Lecture An introduction to computer science using java (2nd Edition): Chapter 9 - S.N. Kamin, D. Mickunas, E. Reingold

Chapter 9 - Nested loops and two-dimensional arrays. In this chapter we will: show how nested loops are useful, introduce two-dimensional arrays, describe the use of two-dimensional arrays to represent grids of information, show how computer graphics are generated using pixels.

Chapter 9 Nested Loops and

Two-Dimensional Arrays

Lecture Slides to Accompany

An Introduction to Computer Science Using Java (2nd Edition)

by S.N Kamin, D Mickunas, E Reingold

Chapter Preview

In this chapter we will:

• show how nested loops are useful

• introduce two-dimensional arrays

• describe the use of two-dimensional arrays to represent grids of information

• show how computer graphics are generated using pixels

Nested for Loops

• Nested loops frequently used to process two-dimensional arrays

• Often body of inner loop is where the main computation is done

• Example:

for (i = 0; i < m; I++) {

before inner loop

for (j = 0; j < n; j++)

body of inner loop after inner loop

};

Dependent for Loops

• Sometimes the extent of the inner nested loop will depend on the index value of the outer loop

• Example:

for (i = 0; i < 3; i++) {

out.print(“i= “; + i + “: j = “); for (j = 0; j <= i; j++) {

out.print(“ “ + j);

}

}

Nested Loop Contained in Other

Statements

for (int i = 1; i <= 10; i++) {

if (i % 2 == 0) // i even

for (int j = 1; j <= i/2; j++)

out.print(“*”);

else // i odd

for (int k = 1; k <= 5 – i/2; k++)

out.print(“#”);

out.println();

}

Output

#####

*

####

**

###

***

##

****

#

*****

Two-Dimensional Arrays

• Declaration similar to one dimensional arrays

• Need to specify both the number of rows and columns during allocation

• Example:

final int

COLS = 6,

ROWS = 5;

double[][]

energyTable = new double[ROWS][COLS]

Computing Row Totals

double [] yearTotals = new double[ROWS];

for (y = 0; y < ROWS; y++) {

// compute total for year y

yearTotals[y] = 0.0;

for (s =0; s < COLS; s++)

yearTotals[y] =

yearTotals[y] + energyTotal[y][s]; }

Populating energyTable

int y, s;

// reads 30 numbers needed to fill

// energyTable one row at a time

for (y = 0; y < ROWS; y++)

for (s = 0; s < COLS; s++)

energyTable[y][s] = in.readDouble();

Initializing Two-Dimensional

Arrays

double[][] energyTable =

{

{18.9, 19.4, 34.2, 3.9, 5.7, 0.3}, {19.1, 19.3, 33.6, 3.0, 6.2, 0.2}, {18.8, 19.6, 32.9, 3.1, 6.6, 0.2}, {18.9, 20.3, 33.5, 2.8, 6.7, 0.2}, {19.6, 20.8, 33.8, 3.1, 6.5, 0.2} };

Arrays of Arrays

When we write

energyTable = new double[ROWS][COLS];

This is shorthand for

energyTable = new double[ROWS][];

for (int i = 0; i < ROWS; i++)

energyTable[I] = new double[COLS];

Computer Graphics

• Computer graphics is the study of methods of representing and manipulating images

• A two-dimensional array can be used to

represent the image to be displayed

• This array is called a frame buffer ; it has one

entry for each pixel giving its color

• The number of pixels in the frame buffer is

the resolution the display device

Bresenham’s Line Drawing

Algorithm

1 Draw (x0, y0) and compute p0 = 2 y - x

2 Repeat for values of i from 1 to x – 1 :

a) Calculate xi+1 = xi + 1 b) Calculate yi+1 = yi + 1, if pi > 0

yi , otherwise c) Draw a pixel at (xi+1, yi+1)

d) Compute pi = pi + 2 y - 2 x(yi+1 - yi)

Two-Dimensional Arrays

and length

• A.length is number of rows in two-dimensional array A

• A[i].length is number of columns in row i from

two-dimensional array A

• In Java rows can be of different lengths

• Example:

int[][] A = new int[5][];

for (int i = 0; i < 5; i++) {

A[i] = new int[i + 1];

}