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Arrays, Loops
weeks 4-6
(change from syllabus for week 6)
Chapter 4
Arrays
• 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
word
0
1
2
3
char[ ] word = new char[4];
two parts to an array:
index -- integer
element – type inside
array
word[3] = 'o';
word[0] = 'h';
'h'
0
1
2
3
'h'
'e'
'o'
0
1
2
3
'h'
'e'
'r'
'o'
0
1
2
3
word[2] = 'r';
word[1] = 'e';
'h'
'e'
0
1
2
3
Array manipulation
'h'
'e'
'r'
'o'
0
1
2
3
• Can use variables for index OR elements
int i=3; char new = 'd';
word[i] = new;
'h'
'e'
'r'
'd'
0
1
2
3
• can find length
word.length // is 4
• largest index is always length – 1
• word[4] is RUN time error
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( );
Repetition in arrays
• arrays often do the same thing
(e.g., for each Circle in array, create a
memorize this line
Circle)
for (int i=0; i<circles.length; i++)
circles[i] = new Circle( );
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
“while” structures
• It repeats a set of statements while a condition is true.
while (condition)
{
execute these statements;
}
“while” structures
• It repeats a set of statements while a condition is true.
while ( condition ) 1
{
execute these statements;
}
2
3
The dynamics of “while”
1. Evaluate condition:
• if TRUE go to 2
• If FALSE go to 3
2. Execute statements, and then go to 1
3. Continue with next statement.
“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;
1
initialize variables
in conditional
while ( speed < speedLimit )
2
{
modify variables
speed = speed + 1;
in conditional
}
// since we don’t want a ticket…
speed = speed - 1;
“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).
Quiz 4 (parts 3 and 4)
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.
“for” structures
• It (also) repeats statements while a condition is true.
1
2
4
loop
initial
modify
for ( statement ; condition; statement )
{
statements;
3
}
5
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 4
4. Modify condition variables; then go to 2
5. Continue with next statements.
“for” structures: Exercises
1. 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: Exercises
3. 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 element
circles.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
Remember the ()s
unlike arrays
• 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
ArrayLists are ugly
• for loop with index
for (int i=0;i<circles.size( ); i++)
Cast to class that
{
you put in there
Circle c = (Circle) circles.get(i);
c.changeColor(“red”);
}
• When using containers in the past, always
had to cast object when removing them
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++)
No “(Circle)”
{
Circle c = circles.get(i);
c.changeColor(“red”);
}
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.