//Array2DTest.java
// demo of Java two-dimensional arrays

import javax.swing.*;

public class Array2DTest {

    public static void main(String[] args) {
	//2D array of ints: 10 rows, 20 columns, like a table.
	//10*20=200 elements.  
	//Each row has 20 elements.  Each row is itself an array.
	//Syntax is two pairs of brackets.
	int [][] my2DArray = new int[10][20];

	//nested for loops to access all elements.
	//outer loop loops over rows,
	//inner loop loops over columns (elements of that row).
	for (int i=0; i<10; i++) {  //i is conventional for row index
	    for (int j=0; j<20; j++) { //j is conventional for column index
		//dummy value that is offset from base of the array for illustration purposes
		my2DArray[i][j] = i*20 + j;    //indexes are [row][column]
		System.out.print(" "+my2DArray[i][j]);
	    }
	    System.out.println();   //end of the row
	}


	//row and column sizes can be variables:  size determined at runtime.

        String input = JOptionPane.showInputDialog( "Enter number of rows" );
        int rows = Integer.parseInt( input );
        input = JOptionPane.showInputDialog( "Enter number of columns" );
        int cols = Integer.parseInt( input );

	System.out.println();   

	//re-instantiate the 2D array
	my2DArray = new int[rows][cols];

	for (int i=0; i<rows; i++) {    //row index: 0 thru rowSize-1
	    for (int j=0; j<cols; j++) {    //column index: 0 thru colSize-1
		my2DArray[i][j] = (int)(Math.random()*100);  //random int 0-99
		System.out.print(" "+my2DArray[i][j]);
	    }
	    System.out.println();   
	}

	System.out.println();   
	//call of method whose parameter is 2D array of ints
	System.out.println("Sum = "+arraySum(my2DArray));   


	//access a particular element
        input = JOptionPane.showInputDialog( "Enter row index" );
        int r = Integer.parseInt( input );
        input = JOptionPane.showInputDialog( "Enter column index" );
        int c = Integer.parseInt( input );
	//
	JOptionPane.showMessageDialog(null,"Value is currently: "+my2DArray[r][c]);
        input = JOptionPane.showInputDialog( "Enter new value for that element" );
        int newValue = Integer.parseInt( input );
	my2DArray[r][c] = newValue;
	JOptionPane.showMessageDialog(null,"Value is now: "+my2DArray[r][c]);


    }

    //method to sum all elements of a 2D array of ints
    public static int arraySum(int[][] A) {
	int sum = 0;
	for (int i=0; i<A.length; i++)     //length of 2D array is #rows
	    for (int j=0; j<A[i].length; j++)     //length of Ith row
		sum += A[i][j];
	return sum;
    }
}