Department of Computer Science
Faculty of Physical Sciences
Ahmadu Bello University, Zaria

COSC 211 : Object Oriented Programming I - LAB08

Objectives:

To gain experience with:

• Declaring, creating and accessing both regular and rugged 2-D arrays
• Using 2-D arrays in methods (as parameters and as return type)
• Cloning 2-D arrays

1.  Brief Review of 2-D Arrays

Why 2D-arrays?

    A 2D-array is used to store information that would otherwise require parallel 1D-arrays to store.

What is a Java 2D-array?

   A Java 2D-array is an array of arrays of possibly different sizes.

Declaring 2D-arrays:

A 2D-array in which each row has the same number of elements is declared as:

                type[ ][ ]  arrayName  =  new  type[numberOfRows][numberOfColumns];

Example:

                double[ ][ ]  quiz  =  new  double[6][7];

The above declaration is equivalent to either of the following:

Notes:

It is obvious from the above declarations that each row of a 2-D array, quiz[i] is an independent 1-D array.  Thus, we could use it wherever a 1-D array is expected.  For example, if there is a method average with the following header:

public static double average(double[] a) {

. . .

}

Then to get the average of the i^th row, we could call it as:

double avg = average(quiz[i]);

Declaring rugged 2D-arrays:

Because each row in a 2-D array is an independent 1-D array, it follows that the rows of a 2-D array do not need to be all of the same size.  For example, if in the above declaration, we change the declaration of rows 0 and 4 as follows,

quiz[0]  =  new  double[3];

quiz[4]  =  new  double[5];

Then we have the following rugged array.

Accessing individual elements:

Individual element of a 2-D array is accessed by specify the index of row and column. For example

quiz[2][5] = 95.0;

Declaring, Creating and Initializing a 2D-array in one step:

If we have the values, we could directly declare, create and initialize a 2-D array in one step.  For example:

int[][] a = { {1, 0, 12, -1},

        {7, -3, 2, 5 },

        {-5, -2, 2, 9} };

Again, the rows do not have to be all of the same size.  For example:

int[][] a = { {1, 0, 12, -1},

        {7, 5 },

        {-5, -2, 2, 9} };

Example 1: The following reads a rugged array one row at a time and then prints it.

import java.util.Scanner;

import java.util.StringTokenizer;

public class Rugged2DArray {

   public static void main(String[] args) {

      Scanner stdin = new Scanner(System.in);

      int[][] a = new int[3][];

      a[0] = new int[5];

      a[1] = new int[3];

      a[2] = new int[4];

      int row, column;

      for(row = 0; row < a.length; row++)     {

        System.out.println("Enter "+a[row].length + " elements for row#" + (row + 1));

        String inputLine = stdin.nextLine();

        StringTokenizer tokenizer = new StringTokenizer(inputLine);

        for(column = 0; column < a[row].length; column++)

         a[row][column] = Integer.parseInt(tokenizer.nextToken());

       }

       System.out.println("\nThe array elements are:\n");

       for(row = 0; row < a.length; row++) {

             for(column = 0; column < a[row].length; column++)

                System.out.print(a[row][column] + "    ");

             System.out.println();

       }

       System.out.println("\n\n");

   }

}

2.  Using 2-D arrays in methods

    Like 1-D array, 2D-array can be passed as parameter to a method.  Also, a method can return a 2-D array.

Example 2: The demonstrates array cloning.

import java.util.Scanner;

import java.util.StringTokenizer;

public class Matrix {

  static Scanner stdin = new Scanner(System.in);  

  public static double[][] sum(double[][] a, double[][] b) {

     int row = a.length;

     int col = a[0].length;

     double[][] c = new double[row][col];

     for (int i=0; i<row; i++) 

      for (int j=0; j<col; j++) 

        c[i][j] = a[i][j] + b[i][j];

        return c;

  }

  public static double[][] createArray(int row, int col) {

     double[][] array = new double[row][col];

     System.out.println("Enter a "+row+" by "+col+" matrix row-wise");

     for (int i=0; i<row; i++) {

        String input = stdin.nextLine();

        StringTokenizer st = new StringTokenizer(input);

         for (int j=0; j<col; j++) 

         array[i][j] = Double.parseDouble(st.nextToken());

     }  

        return array;

  }

  public static void print(double[][] a) {

     int row = a.length;

     int col = a[0].length;

     for (int i=0; i<row; i++) {

      for (int j=0; j<col; j++) 

        System.out.print(a[i][j]+ "\t");

