summary of last week / Lab6A

int x = 14;GreenfootImage myImage = new GreenfootImage();this.setImage( myImage );this.setLocation( 100, 100 );myImage.setColor( Color.YELLOW );myImage.fill( );myImage.setColor( Color.BLACK );myImage.drawString( String.valueOf( x ), 8, 20 );

100

100 14

public void writeToScreen( String text, int x, int y ) { GreenfootImage myImage = new GreenfootImage(100,

50);

setImage( myImage ); setLocation( x, y );

myImage.setColor(Color.BLACK); myImage.fill();

myImage.setColor(Color.WHITE); myImage.drawString( text, 8, 20); }

public void writeToScreen( int num, int x, int y ) { GreenfootImage myImage = new GreenfootImage(100, 50);

setImage( myImage ); setLocation( x, y );

myImage.setColor(Color.BLACK); myImage.fill();

myImage.setColor(Color.WHITE); myImage.drawString( String.valueOf(num), 8,

20); }

public void writeToScreen( boolean var, int x, int y ) { GreenfootImage myImage = new GreenfootImage(100,

50);

setImage( myImage ); setLocation( x, y );

myImage.setColor(Color.BLACK); myImage.fill();

myImage.setColor(Color.WHITE); myImage.drawString( String.valueOf(var), 8, 20); }

public void writeToScreen( double num, int x, int y ) { GreenfootImage myImage = new GreenfootImage(100, 50);

setImage( myImage ); setLocation( x, y );

myImage.setColor(Color.BLACK); myImage.fill();

myImage.setColor(Color.WHITE); myImage.drawString( String.valueOf(num), 8,

20); }

import java.awt.*;import greenfoot.*;public class Utilities{public void writeToScreen( int num, int x, int y ) { ...... code here }public void writeToScreen( String text, int x, int y ) { ...... code here } public void writeToScreen( double num, int x, int y ) { ...... code here } public void writeToScreen( boolean var, int x, int y ) { ...... code here }}

Utilities myScreen = new Utilities( );

myScreen.writeToScreen( "hello", 100, 100) myScreen.writeToScreen( 10, 200, 200 );myScreen.writeToScreen( 98.6, 300, 300 );myScreen.writeToScreen( true, 400, 400 );

Concatenation (Sequencing)

Computers perform one small step at a time, in a stated order.

z = 0;x = 15;y = 12;z = x + y;image.drawString( String.valueOf( z ), 10, 10) ;

Variables

• x, y, z• i, j, k• TotalCountOfNewEmployees• Geekish_Designations_0123

They Stand for Something

The ESSENCE of a variable

a = 15;b = 12;c = a+b; What is c?

b = 1288;c = a+b; What is c now?a, b, and c hold values and are

CHANGEABLE

x = 14;

• Could mean 14 (not likely)• Could mean 13.999995• Could mean 14.000001• Could mean 20 (honestly), might not be

base-10, more later• Could mean “Fourteen”, which has no

meaning other than to be read by humans

“Typing” a variable

int X = 0;X = 14;Therefore, 14 means 14

double X = 0.0; X = 14.0000;Therefore, X has meaning, 14 to 4 places

Variable Types (primitive, common)

• int : Integers – meant to be a whole number (used for counting)

• double : Floating Pt. – meant to be a detailed number (used for measuring) – called a DOUBLE

• boolean : true or false, meant to hold a state of something

• char : a single Character – has no meaning other than to be read by a human

• String : Strings of Characters – have no meaning other than to be read by a human

Illegal

int Y;Y = 14.001

String Q;Q = 10;

double z;z = “hello”;

Illegal

int x;double y;String z;x = 12;y = 16.001z = x + y;

3 categories of ILLEGAL1. Variables set to a value wrong for its type:

int x;x = true;

2. Variables of different types used in the same operation:int x, y;String z;z = x * y; (* = multiply)

3. Operation wrong for variable type:boolean Q, R, S;Q = R / S; (/ = divide)

boolean types• boolean – true or false (really means

presence or absence), used to signify a condition, or “state” of being.

boolean X, Y, Z;X = true; (means “X is present”)Y = false; (means “Y is NOT present”)Z = X OR Y (not code / what is Z?, i.e. does

Z signify some presence?)Z = X AND Y (not code / what is Z?)

comparing

int x = 0; int y = 0;int z = 0;if (x < 15 ) { .....if (y >= 25)

if ( z = = 0 ) // note TWO equal signs

boolean IsOK = false;

if ( IsOK = = true ) { .....

Operators=assignment

if ( ) or while ( ) operators:

= = comparison if equal

> comparison greater than

>= comparison greater than or equal to

< comparison less than

<= comparison less than or equal to

Operators

+ add- subtract* multiply/ divide

|| OR&& AND

examples

if ( ( x == 0 ) || ( y >= 100 ) )// either has to be true

if ( ( z < 6 ) && ( q == 40 ) )// both have to be true