10/15/09 - Two-dimensional Arrays (Tables)

In Java, we can create two-dimensional arrays (which have both rows and columns) by declaring:

int [][] myArray = new int [2][3];

This creates an array in memory with two rows (the first dimension) and three columns (the second dimension).

The following code will assign the values shown in the diagram below:

     myArray[1][1] = 5;
     myArray[0][2] = -7;
     myArray[0][0] = 3;
     myArray[1][2] = 4;
     myArray[1][0] = -6;
     myArray[0][1] = 8;

The number of rows in the array is given by:

     myArray.length

The number of columns in a particular row is given by specifying the row index in brackets and then finding the length:

     myArray[0].length     // gives the number of columns in row 0
     myArray[1].length     // gives the number of columns in row 1

The following code will print out all the values stored in the array, row by row:

     for (int row = 0; row < myArray.length; ++row) {
          for (int col = 0; col < myArray[row].length; ++col) {
               System.out.print(myArray[row][col] + "    ");
          }
          System.out.println();
     }

The output of the code above would be:

     3    8    -7
     -6    5    4

The following code will print out all the values stored in the array, column by column:

     int row = 0;
     for (int col = 0; col < myArray[row].length; ++col) {
          for (row = 0; row < myArray.length; ++row) {
               System.out.print(myArray[row][col] + "    ");
          }
          System.out.println();
     }

The output of the code above would be:

     3    -6
     8    5
     -7    4

The following code will find and print the sum of the values in row 1 of the array:

     int sum = 0;
     int row = 1;
     for (int col = 0; col < myArray[row].length; ++col) {
          sum += myArray[row][col];
     }
     System.out.println("The sum of the values in row " + row + " is " + sum);

The output of the code above would be:

     The sum of the values in row 1 is 4

The following code will find and print the sum of the values in column 2 of the array:

     int sum = 0;
     int col = 2;
     for (int row = 0; row < myArray.length; ++row) {
          sum += myArray[row][col];
     }
     System.out.println("The sum of the values in column " + col + " is " + sum);

The output of the code above would be:

     The sum of the values in column 2 is -3

			Read pp. 409-430 of the text.

Read the following document (double click the document to see both pages) about Magic Squares, and write a program that creates and displays a 7 by 7 Magic Square using the method described. What's special about a magic square is that the sum of the numbers in any row, column or diagonal is the same. For example, the 3 by 3 Magic Square shown below has row sums, column sums and diagonal sums of 15:

The 5 by 5 Magic Square shown below has row sums, column sums and diagonal sums of 65:

And so forth.