8/8: Sorting and Searching Arrays

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.

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.

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.

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.

8/8: Sorting and Searching Arrays