Transcript Week 8—10/25/11) Arrays

Arrays
How used???
• With loops
• while…
• do…while
• for…
Why used???
• To accommodate large volumes of data in
memory at once, so that analysis can be
performed on them
Declaring an Array
Double[] salesFigure;
Long[] idNum;
Employee oneWorker;
After you declare an array…
• You still need to set aside memory space
oneWorker = new Employee;
• The statement above actually creates the
oneWorker object and sets aside memory
space for it.
We do something similar for arrays
double[] salesFigure;
• To declare it. Then…
salesFigure = new double[20];
• Or, in a single statement…
double[] salesFigure = new double[20];
• In the above statement, you are both
declaring and creating the array
salesFigure[] in which 20 storage locations
are set aside for it
Note that this is different from C++
double salesFigure [20];
• Is acceptable in C++, but causes a
compiler error in Java
Distinguishing between storage
locations
• You use a subscript to distinguish between
storage locations
salesFigure[0]
salesFigure[1]
salesFigure[2]
salesFigure[3]
• A subscript is an integer contained within
brackets that indicates one of the arrays’ storage
locations
• Legal subscripts would be 0 through 19
• Many programming languages begin with 1 as
the first subscript—not so, Java
Array names..
• Contain references or memory addresses,
not the actual contents or value
• This is true of all Java objects
• This is in contrast to a variable that was a
primitive type; such variables actually hold
the value
Initializing an Array
Int[] someNums = new int[10];
• Here, someNums contains a reference or
pointer address to the starting location of
the array
• The array is initialized to all 0’s because it
is a numeric array
• Char array elements are initialized to
‘\u0000’ and boolean array elements are
initialized to false
Initializations occur at…
• Compile time whereas assignments occur at
execution time
Int[] someNums = new int[10];
• Causes 10 0’s to be placed in the ten storage
locations, whereas
someNums[0] = 46;
• Causes the first such storage location to be
assigned the integer value 46 at execution
time
Nonzero initializations
Int[] tenMult = {10, 20, 30, 40, 50, 60};
• The size of the array is determined by the
number of elements you put in the list.
This causes the array tenMult[] to be
declared and created and initialized
• Here you did not need to use the keyword
new or the name of the constructor
The power of arrays becomes
• Apparent when you use subscripts that are
variables rather than subscripts that are
constants
• Consider tenMult[]
Subscripts starting from 0…
• For convenience, you can think of these
subscripts as offsets from the starting location of
the array.
• Int[] tenMult = {10, 20, 30, 40, 50, 60};
tenMult[0] = +=3;
tenMult[1] = +=3;
tenMult[2] = +=3;
tenMult[3] = +=3;
tenMult[4] = +=3;
What is tenmult[3] now???
The same assignments using for..
for(i=0; i < 5; i++)
tenMult[i] += 3;
• Does the same thing as the previous five
statements
• Can also use a symbolic constant as follows:
final int FIVE = 5;
for(i=0; i < FIVE; i++)
tenMult[i] += 3;
The following is also common…
for(i=0; i < tenMult.length; i++) tenMult[i] += 3;
• Here length is not a method, but a field, an
instance variable that is automatically assigned
for every array you create
for(i=0; i < tenMult.length(); i++) tenMult[i] += 3;
• Is Wrong!!!
Enhanced for loop
for(int sub = 0; sub < scoreArray.length; ++sub)
System.out.println(scoreArray[sub]);
For(int val : scoreArray) system.out.println(val);
The second statement does the same thing as
the first
Using Subscripts with an Array
• Is the only way to go for large arrays
Declaring an Array of Objects
Instead of …
Employee painter, electrician, plumber;
Employee bricklayer, roofer, carpetman;
Employee[] emp = new employee[6]
Here an array named emp holds six
employee objects
However, these statements do not actually
construct those Employee objects
The above assumes…
Public class Employee
{
private int empNum;
private double empSal;
Employee (int e, double s)
{
empNum = e;
empSal = s;
}
public int getEmpNum()
{
return empNum;
}
public double getSalary()
{
return empSal;
}
}
Instead….
