Fibonacci Sequence Fibonacci sequence is a sequence of numbers defined by f 1 = 1 f 2 = 1 f n = f n-1 + f n-2 First ten terms – 1, 1, 2, 3, 5, 8, 13, 21, 34, 55

01: import java.util.Scanner; 02: 03: /** 04: This program computes Fibonacci numbers using a recursive 05: method. 06: */ 07: public class FibTester 08: { 09: public static void main(String[] args) 10: { 11: Scanner in = new Scanner(System.in); 12: System.out.print("Enter n: "); 13: int n = in.nextInt(); 14: 15: for (int i = 1; i <= n; i++) 16: { 17: long f = fib(i); 18: System.out.println("fib(" + i + ") = " + f); 19: } 20: } 21: 22: /** 23: Computes a Fibonacci number. n an integer the nth Fibonacci number 26: */ 27: public static long fib(int n) 28: { 29: if (n <= 2) return 1; 30: else return fib(n - 1) + fib(n - 2); 31: } 32: } A tester program used to generate and print Fibonacci numbers Note that the method fib(int n) calls itself recursively

Recursion A recursive computation solves a problem by using the solution of the same problem with simpler values For recursion to terminate, there must be special cases for the simplest inputs. – To complete our example, we must handle n <= 2 If (n <= 2) return 1; Two key requirements for recursion success: – Every recursive call must simplify the computation in some way – There must be special cases to handle the simplest computations directly

Sorting and Searching Goals – To study several sorting and searching algorithms – To appreciate that algorithms for the same task can differ widely in performance – To understand the big-Oh notation – To learn how to estimate and compare the performance of algorithms – To learn how to measure the running time of a program

// Sort an array's values into ascending order. import java.awt.*; import javax.swing.*; public class BubbleSort extends JFrame { public BubbleSort() { JTextArea outputArea = new JTextArea(); Container container = getContentPane(); container.add( outputArea ); int array[] = { 2, 6, 4, 8, 10, 12, 89, 68, 45, 37 }; String output = "Data items in original order\n"; // append original array values to String output for ( int counter = 0; counter < array.length; counter++ ) output += " " + array[ counter ]; bubbleSort( array ); // sort array output += "\n\nData items in ascending order\n"; // append sorted\ array values to String output for ( int counter = 0; counter < array.length; counter++ ) output += " " + array[ counter ]; outputArea.setText( output ); setSize( 375, 200 ); setVisible( true ); } Bubble Sort

// sort elements of array with bubble sort public void bubbleSort( int array2[] ) { // loop to control number of passes for ( int pass = 1; pass array2[ element + 1 ] ) swap( array2, element, element + 1 ); } } } // swap two elements of an array public void swap( int array3[], int first, int second ) { int hold; // temporary holding area for swap hold = array3[ first ]; array3[ first ] = array3[ second ]; array3[ second ] = hold; } public static void main( String args[] ) { BubbleSort application = new BubbleSort (); application.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); } } // end class BubbleSort Bubble Sort

public class InsertionSorter{ public InsertionSorter(int[] anArray) { a = anArray; } public void sort() { for (int i = 1; i 0 && a[j - 1] > next) { a[j] = a[j - 1]; j--; } // Insert the element a[j] = next; } } private int[] a; } Insertion Sort

public class SelectionSortTester { public static void main(String[] args) { int[] a = ArrayUtil.randomIntArray(20, 100); ArrayUtil.print(a); SelectionSorter sorter = new SelectionSorter(a); sorter.sort(); ArrayUtil.print(a); } } public class SelectionSorter{ public SelectionSorter(int[] anArray) { a = anArray; } public void sort() { for (int i = 0; i < a.length - 1; i++) { int minPos = minimumPosition(i); swap(minPos, i); } } //Finds the smallest element in a tail range of the array. private int minimumPosition(int from) { int minPos = from; for (int i = from + 1; i < a.length; i++) if (a[i] < a[minPos]) minPos = i; return minPos; } private void swap(int i, int j) { int temp = a[i]; a[i] = a[j]; a[j] = temp; } private int[] a; } Selection Sort

Complexity of the algorithms All the previous algorithms have time complexity (number of steps of computation) O(n 2 ) where n is the number of numbers to sort – See next slide for an example Better (faster) algorithms? – E.g., Merge Sort

