Sorting Algorithms
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Sort Algorithms 15-110 - Friday 2/28 Learning Objectives
Sort Algorithms 15-110 - Friday 2/28 Learning Objectives • Recognize how different sorting algorithms implement the same process with different algorithms • Recognize the general algorithm and trace code for three algorithms: selection sort, insertion sort, and merge sort • Compute the Big-O runtimes of selection sort, insertion sort, and merge sort 2 Search Algorithms Benefit from Sorting We use search algorithms a lot in computer science. Just think of how many times a day you use Google, or search for a file on your computer. We've determined that search algorithms work better when the items they search over are sorted. Can we write an algorithm to sort items efficiently? Note: Python already has built-in sorting functions (sorted(lst) is non-destructive, lst.sort() is destructive). This lecture is about a few different algorithmic approaches for sorting. 3 Many Ways of Sorting There are a ton of algorithms that we can use to sort a list. We'll use https://visualgo.net/bn/sorting to visualize some of these algorithms. Today, we'll specifically discuss three different sorting algorithms: selection sort, insertion sort, and merge sort. All three do the same action (sorting), but use different algorithms to accomplish it. 4 Selection Sort 5 Selection Sort Sorts From Smallest to Largest The core idea of selection sort is that you sort from smallest to largest. 1. Start with none of the list sorted 2. Repeat the following steps until the whole list is sorted: a) Search the unsorted part of the list to find the smallest element b) Swap the found element with the first unsorted element c) Increment the size of the 'sorted' part of the list by one Note: for selection sort, swapping the element currently in the front position with the smallest element is faster than sliding all of the numbers down in the list. -
Quick Sort Algorithm Song Qin Dept
Quick Sort Algorithm Song Qin Dept. of Computer Sciences Florida Institute of Technology Melbourne, FL 32901 ABSTRACT each iteration. Repeat this on the rest of the unsorted region Given an array with n elements, we want to rearrange them in without the first element. ascending order. In this paper, we introduce Quick Sort, a Bubble sort works as follows: keep passing through the list, divide-and-conquer algorithm to sort an N element array. We exchanging adjacent element, if the list is out of order; when no evaluate the O(NlogN) time complexity in best case and O(N2) exchanges are required on some pass, the list is sorted. in worst case theoretically. We also introduce a way to approach the best case. Merge sort [4]has a O(NlogN) time complexity. It divides the 1. INTRODUCTION array into two subarrays each with N/2 items. Conquer each Search engine relies on sorting algorithm very much. When you subarray by sorting it. Unless the array is sufficiently small(one search some key word online, the feedback information is element left), use recursion to do this. Combine the solutions to brought to you sorted by the importance of the web page. the subarrays by merging them into single sorted array. 2 Bubble, Selection and Insertion Sort, they all have an O(N2) In Bubble sort, Selection sort and Insertion sort, the O(N ) time time complexity that limits its usefulness to small number of complexity limits the performance when N gets very big. element no more than a few thousand data points. -
Lecture 11: Heapsort & Its Analysis
Lecture 11: Heapsort & Its Analysis Agenda: • Heap recall: – Heap: definition, property – Max-Heapify – Build-Max-Heap • Heapsort algorithm • Running time analysis Reading: • Textbook pages 127 – 138 1 Lecture 11: Heapsort (Binary-)Heap data structure (recall): • An array A[1..n] of n comparable keys either ‘≥’ or ‘≤’ • An implicit binary tree, where – A[2j] is the left child of A[j] – A[2j + 1] is the right child of A[j] j – A[b2c] is the parent of A[j] j • Keys satisfy the max-heap property: A[b2c] ≥ A[j] • There are max-heap and min-heap. We use max-heap. • A[1] is the maximum among the n keys. • Viewing heap as a binary tree, height of the tree is h = blg nc. Call the height of the heap. [— the number of edges on the longest root-to-leaf path] • A heap of height k can hold 2k —— 2k+1 − 1 keys. Why ??? Since lg n − 1 < k ≤ lg n ⇐⇒ n < 2k+1 and 2k ≤ n ⇐⇒ 2k ≤ n < 2k+1 2 Lecture 11: Heapsort Max-Heapify (recall): • It makes an almost-heap into a heap. • Pseudocode: procedure Max-Heapify(A, i) **p 130 **turn almost-heap into a heap **pre-condition: tree rooted at A[i] is almost-heap **post-condition: tree rooted at A[i] is a heap lc ← leftchild(i) rc ← rightchild(i) if lc ≤ heapsize(A) and A[lc] > A[i] then largest ← lc else largest ← i if rc ≤ heapsize(A) and A[rc] > A[largest] then largest ← rc if largest 6= i then exchange A[i] ↔ A[largest] Max-Heapify(A, largest) • WC running time: lg n. -
CS 758/858: Algorithms
