Arrays, Loopsweeks 4-6

(change from syllabus for week 6)

Chapter 4

• Each variable only holds one item

• if > 1 item wanted, need an array

• array that holds a word

• arrays hold elements all of the same type

char[ ] word = new char[4];

• holds 4 elements of type char

Arrays

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word

char[ ] word = new char[4];

two parts to an array:

index -- integer

element – type inside array

'h' 'e'

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'h'

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word[1] = 'e';

'h' 'e' 'o''r'

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'h' 'e' 'o'

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word[3] = 'o';

word[2] = 'r';

word[0] = 'h';

• Can use variables for index OR elementsint i=3; char new = 'd';word[i] = new;

• can find lengthword.length // is 4• largest index is always length – 1• word[4] is RUN time error

Array manipulation

'h' 'e' 'o''r'

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'h' 'e' 'd''r'

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arrays and new

char[ ] word;

• creates word that is of type char array that points to nothing

word = new word[4];

• creates array of 4 elements initialized to \u0000 (Java always initializes primitives to 0)

Myarray example

public class Myarray

{

private static Circle[] circles;

private static double[] area;

// other stuff in the class

}

Myarray gets elements allocated

• Create an object

circles = new Circle[4];

area = new double[4];

createcircles()

• createcircles()

circles[0] = new Circle();

array creation summary

• char[ ] word;creates a space named word that contains null• word = new char [4];allocates 4 chars, initialized, word points to them• classes: Circle[ ] mycircles;same as word • mycircles = new Circle[4];allocates 4 spaces that contain null• mycircles[0] = new Circle( );creates an actual circle

Repetition in arrays

• arrays often do the same thing

(e.g., for each Circle in array, create a Circle)

for (int i=0; i<circles.length; i++)

circles[i] = new Circle( );

memorize this line

Do: On the board

• Write code to declare a 4 character word array, then write a loop to initialize chars in word to be 'A'

• Write code to declare a 4 character array, then write a loop to initalize chars in word to be ABCD (do this in a loop). Hint: use a separate variable for the element value (start with 'A')

• Declare an int array with 10 integers and write a loop to put the value of the index into the element (e.g., intarray[3] should have the value 3)

Loops

Notes adapted from Dr. Flores

• It repeats a set of statements while a condition is true.

while (condition) { execute these statements;}

“while” structures

“while” structures• It repeats a set of statements while a condition is true.

while ( condition ) { execute these statements;}

22

11

33The dynamics of “while”

Evaluate condition: • if TRUE go to 2• If FALSE go to 3

Execute statements, and then go to 1Continue with next statement.

The dynamics of “while”Evaluate condition:

• if TRUE go to 2• If FALSE go to 3

Execute statements, and then go to 1Continue with next statement.

1.

2.3.

“while” structures• It repeats a set of statements while a condition is true.

int speedLimit = 55;int speed = 0;

while ( speed <= speedLimit ) { speed = speed + 1;}// since we don’t want a ticket…speed = speed - 1;

• What is the value of “speed” at this point?

“while” structures• It repeats a set of statements while a condition is true.

int speedLimit = 55;int speed = 0;

while ( speed < speedLimit ) { speed = speed + 1;}// since we don’t want a ticket…speed = speed - 1;

initialize variablesin conditional

initialize variablesin conditional

1

modify variablesin conditional

modify variablesin conditional

2

“while” structures• Adding the values of an array of integers

int grades[] = new int[1000];/* the values of the elements are somehow initialized here.*/int i = 0;int sum = 0;

while ( i < grades.length ) { sum += grades[i]; i++;}

System.out.println(“The sum is ” + sum);

“while” structures: Exercises

• Determine the output of the following methods:

public void foo1() { int i=0; while (i <= 20) { System.out.println( i ); i = i + 4; } }

public void foo2() { int i = 20; while (i > 0) { i = i / 2; System.out.println( i ); }}

Quiz 4 (parts 1 and 2)1. Write a method named “countDown” that receives an integer

parameter named “number”, and displays (using System.out.println) all numbers from the number down to 0.• For example, if the parameter was 8, the method should

display numbers 7, 6, 5, 4, 3, 2, 1, 0, in this order and with each number in one line.

2. Write a method named “countEven” that receives an integer parameter named “number”, displays (using System.out.println) all even numbers between 0 and the number received, and returns a integer value with the number of even numbers displayed.• For example, if the parameter was 8, the method should

display numbers 2, 4 and 6, in this order and with each number in one line, and return a value of 3 (which is how many even numbers were between 0 and 8).

3. Write a method named “reverse” that receives an integer parameter named “number” and displays (using System.out.println) the number with all digits reversed. The method should only work with positive parameter values. For example, if the number was 2001, the method should display 1002. (Hint: use modulus by 10 to extract the last digit from the number, and then divide the number by 10 before extracting the next digit).

4. Write a method named “prime” that returns a boolean value indicating whether an integer parameter named number is a prime number (i.e., not divisible by any number except 1 and the number itself). Use only positive integers.