You must call the constructor six times
final int NUM_EMPLOYEES = 6;
final double PAYRATE = 6.35;
For(int x = 0; x < NUM_EMPLOYEES; ++x)
emp[x] = new Employee(101+x, PAYRATE);
For(int x = 0; x < NUM_EMPLOYEES; ++x)
system.out.println(emp[x].getEmpNum() + “ “ +
emp[x].getSalary());
Searching an Array for an Exact
Match
Searching an Array for a Range
Match
Passing Arrays to Methods
• If you pass an entire array to a method,
you are passing by reference, meaning
that the starting address of the array is
passed to the method and the method can
change every element in the array
• If you pass a specific element of an array,
it is like passing a primitive element and is
a pass by value, meaning the actually
value is transferred to the method, so the
method cannot change the original
variable value
Creating Arrays of Strings
String [] deptName = {“Accounting”, “Human
Resources”, “Sales”};
for(int a=0; a<deptName.length; ++a)
System.out.println(deptName[a]);
Sorting Array Elements
• The algorithm must do a comparison between
the ith element in the list and the i+1st element in
the list (adjacent element in the list)
• If the pair of elements are not in the correct
order, the elements must be swapped:
temp = list[i];
List[i] = list[i+1];
List[i+1] = temp;
More sorting
• When numbers are sorted from smallest to
largest, the order is ascending order
• When numbers are sorted from largest to
smallest, the order is descending order
Bubble sort
int[] someNums = {88, 33, 99, 22, 54};
int b, temp
For(b=0; b<someNums.length-1; ++b)
if(someNums[b] > someNums[b+1]
{
temp = someNums[b];
someNums[b] = someNums[b+1];
someNums[b+1] = temp;
}
Using Two-Dimensional and
Multidimensional Arrays
Employee[] someEmps = new Employee[5];
The above statement creates an array of
five employee objects
Sorting Arrays of Objects
Using the Arrays Class
• You can use arrays to store all kinds of
objects.
Two dimensional arrays
Int[] [] someNumbers = new int[3][4];
Creates a two-dimensional array of ints with
a total of 12 elements
Int[] [] someNumbers = {{9,10,11,12},
{1,2,3,4},
{5,6,7,8}};
Creates and initializes a two-dimensional
array of ints with 12 elements
A two dimensional array
• is also called a matrix or table
• Can create even higher dimensional
arrays
That’s it
import java.util.*;
import javax.swing.*;
public class ArraysDemo2
{
public static void main(String[] args)
{
char[] grades = {'A', 'B', 'C', 'D', 'F'};
String entry;
char myGrade;
int position;
entry = JOptionPane.showInputDialog(null,
"Enter student grade");
myGrade= entry.charAt(0);
position = Arrays.binarySearch(grades, myGrade);
if(position >= 0)
JOptionPane.showMessageDialog(null, "Position of " +
myGrade + " is " + position);
else
JOptionPane.showMessageDialog(null, "Invalid grade");
System.exit(0);
}
}
import javax.swing.*;
public class FindDiscount
{
public static void main(String[] args)
{
final int NUM_RANGES = 5;
int[] discountRangeLimit = { 1, 13, 50, 100, 200};
double[] discountRate = {0.00, 0.10, 0.14, 0.18, 0.20};
double customerDiscount;
String strNumOrdered;
int numOrdered;
int sub = NUM_RANGES - 1;
strNumOrdered = JOptionPane.showInputDialog(null,
"How many items are ordered?");
numOrdered = Integer.parseInt(strNumOrdered);
while(sub >= 0 && numOrdered < discountRangeLimit[sub])
--sub;
customerDiscount = discountRate[sub];
JOptionPane.showMessageDialog(null, "Discount rate for " +
numOrdered + " items is " + customerDiscount);
System.exit(0);
}
• }
public class PassArray
{
public static void main(String[] args)
{
final int NUM_ELEMENTS = 4;
int[] someNums = {5, 10, 15, 20};
int x;
System.out.print("At start of main: ");
for(x = 0; x < NUM_ELEMENTS; ++x)
System.out.print(" " + someNums[x] );
System.out.println();
methodGetsArray(someNums, NUM_ELEMENTS);
System.out.print("At end of main: ");
for(x = 0; x < NUM_ELEMENTS; ++x)
System.out.print(" " + someNums[x]);
System.out.println();
}
public static void methodGetsArray(int arr[], int numEls)
{
int x;
System.out.print("At start of method arr holds: ");
for(x = 0; x < numEls; ++x)
System.out.print(" " + arr[x] );
System.out.println();
for(x = 0; x < numEls; ++x)
arr[x] = 888;
System.out.print(" and at end of method arr holds: ");
for(x = 0; x < numEls; ++x)
System.out.print(" " + arr[x] );
