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Two-dimensional arrays in Java

Video on 2D Arrays

To loop through the values in a 2D array you must have two indexes. One index is used to change the row index and one is used to change the column index. You can use nested loops, which is one for loop inside of another, to loop through all the values in a 2D array.

Here is a method in the IntArrayWorker class that totals all the values in a 2D array of integers in a private instance variable (field in the class) named matrix. Notice the nested for loop and how it uses matrix.length to get the number of rows and matrix[0].length to get the number of columns. Since matrix[0] returns the inner array in a 2D array, you can use matrix[0].length to get the number of columns.

public int getTotal()
  {
    int total = 0;
    for (int row = 0; row < matrix.length; row++)
    {
      for (int col = 0; col < matrix[0].length; col++)
      {
        total = total + matrix[row][col];
      }
    }
    return total;

}

Because Java two-dimensional arrays are actually arrays of arrays, you can also get the total using nested "for-each" loops as shown in getTotalNested below. The outer loop will loop through the outer array (each of the rows) and the inner loop will loop through the inner array (columns in that row). You can use a nested for-each loop whenever you want to loop through all items in a 2D array and you don't need to know the row index or column index.

public int getTotalNested()
  {
    int total = 0;
    for (int[] rowArray : matrix)
    {
      for (int item : rowArray)
      {
        total = total + item;
      }
    }
    return total;
 }

Exercises

1. Write a getCount(int i) method in the IntArrayWorker class that returns the count of the number of times the integer value i is found in the matrix. There is already a method to test this in IntArrayWorkerTester. Just uncomment the method testGetCount() (around line 25) and the call to it in the main method of IntArrayWorkerTester.
2. Write a getLargest method in the IntArrayWorker class that returns the largest value in the matrix. There is already a method to test this in IntArrayWorkerTester. Just uncomment the method testGetLargest() (around line 55) and the call to it in the main method of IntArrayWorkerTester.
3. Write a getColTotal(int col) method in the IntArrayWorker class that returns the total of all integers in a specified column. There is already a method to test this in IntArrayWorkerTester (around line 80). Just uncomment the method testGetColTotal() and the call to it in the main method of IntArrayWorkerTester.