5. Write methods 1-4 on a piece of paper handwritten, NOT typed.

Quiz 4 (parts 3 and 4)

“for” structures• It (also) repeats statements while a condition is true.

initialstatement

loopcondition

modifystatement

2211 44

33

55The dynamics of “for”

1. Initialize condition variables.2. Evaluate loop condition:

• if TRUE go to 3• If FALSE go to 5

3. Execute statements; then go to 44. Modify condition variables; then go to 25. Continue with next statements.

The dynamics of “for”1. Initialize condition variables.2. Evaluate loop condition:

• if TRUE go to 3• If FALSE go to 5

3. Execute statements; then go to 44. Modify condition variables; then go to 25. Continue with next statements.

for ( ; ; ){ statements;}

“for” structures: Exercises1. Write a method named “factorial” that calculates the

factorial of an integer parameter named “number” (where factorial is the multiplication of all numbers from 1 to number-1). The method should return an integer number with the result of the factorial, and it should work only with positive numbers (return 0 in the case of non-positive parameter numbers).

2. Write a method named “prime” that returns a boolean value indicating whether an integer parameter named “number” is a prime number (where a prime number is a number that is not divisible without remainder by any other numbers except 1 and the number itself). The method should work only with positive numbers (return false if a negative parameter number is given).Sample list of prime numbers: 2, 3, 5, 7, 11, 13, 19, 23…

“for” structures: Exercises3. Write a method named “digits” that displays (using

System.out.println) the digits of an integer parameter named “number” separated by dashes (“-”). For example, when receiving the number 1234 as a parameter, the method should display “1-2-3-4”.

• Hints: use an array to store each digit as you extract them using the modulus operator. Since these digits are stored in the array in the inverse order as they are found in the number, you will need to print them backwards. Also, note that the last digit does not have a trailing dash!

Containers(ArrayList)

Chapter 4

ArrayList• Like arrays, but have built in operators• Use library (package)

import java.util.ArrayList;• Declare variable

private ArrayList circles;• To create a list

circles = new ArrayList( ); // NOT Circle• To add an element to the list (like Circle c)

circles.add(c)

More ArrayList methods

• To find the size:circles.size();

• To retrieve an element:circles.get(1); // returns the second element

• To remove an elementcircles.remove(1); // removes the second element

ArrayList Overview

• Part of Java Containers– Not fixed size– Can add without having index

• ArrayLists, Sets, Maps – Sets have no duplicates and no order– Maps have key and element association

• Containers have same methods: (size, remove)

• Can put in ANY object – No primitives in containers

More on Containers

• No need to specify type of object:

private ArrayList circles;

circles = new ArrayList( ); // NOT Circle

circles.add(c); • c could be a Circle, a Square, a Student,

ANY object• If c is a Student, c.changeColor(“red”);

will cause runtime error, not syntax error

ArrayLists are ugly

• for loop with index

for (int i=0;i<circles.size( ); i++) {

Circle c = circles.get(i); // this returns the ith circle

c.changeColor(“red”);

}• Syntax error:

– Incompatible types - found java.Lang.Object but expected Circle

– Object is superclass of all objects

Remember the ()s unlike arrays

ArrayLists are ugly

• for loop with index

for (int i=0;i<circles.size( ); i++)

{

Circle c = (Circle) circles.get(i); c.changeColor(“red”);

}

• When using containers in the past, always had to cast object when removing them

Cast to class that you put

in there

ArrayLists and Casting

• Worse, you could put in something that is NOT a circle:

Student s = new Student( );circles.add(s);// more code, then:(Circle) circles.get(i) // where i is s’s index• Runtime error when a circle method is

called• c.changeColor(“red”) // if c is student

Solution: Java 5 Generics

ArrayList<Circle> circles = new ArrayList<Circle>( );

• Now, syntax error for:

Student s = new Student( );

circles.add(s);• Can’t do it

More on Java 5 Generics

• And, no casting:

for (int i=0; i<circles.size( ); i++)

{

Circle c = circles.get(i); c.changeColor(“red”);

}

No “(Circle)”

More on Container Loops

• Containers can use iterators

• New Java 5 for loop

ClassWork

• Write a class ConcentricCircles that holds an ArrayList of Circles.

• Create a constructor that adds 3 circles to the ArrayList. Alternate black and white colors.

• Write method DrawCircle that draws the circles.

Quiz 5• Write a ConcentricCircle class that has the methods: add and draw.

The class should use an ArrayList using generics to • hold the circles.• The constructor should create one black circle in the middle of the

canvas (150, 150) of size 20 (note: the circle will NOT have an xPosition and yPosition of 150.

• Each additional call to add should add a circle with a diameter 15 larger and still centered around the center of the canvas. It should be white if the previous circle was black and black if the previous circle was white. Have a boolean instance variable to keep track of this.

• Draw should make each circle in the ArrayList visible.• Turn in your ConcentricCircle source code.