Transcript ppt - AD Book Enterprises

8/8: Sorting and Searching Arrays
•
•
•
•
Look at PassArray.java
Sorting arrays: the bubble sort method
Searching arrays: the linear search
Searching arrays: the binary search
PassArray.java -- pt. 1
//Fig 7.10: PassArray.java
//Passing arrays and individual elements to methods
import java.awt.Container;
import javax.swing.*;
public class PassArray extends JApplet {
JTextArea outputArea;
String output;
public void init()
{
outputArea = new JTextArea();
Container c = getContentPane();
c.add( outputArea );
PassArray.java -- pt. 2
int a[] = { 1, 2, 3, 4, 5 };
output = "Effects of passing entire " +
"array call-by-reference:\n" +
"The values of the original array are:\n";
for ( int i = 0; i < a.length ; i++ )
output += " " + a[ i ];
modifyArray ( a ); //passing the whole array
output+="\n\nValues of the modified array are:\n";
PassArray.java -- pt. 3
for ( int i = 0; i < a.length ; i++ )
output += " " + a[ i ];
output += "\n\nEffects of passing array " +
"element call-by-value:\n" +
"a[3] before modifyElement: " + a[ 3 ];
modifyElement ( a [ 3 ] );
}
output += "\na[3] after modifyElement: " + a [ 3 ];
outputArea.setText ( output );
PassArray.java -- pt. 4
public void modifyArray ( int b[] )
{
for ( int j = 0 ; j < b.length; j++ )
b [ j ] *= 2;
}
}
public void modifyElement ( int e )
{
e *= 2;
}
Sorting Arrays using Bubble Sort
• Reorganizing an array in some
order (low to high, etc.)
• Bubble Sort compares two
values, switches them in the
array positions if appropriate,
and checks the next two values.
3619
3619
3169
3169
Sorting Arrays using Bubble Sort
• In this case, 3 & 6 are compared,
and NOT switched.
• Then, 6 & 1 are compared, then
switched.
• Then 6 & 9 are compared and
NOT switched.
• This is ONLY ONE PASS
through the array.
3619
3619
3169
3169
Sorting Arrays using Bubble Sort
• Core of sorting: an if structure.
• This is nested inside a for loop
to look at each pair of values in
the array.
• This loop in nested inside
another loop to make multiple
passes through the array.
• Look at SortThem & its source
code.
3619
3619
3169
3169
Searching Arrays: Linear Search
• How do you find a particular element value in an
array?
• One way: a linear search.
• The core structure: an if statement in a for loop.
• The for loop gives movement through the array
• The if structure looks for a matching value in the
elements.
Searching Arrays: Linear Search
• The for loop gives movement through the array
• The if structure looks for a matching value in the
elements.
for ( int n = 0 ; n < array.length ; n++ ) {
if ( array [ n ] == key )
return n ;
}
Searching Arrays: Binary Sort
• Go to the middle of the array.
• See if the number you are looking for is higher or
lower, and go to the middle of that side.
• Repeat until found.
• Note that the array must already be sorted!
Searching Arrays: Linear vs. Binary
• A linear search works well for smaller arrays, and
is simpler to understand and troubleshoot.
• A binary search is better for a large array. It is
more efficient in its searching.
Program of the Day: BinarySearch
• pg. 291 BinarySearch.java
• Once you get it to work, figure out how it works.
• Next time: review searching and learn about
multiple-subscripted arrays.