2D array, you want to do it in row major order, because this is more memory efficient. The reason why is that adjacent elements in a row are stored contiguously (one after another) in memory. Another reason, for an English speaker, is that this is the same order your read a book—top to bottom, left to right.

The following code iterates over arr2D, changing each value to 100:

for (int r=0; r < arr2D.length; r++)

{

for (int c=0; c < arr2D[r].length; c++) {

arr2D[r][c] = 100;

}

}

I'm using the variables "r" and "c" because they iterate of the rows and columns of the array, respectively. The outer loop iterates over all the rows, and the inner loop iterates over all the columns in a single row. Notice how the row index (r) comes before the column index (c). Also, notice how the loop bounds are handled in the interior loop. While the outer loop goes from 0 to arr2D.length, the inner loop goes from 0 to arr2D[r].length, which is then length of row r.

Q3) What code would you use to add 1 to all the elements of arr2D?

Answer 1

Answer:

   for (int r=0; r < arr2D.length; r++)
       {
           for (int c=0; c < arr2D[r].length; c++)
           {
               arr2D[r][c] = arr2D[r][c] + 1;
           }

       }

Explanation:

Add 1 to each of the elements of arr2D and store back to arr2D. When all the elements are initialized by 100 then after add 1 to  each of the elements of arr2D, the elements of the array becomes 101.

Full Program:

class Arr2D
{
   public static void main (String[] args)
   {
       int arr2D[][] = new int[3][3];
      
       for (int r=0; r < arr2D.length; r++)
       {
           for (int c=0; c < arr2D[r].length; c++)
           {
               arr2D[r][c] = 100;
           }

       }
      
       for (int r=0; r < arr2D.length; r++)
       {
           for (int c=0; c < arr2D[r].length; c++)
           {
               arr2D[r][c] = arr2D[r][c] + 1;
           }

       }
      
       for (int r=0; r < arr2D.length; r++)
       {
           for (int c=0; c < arr2D[r].length; c++)
           {
               System.out.print(arr2D[r][c] + " ");
           }
          
               System.out.println();

       }
   }
      
}

Output:

101 101 101
101 101 101
101 101 101