      System.out.println();

     }  

  }

  public static void main(String[] args) throws IOException {

   int row, column;

   System.out.print("Enter number of rows: ");

   row = Integer.parseInt(stdin.readLine());

   System.out.print("Enter number of columns: ");

   column = Integer.parseInt(stdin.readLine());

   double[][] a = createArray(row, column);

   double[][] b = createArray(row, column);

   double[][] c = sum(a,b);

   System.out.println("The sum of the two arrays is");

   print(c);

  }

}

3.  Cloning 2-D arrays

    Like 1-D array, we can make a copy of an array using the clone method. For example, the 2D-array quiz can be clone using:

double[][] quizClone = (double[][]) quiz.clone;

However, the above statement only copies the references to the rows of quiz. The values of the actual elements are not copied.  That is, any changes made to quiz will affect quizClone.  This is called shallow cloning.  The following figure shows the effect of shallow cloning:

To actually make a complete copy of quiz – called deep clonning, we need to also clone each row of quiz and assigned to the corresponding row of quizClone.  Using the staments of the form:    quizClone[i] = (double[]) quiz[i].clone();

Example 3:  The following shows shallow cloning.

public class ShallowCloning {

 public static void main(String args[]) {

    double[][] a = {{1, 2, 3},

                    {4, 5, 6},

                    {7, 8, 9}};

    System.out.println("Printing array a");

    Matrix.print(a);

    double[][] b = (double[][])a.clone();

    System.out.println("Printing array b  - clone of a");

    Matrix.print(b);

    b[1][1] = 20;

    System.out.println("Printing array b after making chages to it");

    Matrix.print(b);

    System.out.println("Printing array a after changing its clone");

    Matrix.print(a);

  }

}

Example 4:  The following shows deep cloning:

public class DeepCloning {

 public static void main(String args[]) {

    double[][] a = {{1, 2, 3},

                    {4, 5, 6},

                    {7, 8, 9}};

    System.out.println("Printing array a");

    Matrix.print(a);

    double[][] b = (double[][])a.clone();

    for (int i=0; i<b.length; i++)

        b[i] = (double[])a[i].clone();

    System.out.println("Printing array b  - clone of a");

    Matrix.print(b);

    b[1][1] = 20;

    System.out.println("Printing array b after making chages to it");

    Matrix.print(b);

    System.out.println("Printing array a after changing its clone");

    Matrix.print(a);

  }

}

4.  Assignments

1.        Modify the Matrix class of Example2 by adding a static method transpose, that returns the transpose of a matrix it receives as parameter.  Now call this method from the main method to get the transpose of the array c (the sum of the two matrices) and then print it.  Fig 1 below shows the sample run of the modified program.

2.       The file attend.txt contains attendance of 15 students in 20 lectures, with 1 representing present and 0 representing absent.  Write a program that reads the data from the file into a 2-D integer array, absences, of size 15 by 21, storing the ID numbers in the first column and the absences in the remaining 20 columns.  Your program should count the number of absences of each student and store the result in a 2-D integer array, numberOfAbsences, of size 15 by 2, storing the ID in the first column and the number of absences in the second.  Finally print the content of the array numberOfAbsences.  Fig 2 above shows the sample run of the program.

3.       Modify assignment 2 above so that the program prints only those students whose number of absences is more than 5.  Your program must define and make use of the following methods:

1.        public static int[][] createArray(String fname,  int rows, int cols) : That receives name of the file and the number of rows and columns as parameter, then opens the file and uses it to create a 2-D array of int and returns it.

2.        public static int[][] countAbsences(double[][] absences) : to counts the absences of each student.

4.  Home Work

A square matrix is said to be a magic square if it satisfies the following conditions:

The elements are unique

adding the elements in each row, each column and each of the two diagonals gives the same result. For example the following is a magic square:

Write a program that first reads the number size of a square matrix, then reads elements of the matrix and prints one of the messages “IS A MAGIC SQUARE” or “IS NOT A MAGIC SQUARE” accordingly.

Your program must include at least the following methods:

static int sumRow(int[][] a,  int i) : That returns the sum of row i

static int sumColumn(int[][] a, int i): That returns the sum of columns i

static int sumDiagonal(int[][] a, boolean mainDiag) : That returns the sum of the main diagonal if mainDiag is true or the sum of the minor diagonal if mainDiag is false.

static boolaean isUnique(int[][] a) : That returns true if all the elments are unique

static boolean isMagic(int[][] a): That returns true if the square is magic and false otherwise.