CS 758/858: Algorithms ■ COVID Prof. Wheeler Ruml Algorithms TA Sumanta Kashyapi This Class Complexity http://www.cs.unh.edu/~ruml/cs758 4 handouts: course info, schedule, slides, asst 1 2 online handouts: programming tips, formulas 1 physical sign-up sheet/laptop (for grades, piazza) Wheeler Ruml (UNH) Class 1, CS 758 – 1 / 25 COVID ■ COVID Algorithms This Class Complexity ■ check your Wildcat Pass before coming to campus ■ if you have concerns, let me know Wheeler Ruml (UNH) Class 1, CS 758 – 2 / 25 ■ COVID Algorithms ■ Algorithms Today ■ Definition ■ Why? ■ The Word ■ The Founder This Class Complexity Algorithms Wheeler Ruml (UNH) Class 1, CS 758 – 3 / 25 Algorithms Today ■ ■ COVID web: search, caching, crypto Algorithms ■ networking: routing, synchronization, failover ■ Algorithms Today ■ machine learning: data mining, recommendation, prediction ■ Definition ■ Why? ■ bioinformatics: alignment, matching, clustering ■ The Word ■ ■ The Founder hardware: design, simulation, verification ■ This Class business: allocation, planning, scheduling Complexity ■ AI: robotics, games Wheeler Ruml (UNH) Class 1, CS 758 – 4 / 25 Definition ■ COVID Algorithm Algorithms ■ precisely defined ■ Algorithms Today ■ Definition ■ mechanical steps ■ Why? ■ ■ The Word terminates ■ The Founder ■ input and related output This Class Complexity What might we want to know about it? Wheeler Ruml (UNH) Class 1, CS 758 – 5 / 25 Why? ■ ■ COVID Computer scientist 6= programmer Algorithms ◆ ■ Algorithms Today understand program behavior ■ Definition ◆ have confidence in results, performance ■ Why? ■ The Word ◆ know when optimality is abandoned ■ The Founder ◆ solve ‘impossible’ problems This Class ◆ sets you apart (eg, Amazon.com) Complexity ■ CPUs aren’t getting faster ■ Devices are getting smaller ■ Software is the differentiator ■ ‘Software is eating the world’ — Marc Andreessen, 2011 ■ Everything is computation Wheeler Ruml (UNH) Class 1, CS 758 – 6 / 25 The Word: Ab¯u‘Abdall¯ah Muh.ammad ibn M¯us¯aal-Khw¯arizm¯ı ■ COVID 780-850 AD Algorithms Born in Uzbekistan, ■ Algorithms Today worked in Baghdad. -
Sorting Algorithms Correcness, Complexity and Other Properties
Sorting Algorithms Correcness, Complexity and other Properties Joshua Knowles School of Computer Science The University of Manchester COMP26912 - Week 9 LF17, April 1 2011 The Importance of Sorting Important because • Fundamental to organizing data • Principles of good algorithm design (correctness and efficiency) can be appreciated in the methods developed for this simple (to state) task. Sorting Algorithms 2 LF17, April 1 2011 Every algorithms book has a large section on Sorting... Sorting Algorithms 3 LF17, April 1 2011 ...On the Other Hand • Progress in computer speed and memory has reduced the practical importance of (further developments in) sorting • quicksort() is often an adequate answer in many applications However, you still need to know your way (a little) around the the key sorting algorithms Sorting Algorithms 4 LF17, April 1 2011 Overview What you should learn about sorting (what is examinable) • Definition of sorting. Correctness of sorting algorithms • How the following work: Bubble sort, Insertion sort, Selection sort, Quicksort, Merge sort, Heap sort, Bucket sort, Radix sort • Main properties of those algorithms • How to reason about complexity — worst case and special cases Covered in: the course book; labs; this lecture; wikipedia; wider reading Sorting Algorithms 5 LF17, April 1 2011 Relevant Pages of the Course Book Selection sort: 97 (very short description only) Insertion sort: 98 (very short) Merge sort: 219–224 (pages on multi-way merge not needed) Heap sort: 100–106 and 107–111 Quicksort: 234–238 Bucket sort: 241–242 Radix sort: 242–243 Lower bound on sorting 239–240 Practical issues, 244 Some of the exercise on pp. -
Bubble Sort: an Archaeological Algorithmic Analysis
Bubble Sort: An Archaeological Algorithmic Analysis Owen Astrachan 1 Computer Science Department Duke University [email protected] Abstract 1 Introduction Text books, including books for general audiences, in- What do students remember from their first program- variably mention bubble sort in discussions of elemen- ming courses after one, five, and ten years? Most stu- tary sorting algorithms. We trace the history of bub- dents will take only a few memories of what they have ble sort, its popularity, and its endurance in the face studied. As teachers of these students we should ensure of pedagogical assertions that code and algorithmic ex- that what they remember will serve them well. More amples used in early courses should be of high quality specifically, if students take only a few memories about and adhere to established best practices. This paper is sorting from a first course what do we want these mem- more an historical analysis than a philosophical trea- ories to be? Should the phrase Bubble Sort be the first tise for the exclusion of bubble sort from books and that springs to mind at the end of a course or several courses. However, sentiments for exclusion are sup- years later? There are compelling reasons for excluding 1 ported by Knuth [17], “In short, the bubble sort seems discussion of bubble sort , but many texts continue to to have nothing to recommend it, except a catchy name include discussion of the algorithm after years of warn- and the fact that it leads to some interesting theoreti- ings from scientists and educators. -