Selection sort on arrays of various sizes

01: /** 02: This class sorts an array, using the merge sort algorithm. 03: */ 04: public class MergeSorter 05: { 06: /** 07: Constructs a merge sorter. anArray the array to sort 09: */ 10: public MergeSorter(int[] anArray) 11: { 12: a = anArray; 13: } 14: 15: /** 16: Sorts the array managed by this merge sorter. 17: */ 18: public void sort() 19: { 20: if (a.length <= 1) return; 21: int[] first = new int[a.length / 2]; 22: int[] second = new int[a.length - first.length]; 23: System.arraycopy(a, 0, first, 0, first.length); 24: System.arraycopy(a, first.length, second, 0, second.length); 25: MergeSorter firstSorter = new MergeSorter(first); 26: MergeSorter secondSorter = new MergeSorter(second); 27: firstSorter.sort(); 28: secondSorter.sort(); 29: merge(first, second); 30: } 31: 32: /** 33: Merges two sorted arrays into the array managed by this 34: merge sorter. first the first sorted array second the second sorted array 37: */ 38: private void merge(int[] first, int[] second) 39: { 40: // Merge both halves into the temporary array 41: 42: int iFirst = 0; 43: // Next element to consider in the first array 44: int iSecond = 0; 45: // Next element to consider in the second array 46: int j = 0; 47: // Next open position in a 48: 49: // As long as neither iFirst nor iSecond past the end, move 50: // the smaller element into a 51: while (iFirst < first.length && iSecond < second.length) 52: { 53: if (first[iFirst] < second[iSecond]) 54: { 55: a[j] = first[iFirst]; 56: iFirst++; 57: } 58: else 59: { 60: a[j] = second[iSecond]; 61: iSecond++; 62: } 63: j++; 64: } 65: 66: // Note that only one of the two calls to arraycopy below 67: // copies entries 68: 69: // Copy any remaining entries of the first array 70: System.arraycopy(first, iFirst, a, j, first.length - iFirst); 71: 72: // Copy any remaining entries of the second half 73: System.arraycopy(second, iSecond, a, j, second.length - iSecond); 74: } 75: 76: private int[] a; 77: } Merge Sort

32: /** 33: Merges two sorted arrays into the array managed by this 34: merge sorter. first the first sorted array second the second sorted array 37: */ 38: private void merge(int[] first, int[] second) 39: { 40: // Merge both halves into the temporary array 41: 42: int iFirst = 0; 43: // Next element to consider in the first array 44: int iSecond = 0; 45: // Next element to consider in the second array 46: int j = 0; 47: // Next open position in a 48: 49: // As long as neither iFirst nor iSecond past the end, move 50: // the smaller element into a 51: while (iFirst < first.length && iSecond < second.length) 52: { 53: if (first[iFirst] < second[iSecond]) 54: { 55: a[j] = first[iFirst]; 56: iFirst++; 57: } 58: else 59: { 60: a[j] = second[iSecond]; 61: iSecond++; 62: } Merge Sort

63: j++; 64: } 65: 66: // Note that only one of the two calls to arraycopy below 67: // copies entries 68: 69: // Copy any remaining entries of the first array 70: System.arraycopy(first, iFirst, a, j, first.length - iFirst); 71: 72: // Copy any remaining entries of the second half 73: System.arraycopy(second, iSecond, a, j, second.length - iSecond); 74: } 75: 76: private int[] a; 77: } Merge Sort

01: /** 02: This program tests the merge sort algorithm by 03: sorting an array that is filled with random numbers. 04: */ 05: public class MergeSortTester 06: { 07: public static void main(String[] args) 08: { 09: int[] a = ArrayUtil.randomIntArray(20, 100); 10: ArrayUtil.print(a); 11: MergeSorter sorter = new MergeSorter(a); 12: sorter.sort(); 13: ArrayUtil.print(a); 14: } 15: } 16: Merge Sort Tester

Merge Sort vs. Selection Sort Selection sort is an O(n 2 ) algorithm Merge sort is an O(nlog(n)) algorithm The nlog(n) function grows much more slowly than n 2

Sorting in a Java Program The Arrays class implements a sorting method To sort an array of integers int[] a =... ; Arrays.sort(a); That sort method uses the Quicksort algorithm

import java.util.Random; import java.util.Arrays; public class BuiltInSort{ public static void main(String[] args){ String[] alphas = {"zulu", "yankee", ”x-ray", "whisky", "victor", "uniform", "tango", "sierra"}; System.out.print("Initial : "); printArray(alphas); sortArray(alphas, true); // true refers to printing after each pass System.out.print("Final : "); printArray(alphas); } public static void printArray( String[] ra){ for (int i = 0 ; i < ra.length ; i++ ){ System.out.print(ra[i]); System.out.print("\t"); } System.out.println(""); // Start a new line } public static void sortArray(String[] ra, boolean printAfterPass){ Arrays.sort(ra); } } Using built-in sort

Searching Linear search: also called sequential search Examines all values in an array until it finds a match or reaches the end Number of visits for a linear search of an array of n elements: – The average search visits n/2 elements – The maximum visits is n A linear search locates a value in an array in O(n) steps

01: /** 02: A class for executing linear searches through an array. 03: */ 04: public class LinearSearcher 05: { 06: /** 07: Constructs the LinearSearcher. anArray an array of integers 09: */ 10: public LinearSearcher(int[] anArray) 11: { 12: a = anArray; 13: } 14: 15: /** 16: Finds a value in an array, using the linear search 17: algorithm. v the value to search the index at which the value occurs, or -1 20: if it does not occur in the array 21: */ 22: public int search(int v) 23: { 24: for (int i = 0; i < a.length; i++) 25: { 26: if (a[i] == v) 27: return i; 28: } 29: return -1; 30: } 31: 32: private int[] a; 33: } Linear Search

