Week 10 - 2D Array Example

2D Arrays

A 2-D array corresponds to a mathematical matrix:

		11	12	13
		21	22	23
		31	32	33

One simple matrix operation transposes the matrix elements:

		11	21	31
		12	22	32
		13	23	33

MatrixTranspose.java

// MatrixTranspose:  This class shows how to transpose a 2D square matrix.
//                   It's a complete solution for the example discussed
//                   in lecture.

import java.io.*;
public class MatrixTranspose {
	public static void main (String[] args) throws IOException {
		BufferedReader in = new BufferedReader(new InputStreamReader
						(System.in));

		// User enters one dimension of the 2D array.
		System.out.print ("Enter array size: ");
		int size = Integer.parseInt(in.readLine());
		System.out.println ("   Array is " + size + "x" + size);

		// Create a 2D array.
		int[][] a = new int[size][size];

		// User enters values, which are put into the array.
		System.out.print ("Enter " + size*size + " integer values");
		System.out.println (" (one per line): ");
		for (int r = 0; r < a.length; r++) {
			for (int c = 0; c < a[0].length; c++) {
				a[r][c] = Integer.parseInt(in.readLine());
			}
		}

		// Print original array.
		System.out.println ("Original array:");
		for (int r = 0; r < a.length; r++) {
			for (int c = 0; c < a[0].length; c++) {
				System.out.print (a[r][c] + " ");
			}
			System.out.println();
		}

		// Transpose array.
		int startc = 1;
		for (int r = 0; r < a.length; r++) {
			for (int c = startc; c < a[0].length; c++) {
				int temp = a[r][c];
				a[r][c] = a[c][r];
				a[c][r] = temp;
			}
			startc++;
		}

		// Print transposed array.
		System.out.println ("Transposed array:");
		for (int r = 0; r < a.length; r++) {
			for (int c = 0; c < a[0].length; c++) {
				System.out.print (a[r][c] + " ");
			}
			System.out.println();
		}
	}
}

Output for MatrixTranspose.java

Enter array size: 3
   Array is 3x3
Enter 9 integer values (one per line):
6
7
8
4
3
1
2
9
5
Original array:
6 7 8
4 3 1
2 9 5
Transposed array:
6 4 2
7 3 9
8 1 5