Lecture 6 What is the difference in pictures

Lecture 6

What is the difference in pictures ? ? ?

Today’s topic • Let’s discuss about the most common errors when writing a program – Based on my experience and what I found from you guys • Control program flow – Loop One of the most important and interesting topics in this course

Review (sub-methods) public class Turtle { public void draw. Line. At( int x, int y, int len) { pen. Up(); move. To( x, y); pen. Down(); forward(len); pen. Up(); } } public void draw. T( int len ) { draw. Line. At(400, 200, len); turn( 90 ); draw. Line. At(350, 200, len); } Turtle class contains 2 methods in this example Methods can be used within a method If we run t. draw. T( 50 )

Review (sub-methods) public class Turtle { public void draw. Line. At( int x, int y, int len) { Why is this good? ? ? pen. Up(); move. To( x, y); pen. Down(); What is the benefit? ? ? forward(len); pen. Up(); } What if we write draw. T } public void draw. T( int len ) { draw. Line. At(400, 200, len); turn( 90 ); draw. Line. At(350, 200, len); } without using draw. Line. At

Review (sub-methods) public class Turtle { public void draw. T( int len ) { pen. Up(); move. To( 400, 200 ); pen. Down(); forward(len); pen. Up(); turn( 90 ); pen. Up(); move. To( 350, 200 ); pen. Down(); forward(len); pen. Up(); } } Do you like this? - We can avoid to write repeatedly shown statements - We can wrap them up as a method - Also, it is easier to understand how program works

Review (main method) When we run a program, it always starts from main method. public class Test { public static void main(String[] args) { } }

Review (main method) When we run a program, it always starts from main method. public class Test { public static void main(String[] args) { World w = new World(); Turtle a = new Turtle(w); Turtle b = new Turtle(w); a. move. To(100, 100); b. move. To(300, 300); } }

What is wrong in this program? ? ? public class Turtle { public void draw. Square() { forward(100) turn(90) forward(100) Semicolon at the end turn(90) of each statement forward(100) turn(90) forward(100) } }

What is wrong in this program? ? ? Public Class Turtle { Public Void draw. Square() { forward(100); Key word such as turn(90); public, class, void, etc forward(100); turn(90); Case sensitive!!! forward(100); turn(90); forward(100); Textbook Page 63 } }

What is wrong in this program? ? ? public class Turtle { public void draw. Square() { forward(100); A pair of brackets turn(90); If open, then close it forward(100); A class or a method turn(90); forward(100); should be closed turn(90); by brackets forward(100); }

What is wrong in this program? ? ? public class Turtle { public void draw. Square { forward(100); turn(90); forward(100); } } Parenthesis are required after method name The method may receive parameters

How about this? ? ? public class Turtle { public void draw. Square() { forward(100); turn(90); forward(100); } It works!!! } But please do not write a program like this. Hard to read (understand) Please be artistic.

How about this? ? ? public class Turtle { public void draw. Square() { forward(100); turn(90); } } Suffering!!! Finding this error makes me sick 3 days But we have to find.

More … public class Turtle { public void draw. Square( int len ) { forward(len); turn(90); forward(len); } } public class Test { public static void main(String[] args) { World w = new World(); Turtle t = new Turtle(w); t. draw. Square(); } } The number of parameters should be the same as that of method definition

More … public class Turtle { public void draw. Square() { // omit …. . } public void draw. Square( int len ) { // omit …. . } } public class Test { public static void main(String[] args) { World w = new World(); Turtle t 1 = new Turtle(w); Turtle t 2 = new Turtle(w); } } t 1. draw. Square(50); t 2. draw. Square(); This is fine

• Keys to be a good programmer – Keep writing a program (Every day!!!!!) – Try to find the errors by yourself • Experience will train you more than just being helped

Repetition Statements (Loops) • Repetition statements allow us to run a statement or a block of statements multiple times • Often we call them as loops

Repetition Statements (Loops) • Java has three kinds of repetition statements: for loop while loop do loop • The programmer should choose the appropriate loop for the situation

Example of for Loop public class Turtle { public void draw. Square( int len ) { for(int i=0; i < 4; i++) { Repeat these forward( len ); statements turn(90); 4 times } } } Count starts from 0 Add one for each repeat 4 is not included

Example of for Loop public class Turtle { public void print. Number() { int num = 20; } } for(int i=0; i < 10; i++) { System. out. pritnln( num ); } How many 20’s are printed out?

Example of for Loop public class Turtle { public void print. Number() { int num = 20; } } for(int i=5; i < 10; i++) { System. out. pritnln( num ); } How about this?

Example of for Loop public class Turtle { public void print. Number() { int num = 20; } } for(int a=0; a < 10; a++) { System. out. pritnln( num ); } You can change and use any variable name for a loop counter

Example of for Loop public class Turtle { public void print. Number( int num ) { for(int i=0; i < 10; i++) { System. out. pritnln( num ); } } } You can specify the printed number when you use this method

• Exercise!!!

You may have this method already public class Turtle { public void draw. Square( int len ) { forward(len); turn(90); forward(len); } }

Exercise 1 public class Test { public static void main(String[] args) { World w = new World(); Turtle t = new Turtle(w); } } for(int k=0; k < 20; k++) { t. draw. Square(100); t. turn(20); }

Exercise 2 public class Test { public static void main(String[] args) { World w = new World(); Turtle t = new Turtle(w); } } for(int k=0; k < 20; k++) { t. draw. Square( k * 10 ); t. turn(20); }

• set. Pen. Color(Color. xxx) where xxx will be RED, GREEN, BLUE, etc (textbook page 43) Note: please add a following line at a top of the class to use the Color method import java. awt. *; • get. XPos() will get the position of x axis • get. YPos() will get the postion of y axis

E x e r c i s e 3 import java. awt. *; public class Test { public static void main(String[] args) { World w = new World(); Turtle t = new Turtle(w); t. set. Pen. Color( Color. RED ); for(int k=0; k < 10; k++) { t. pen. Up(); t. move. To( t. get. XPos()+5, t. get. YPos()+5 ); t. pen. Down(); t. draw. Square( k * 10 ); } } }

Go back to Exercise 1 public class Test { public static void main(String[] args) { World w = new World(); Turtle t = new Turtle(w); } } for(int k=0; k < 20; k++) { t. draw. Square(100); t. turn(20); } Can be written as a method

Exercise 1 (Another way) public class Turtle { public void draw. Picture() { for(int k=0; k < 20; k++) { } } } draw. Square(100); turn(20); public class Test { public static void main(String[] args) { World w = new World(); Turtle t = new Turtle(w); t. draw. Picture() } }

Exercise 2 (Another way) public class Turtle { public void draw. Picture() { for(int k=0; k < 20; k++) { public class Test { public static void main(String[] args) { World w = new World(); Turtle t = new Turtle(w); draw. Square( k * 10 ); turn(20); } } } t. draw. Picture() } }

Exercise 3 (another way) public class Turtle { public void draw. Picture() { set. Pen. Color( Color. RED ); for(int k=0; k < 10; k++) { pen. Up(); move. To( get. XPos()+5, get. YPos()+5 ); pen. Down(); draw. Square( k * 10 ); } } } public class Test { public static void main(String[] args) { World w = new World(); Turtle t = new Turtle(w); t. draw. Picture() } }