01: import java.util.Scanner; 02: 03: /** 04: This program tests the linear search algorithm. 05: */ 06: public class LinearSearchTester 07: { 08: public static void main(String[] args) 09: { 10: // Construct random array 11: 12: int[] a = ArrayUtil.randomIntArray(20, 100); 13: ArrayUtil.print(a); 14: LinearSearcher searcher = new LinearSearcher(a); 15: 16: Scanner in = new Scanner(System.in); 17: 18: boolean done = false; 19: while (!done) 20: { 21: System.out.print("Enter number to search for, -1 to quit: "); 22: int n = in.nextInt(); 23: if (n == -1) 24: done = true; 25: else 26: { 27: int pos = searcher.search(n); 28: System.out.println("Found in position " + pos); 29: } 30: } 31: } 32: } Tester

Binary Search Locates a value in a sorted array by – Determining whether the value occurs in the first or second half – Then repeating the search in one of the halves

01: /** 02: A class for executing binary searches through an array. 03: */ 04: public class BinarySearcher 05: { 06: /** 07: Constructs a BinarySearcher. anArray a sorted array of integers 09: */ 10: public BinarySearcher(int[] anArray) 11: { 12: a = anArray; 13: } 14: Binary Search

15: /** 16: Finds a value in a sorted array, using the binary 17: search algorithm. v the value to search the index at which the value occurs, or -1 20: if it does not occur in the array 21: */ 22: public int search(int v) 23: { 24: int low = 0; 25: int high = a.length - 1; 26: while (low <= high) 27: { 28: int mid = (low + high) / 2; 29: int diff = a[mid] - v; 30: 31: if (diff == 0) // a[mid] == v 32: return mid; 33: else if (diff < 0) // a[mid] < v 34: low = mid + 1; 35: else 36: high = mid - 1; 37: } 38: return -1; 39: } 40: 41: private int[] a; 42: } 43: Binary Search A O(log(n)) algorithm

import java.awt.*; import javax.swing.*; import java.awt.event.*; public class ObjectsSort extends JApplet implements ActionListener{ JTextArea outputArea; JLabel l1, l2; JTextField t1, t2; JButton b1, b2; Student s, array[]; int stCount=0; public void init() { outputArea = new JTextArea(10,15); l1=new JLabel("Enter student name"); l2=new JLabel("Enter student’s mark"); t1= new JTextField(10); t2= new JTextField(10); JPanel p1=new JPanel(); b1= new JButton("Enter student data"); b2= new JButton("Display students"); array = new Student [100]; Container container = getContentPane(); b1.addActionListener(this); b2.addActionListener(this); Sorting an array of objects

p1.setLayout(new GridLayout( 3, 2 ) ); p1.add(l1); p1.add(t1); p1.add(l2); p1.add(t2); p1.add(b1); p1.add(b2); container.add( p1, BorderLayout.NORTH); container.add( new JScrollPane(outputArea), BorderLayout.CENTER ); } public void actionPerformed(ActionEvent e){ String s1="", s2=""; if (e.getSource()==b1){ s1=t1.getText(); s2=t2.getText(); int mark= Integer.parseInt(s2); array[stCount++] = new Student (s1, mark); t1.setText(""); t2.setText(""); } else if (e.getSource()==b2) { String output = "Student records with marks in descending order:\n"; bubbleSort( array ); // sort array // append sorted array values to String output for ( int counter = 0; counter < stCount; counter++ ) output +=array[counter].toString(); outputArea.setText( output ); } } Sorting an array of objects

// sort elements of array with marks in descending order public void bubbleSort( Student array2[] ) { // loop to control number of passes for ( int pass = 1; pass < stCount; pass++ ) { // loop to control number of comparisons for ( int element = 0; element < stCount - 1; element++ ) { // compare side-by-side elements and swap them if first // element is greater than second element if ( array2[ element ].getMark() < array2[ element + 1 ].getMark() ) swap( array2, element, element + 1 ); } } } // swap two elements of an array.. we swap references only public void swap( Student array3[], int first, int second ) { Student hold; // temporary holding reference for swap hold = array3[ first ]; array3[ first ] = array3[ second ]; array3[ second ] = hold; } } // end of class Sorting an array of objects

class Student { private String name; private int mark; public Student (String n, int m){ name=n; mark=m; } public int getMark() { return mark; } public String getName() { return name; } public String toString() { return name + " has " + mark + "\n"; } } Sorting an array of objects

Sorting with names? Use if (array2[ element ].getName().compareTo(array2[ element + 1 ].getName()) > 0 )

The Comparable Interface Several classes in Java (e.g. String and Date) implement Comparable You can implement Comparable interface for your own classes public class Coin implements Comparable { public int compareTo(Object otherObject) { Coin other = (Coin) otherObject; if (value < other.value) return -1; if (value == other.value) return 0; return 1; }... }

Sorting with Comparable s Once your class implements Comparable, simply use the Arrays.sort method: Coin[] coins = new Coin[n]; // Add coins... Arrays.sort(coins);