System.out.println();
}
•
}
import javax.swing.*;
public class SearchList
{
public static void main(String[] args)
{
String[] deptName = {"Accounting", "Human Resources", "Sales"};
String dept;
int x;
boolean deptWasFound = false;
dept = JOptionPane.showInputDialog(null,
"Enter a department name");
for(x = 0; x < deptName.length; ++x)
if(dept.equals(deptName[x]))
deptWasFound = true;
if(deptWasFound)
JOptionPane.showMessageDialog(null, dept +
" was found in the list");
else
JOptionPane.showMessageDialog(null, dept +
" was not found in the list");
System.exit(0);
}
}
import java.util.*;
public class ArraysDemo
{
public static void main(String[] args)
{
int[] myScores = new int[5];
display("Original array:
", myScores);
Arrays.fill(myScores, 8);
display("After filling with 8s:
", myScores);
myScores[2] = 6;
myScores[4] = 3;
display("After changing two values: ", myScores);
Arrays.sort(myScores);
display("After sorting:
", myScores);
}
public static void display(String message, int[] array)
{
int sz = array.length;
System.out.print(message);
for(int x = 0; x < sz; ++x)
System.out.print(array[x] + " ");
System.out.println();
}
}
import javax.swing.*;
public class FindPrice
{
public static void main(String[] args)
{
final int NUMBER_OF_ITEMS = 10;
int[] validValues = {101, 108, 201, 213, 266,
304, 311, 409, 411, 412};
double[] prices = {0.29, 1.23, 3.50, 0.69, 6.79,
3.19, 0.99, 0.89, 1.26, 8.00};
String strItem;
int itemOrdered;
double itemPrice = 0.0;
boolean validItem = false;
strItem = JOptionPane.showInputDialog(null,
"Enter the item number you want to order");
itemOrdered = Integer.parseInt(strItem);
for (int x = 0; x < NUMBER_OF_ITEMS; ++x)
{
if(itemOrdered == validValues[x])
{
validItem = true;
itemPrice = prices[x];
}
}
if(validItem)
JOptionPane.showMessageDialog(null, "The price for item " +
itemOrdered + " is $" + itemPrice);
else
JOptionPane.showMessageDialog(null,
"Sorry - invalid item entered");
System.exit(0);
}
•
}
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
public class JHello7 extends JApplet implements ActionListener
{
JLabel greeting = new JLabel("Hello. Who are you?");
Font headlineFont = new Font("Helvetica", Font.BOLD, 36);
JTextField answer = new JTextField(10);
JButton pressMe = new JButton("Press me");
JLabel personalGreeting = new JLabel("");
Container con = getContentPane();
public void init()
{
greeting.setFont(headlineFont);
personalGreeting.setFont(headlineFont);
con.add(greeting);
con.add(answer);
con.add(pressMe);
con.setLayout(new FlowLayout());
pressMe.addActionListener(this);
answer.addActionListener(this);
}
public void actionPerformed(ActionEvent e)
{
String name = answer.getText();
personalGreeting.setText("Hello, " + name);
con.add(personalGreeting);
validate();
}
•
}
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
public class JHello8 extends JApplet implements ActionListener
{
JLabel greeting = new JLabel("Hello. Who are you?");
Font headlineFont = new Font("Helvetica", Font.BOLD, 36);
JTextField answer = new JTextField(10);
JButton pressMe = new JButton("Press me");
JLabel personalGreeting = new JLabel("");
Container con = getContentPane();
public void init()
{
greeting.setFont(headlineFont);
personalGreeting.setFont(headlineFont);
con.add(greeting);
con.add(answer);
con.add(pressMe);
con.setLayout(new FlowLayout());
pressMe.addActionListener(this);
answer.addActionListener(this);
}
public void actionPerformed(ActionEvent e)
{
remove(pressMe);
remove(answer);
String name = answer.getText();
personalGreeting.setText("Hello, " + name);
con.add(personalGreeting);
validate();
}
}
public class PassArrayElement
{
public static void main(String[] args)
{
final int NUM_ELEMENTS = 4;
int[] someNums = {5, 10, 15, 20};
int x;
System.out.print("At start of main: ");
for(x = 0; x < NUM_ELEMENTS; ++x)
System.out.print(" " + someNums[x] );
System.out.println();
for(x = 0; x < NUM_ELEMENTS; ++x)
methodGetsOneInt(someNums[x]);
System.out.print("At end of main: ");
for(x = 0; x < NUM_ELEMENTS; ++x)
System.out.print(" " + someNums[x]);
System.out.println();
}
public static void methodGetsOneInt(int one)
{
System.out.print("At start of method one is: " + one);
one = 999;
System.out.println(" and at end of method one is: " + one);
}
}