CS 1713/1711 Introduction to Computer Science Lecture 38

CS 1713 Lecture 38: Two-Dimensional Arrays
November 29, 1999

Two dimensional arrays:
- Are arrays of arrays.
- Used to represent tables and for matrices.
- Syntax uses [row][column].
- Rows can have different number of items.
- Number of rows in x is x.length.
- Number of columns in the i-th row is x[i].length
Example: Declare a variable called ticTac that holds a two-dimensional array of char. In a separate statement create a 3x3 array for ticTac.
```
     char [][] ticTac;
     ticTac = new char[3][3];
```
Example: Set the entry of ticTac in the second row, third column to an 'x'.
```
     ticTac[1][2] = 'x';
```

Example: Create a TicTacToe class.

    public class TicTacToe {
       private char [][] ticTac;
       private int myBoardSize;

       public TicTacToe (int board) {
          myBoardSize = board;
          ticTac = new char[board][board];
       }

       //accessors
       public int getBoardSize() {
          return myBoardSize;
       }

       public void printBoard() {
          System.out.println("/n The current board:");
          for (int i = 0; i < ticTac.length; i++) {
              for (int j = 0, j < ticTac[i].length; j++) 
                  System.out.print(ticTac[i][j] + " ");
              System.out.println()
          }
       }
    }

Example: Write a method to see if row of ticTac has all entries that match thisChar.

     public boolean rowMatches(int row, char thisChar) { 
        for (int j = 0; j < ticTac[row].length)
            if (ticTac[row][j] != thisChar)
               return false;
        return true;
     }

Example: Create a two dimensional array with the following values in it:

       6.0 4.5 3.0 1.0
       7.8 5.3 3.2 1.8
       6.2 3.7 7.8 9.9

The following code does explicit initialization of the array:

     double [][] matrix = {{6.0, 4.5, 3.0, 1.0}, {7.8, 5.3, 3.2, 1.8}, 
                           {6.2, 3.7, 7.8, 9.9}};

Example: Add up all of the entries in matrix

     int sum = 0;
     for (int i = 0 i < matrix.length; i++)
         for (int j = 0; j < matrix[i].length; j++)
              sum += matrix[i][j];

Example: Create a two dimensional array whose i-th row contains the integers from 0 to i. Assume the number of rows is n.

     int [][] triangle = new int [n][];
     for (int i = 0; i < n; i++) {
         triangle[i] = new int [i+1];
         for (int j = 0; j < triangle[i].length; j++)
             triangle[i][j] = j;
     }

Example: The Game of Life is played on a rows by columns board consisting of cells. The cell class is given in the description for Laboratory 12.
- Basic idea of game of life.
- How do you create a class Life representing the board?
- How do you access individual elements?
- Write a printNeighbors accessor method.

Objective: Learn to create and use two-dimensional arrays.

Last revision: November 29, 1999 at 3:30 pm