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COMP 110 Introduction to Search Algorithms

This text provides a review of array sorting and searching algorithms, including sequential search and binary search. It also introduces the concept of sorting arrays in ascending order.

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COMP 110 Introduction to Search Algorithms

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  1. COMP 110Introduction to Search Algorithms Catie Welsh April 20, 2011

  2. Announcements • Program 4 due Wed, April 27th by 11:59pm • Final exam, comprehensive • Friday, May 6th, 12pm • No class Friday - Holiday

  3. Questions?

  4. Today in COMP 110 • Search Algorithms

  5. Review: Array • The array itself is referred to by the name scores (in this particular case) Indices the array scores scores[3]

  6. Review: Arrays System.out.println("Enter 5 basketball scores:"); int[] scores = newint[5]; intscoreSum = 0; for (inti = 0; i < scores.length; i++) { scores[i] = keyboard.nextInt(); scoreSum += scores[i]; } double average = (double) scoreSum / scores.length; System.out.println("Average score: " + average); for (inti = 0; i < scores.length; i++) { if (scores[i] > average) System.out.println(scores[i] + ": above average"); elseif (scores[i] < average) System.out.println(scores[i] + ": below average"); else System.out.println(scores[i] + ": equal to the average"); }

  7. Introduction to sorting • Given an array of numbers, sort the numbers into ascending order • Input array: • Sorted array:

  8. Printing a 1-D array publicstaticvoid printArray(int[] array) { for(int i = 0; i < array.length; i++) System.out.print(array[i] + " "); System.out.println( ); }

  9. Searching an Array • Get in groups of 2-3 people • Write an algorithm to search for an item in an unsorted list • Start with find the index of 5 in the list • 3 6 2 1 7 8 5 9 10 3 • Then turn that into a Java method with the following header: public int findIndex(int x, int[] array) • Finally, think about a more efficient way to do that with a sorted array – write pseudocode • Hand in with names on it

  10. Sequential Search publicint findIndex(int x, int[] array) { for(int i =0; i< array.length; i++) { if(array[i] == x) return i; } return -1; }

  11. What if the array is sorted? • Do we need to start at the beginning and look all the way through? • What if we start in the middle of the array? • int midPoint = (array.length / 2); • Since the array is sorted: • If array[midPoint] > x, we know x must be in first half of array • Else if array[midPoint] > x, x must be in 2nd half of array • Else if array[midPoint] == x, return midPoint • This is called a binary search

  12. Binary Search publicint binarySearch(int x, int[] array) { int min= 0; int max= array.length-1; while (min <= max) { int mid = (min+ max) / 2; if(array[mid] > x) max = mid-1; else if(array[mid] < x) min = mid + 1; else //must be equal return mid; } return -1; }

  13. Monday • Review of Arrays, Inheritance and Sorting/Search Algorithms

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