11/30: 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.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.

1 st Program of the Day: BinarySearch pg. 291 BinarySearch.java Once you get it to work, figure out how it works.

Multiple-Subscripted Arrays About BinarySearch.java Multiple-Subscripted Arrays Program of the Day

BinarySearch.java pt.1 //fig. 7.13: BinarySearch.java: binary search of an array import java.awt.*; import java.awt.event.*; import java.text.*; import javax.swing.*; public class BinarySearch extends JApplet implements ActionListener { JLabel enterLabel, resultLabel; JTextField enter, result; JTextArea output; int a[]; String display = "";

BinarySearch.java pt.2 public void init () { Container c = getContentPane(); c.setLayout ( new FlowLayout() ); enterLabel = new JLabel ( "Enter key" ); c.add( enterLabel ); enter = new JTextField( 5 ); enter.addActionListener( this ); c.add( enter ); resultLabel = new JLabel ( "Result" ); c.add( resultLabel );

BinarySearch.java pt.3 result = new JTextField( 22 ); result.setEditable( false ); c.add( result ); output = new JTextArea( 6, 60 ); output.setFont(new Font("Courier",Font.PLAIN,12 ) ); c.add( output ); //create array and fill it with even numbers 0-28 a = new int[ 15 ]; for ( int i = 0; i < a.length; i++ ) a[ i ] = 2 * i; }

BinarySearch.java pt.4 public void actionPerformed ( ActionEvent e ) { String searchKey = e.getActionCommand(); //initialize display string for new search display = "Portions of array searched\n"; //perform the binary search int element = binarySearch ( a, Integer.parseInt ( searchKey ) ); output.setText( display ); if ( element != -1 ) result.setText( "Found it in slot " + element ); else result.setText( "Value not found" ); }

BinarySearch.java pt.5 //binary search method public int binarySearch ( int array[], int key ) { int low = 0; //low subscript int high = array.length - 1;//high subscript int middle; //middle subscript while ( low <= high ) { middle = ( low + high ) / 2; //The next line displays the part of the array still //available for searching through for the key. buildOutput ( low, middle, high );

BinarySearch.java pt.6 if ( key == array[middle] ) //a match return middle; else if ( key < array[middle] ) high = middle - 1; //search lower array else low = middle + 1; } return -1; //searchKey not found }

BinarySearch.java pt.7 //build 1 row of output w/ part of the array being processed. void buildOutput ( int low, int mid, int high ) { DecimalFormat twoDigits = new DecimalFormat ( "00" ); for ( int i = 0; i < a.length; i++ ) { if ( i high ) display += " "; else if ( i == mid ) //mark middle element in output display += twoDigits.format ( a [ i ] ) + "* "; else display += twoDigits.format ( a [ i ] ) + " "; } display += "\n"; }

Multiple-Subscripted Arrays Used to represent a table of values, not just a row. By convention, the 1st number = row number, and the 2nd number = column number. EX: int sample[] [] = new int [ 4 ] [ 2 ]; might have values like this: col 0col 1 row 031 row 120 row 268 row 375

Multiple-Subscripted Arrays Another way to remember: “first the row, then the position” int sample [] [] = { {3,1},{2,0},{6,8},{7,5} }; col 0col 1 row 031 row 120 row 268 row 375

Multiple-Subscripted Arrays EX: int sample[] [] = new int [ 4 ] [ 2 ]; sample [3][0] = 7 sample [1][1] = 0 sample [0][1] = ? sample [2][0] = ? col 0col 1 row 031 row 120 row 268 row 375

Multiple-Subscripted Arrays But how do you use them? Think of them as arrays of arrays (lists of lists). for ( int i = 0; i < id.length; i++ ) { for ( int j = 0; j < id[i].length ; j++ ){ g.drawString(“ “ + (id[ i ][ j ]),x,y); } see StringArray.java

2 nd Program of the Day pg. 297 DoubleArray.java