Objectives of the lesson

Transcription

1 Learning Outcome 1) DEMONSTRATE KNOWLEDGE AND UNDERSTANDING OF THE PROCEDURAL APPROACH TO SOFTWARE DEVELOPMENT. Knowledge & Understanding 2) DEVELOP A PROBLEM BASED STRATEGY FOR CREATING AND APPLYING PROGRAMMED SOLUTIONS USING A PROCEDURAL PARADIGM. Analysis Problem Solving 3) DEMONSTRATE THE ABILITY TO DESIGN, CREATE, EDIT, COMPILE, RUN, DEBUG AND TEST PROGRAMS USING AN APPROPRIATE DEVELOPMENT ENVIRONMENT. Application Objectives of the lesson Explain why programs need to search data List three searching algorithms Identify the key components of a linear search Explain the difference between a linear search and linear search sorted Identify the key components of a binary search Demonstrate understanding by adapting a linear search algorithm to solve the tutorial problem 1

2 Look out the for the answers to the following questions What is a linear search? What real world activity could you compare it to? Does the array need to be sorted for a linear search? What two parameters/values are passed to the linearsearch method? What two conditions will stop a linear search? When does the linearsearch method return -1? Array to be searched Index Contents LinearSearch //Checking to see if a specific item is contained within an UNSORTED list(array) import java.util.scanner; public class LinearSearch{ public static void main(string args[]){ Scanner input = new Scanner(System.in); //Create an array of items to search through int list[] = {7,20,8,2,19,15,14}; //Prompt the user to search for a particular item System.out.println("Minimum index = 0 and maximum index = " + (list.length-1)); System.out.println("Value to search for?"); int searchvalue = input.nextint(); //Search for the value entered by the user and store the result of that search in index int index = linearsearch(list, searchvalue); //If the value returned is not -1 (it must have found the number in the list) if (index!=-1){ System.out.println("Linear search: position " + index); else{ //The value returned must be -1 System.out.println(searchValue + " was not found in the list"); }//end else }//end main 2

3 Linear search - start at the beginning and check each index position until either: a match is found (therefore return the index of the match) or, we run out of items to check (therefore return -1 to indicate this) arr = the array we are searching item = the value we are searching for public static int linearsearch(int arr[], int item){ //The values of loop i match the index positions of the array for (int i = 0; i < arr.length; i++){ //Compare the array at position i with the value being searched for if (arr[i] == item){//if the value is found return i; // Search value found }//end for return -1;//Search value not in list }//end linearsearch }//end class 3

4 4

5 Look out the for the answers to the following questions What is the difference between a linear search and a linear search sorted? What real world example might you compare it to? What three conditions will stop a linear search? When does the linearsearchsorted method return -1? Array to be searched Index Contents LinearSearchSorted Checking to see if a specific item is in the list - if the list is already sorted/ordered This method is simple but in most cases inefficient. import java.util.scanner; public class LinearSearchSorted{ public static void main(string args[]){ Scanner input = new Scanner(System.in); //Create an array of sorted/ordered items to search through //(normally the sort would be done by an algorithm int list[] = {2,4,7,8,12,14,15}; //Prompt the user to search for a particular item System.out.println("Minimum index = 0 and maximum index = " + (list.length-1)); System.out.println("Value to search for?"); int searchvalue = input.nextint(); //Search for the value entered by the user and store the result of that search in index int index = linearsearchsorted(list, searchvalue); //If the value returned is not -1 (it must have found the number in the list) if (index!=-1){ System.out.println("Linear search: position " + index); else{//the value returned must be -1 System.out.println(searchValue + " was not found in the list"); }//end else }//end main 5

6 Linear search - start at the beginning and check each index position until either: a match is found (therefore return the index of the match) or, the current value of the array is larger than the value we are looking for or, we run out of items to check (therefore return -1 to indicate this) arr = the array we are searching item = the value we are searching for public static int linearsearchsorted(int arr[], int item){ //The values of loop i match the index positions of the array for (int i = 0; i < arr.length; i++){ //Compare the array at position i with the value being searched for if (arr[i] == item){//if the value is found return i; //Search value found else if (arr[i]>item){ return -1; //Search value not in the list - it cannot be //because we would have found it by now in an //ordered list }//end else if }//end for return -1;//Search value not in list }//end linearsearchsorted }//end class 6

7 7

8 Look out the for the answers to the following questions What is a binary search? What real world activity might you compare it to? What two conditions will stop a binary search? What is the significance of the low, mid and high variables in the binarysearch method? With a large list of sorted items is a binary search better or worse than a linear search? When is a linear search more appropriate than a binary one? BinarySearch 8

9 9

10 Array to be searched Index Contents import java.util.scanner; public class BinarySearch{ public static void main(string args[]){ Scanner input = new Scanner(System.in); //The array below emulates a list that has been sorted/ordered int list[] = {1,2,5,6,7,8,9,10,11,15,16,20,25}; //Prompt the user to search for a particular item System.out.println("Minimum index = 0 and maximum index = " + (list.length-1)); System.out.println("Value to search for?"); int searchvalue = input.nextint(); System.out.println("\nPerforming binary search - the info below shows how it does it:"); int index = binarysearch(list, searchvalue); //If the value returned is not -1 (it must have found the number in the list) if (index!=-1){ System.out.println("Binary search: index " + index); else{ //The value returned must be -1 System.out.println(searchValue + " was not found in the list"); }//end else }//end main /*Binary search *arr = the array we are searching *item = the value we are searching for */ public static int binarysearch(int arr[], int item){ int low = 0; //Lowest index int high = arr.length - 1; //Highest index while (high >= low){ //Middle index is the half way point between low and high int mid = (low + high) / 2; //Compare the array at position mid with the item being searched for //If the value we are looking for (item) is less than the value at //index mid of the array if (item < arr[mid]){ //Make the highest index one less than the current middle index high = mid - 1; //If the value we are looking for (item) is more than the value at //index mid of the array else if (item > arr[mid]){ //Make the lowest index one more than the current middle index low = mid + 1; }//end else if else{ //Found it - item must be equal to the value of the array at position mid return mid; //Return the index position of mid }//end else }//end while //The loop has ended and the item has not been found return -1; }//end binarysearch }//end class 10