Data Structures & Algorithms
DATA STRUCTURES & ALGORITHMS Tutorial 6 Questions SORTING ALGORITHMS Required Questions Question 1. Many operations can be performed faster on sorted than on unsorted data. For which of the following operations is this the case? a. checking whether one word is an anagram of another word, e.g., plum and lump b. findin the minimum value. c. computing an average of values d. finding the middle value (the median) e. finding the value that appears most frequently in the data Question 2. In which case, the following sorting algorithm is fastest/slowest and what is the complexity in that case? Explain. a. insertion sort b. selection sort c. bubble sort d. quick sort Question 3. Consider the sequence of integers S = {5, 8, 2, 4, 3, 6, 1, 7} For each of the following sorting algorithms, indicate the sequence S after executing each step of the algorithm as it sorts this sequence: a. insertion sort b. selection sort c. heap sort d. bubble sort e. merge sort Question 4. Consider the sequence of integers 1 T = {1, 9, 2, 6, 4, 8, 0, 7} Indicate the sequence T after executing each step of the Cocktail sort algorithm (see Appendix) as it sorts this sequence. Advanced Questions Question 5. A variant of the bubble sorting algorithm is the so-called odd-even transposition sort . Like bubble sort, this algorithm a total of n-1 passes through the array. Each pass consists of two phases: The first phase compares array[i] with array[i+1] and swaps them if necessary for all the odd values of of i. -
13 Basic Sorting Algorithms
Concise Notes on Data Structures and Algorithms Basic Sorting Algorithms 13 Basic Sorting Algorithms 13.1 Introduction Sorting is one of the most fundamental and important data processing tasks. Sorting algorithm: An algorithm that rearranges records in lists so that they follow some well-defined ordering relation on values of keys in each record. An internal sorting algorithm works on lists in main memory, while an external sorting algorithm works on lists stored in files. Some sorting algorithms work much better as internal sorts than external sorts, but some work well in both contexts. A sorting algorithm is stable if it preserves the original order of records with equal keys. Many sorting algorithms have been invented; in this chapter we will consider the simplest sorting algorithms. In our discussion in this chapter, all measures of input size are the length of the sorted lists (arrays in the sample code), and the basic operation counted is comparison of list elements (also called keys). 13.2 Bubble Sort One of the oldest sorting algorithms is bubble sort. The idea behind it is to make repeated passes through the list from beginning to end, comparing adjacent elements and swapping any that are out of order. After the first pass, the largest element will have been moved to the end of the list; after the second pass, the second largest will have been moved to the penultimate position; and so forth. The idea is that large values “bubble up” to the top of the list on each pass. A Ruby implementation of bubble sort appears in Figure 1. -
Quick Sort Algorithm Song Qin Dept
Quick Sort Algorithm Song Qin Dept. of Computer Sciences Florida Institute of Technology Melbourne, FL 32901 ABSTRACT each iteration. Repeat this on the rest of the unsorted region Given an array with n elements, we want to rearrange them in without the first element. ascending order. In this paper, we introduce Quick Sort, a Bubble sort works as follows: keep passing through the list, divide-and-conquer algorithm to sort an N element array. We exchanging adjacent element, if the list is out of order; when no evaluate the O(NlogN) time complexity in best case and O(N2) exchanges are required on some pass, the list is sorted. in worst case theoretically. We also introduce a way to approach the best case. Merge sort [4] has a O(NlogN) time complexity. It divides the 1. INTRODUCTION array into two subarrays each with N/2 items. Conquer each Search engine relies on sorting algorithm very much. When you subarray by sorting it. Unless the array is sufficiently small(one search some key word online, the feedback information is element left), use recursion to do this. Combine the solutions to brought to you sorted by the importance of the web page. the subarrays by merging them into single sorted array. 2 Bubble, Selection and Insertion Sort, they all have an O(N2) time In Bubble sort, Selection sort and Insertion sort, the O(N ) time complexity that limits its usefulness to small number of element complexity limits the performance when N gets very big. no more than a few thousand data points. -
Selection Sort
CS50 Selection Sort Overview Key Terms Sorted arrays are typically easier to search than unsorted arrays. One algorithm to sort • selection sort is bubble sort. Intuitively, it seemed that there were lots of swaps involved; but perhaps • array there is another way? Selection sort is another sorting algorithm that minimizes the • pseudocode amount of swaps made (at least compared to bubble sort). Like any optimization, ev- erything comes at a cost. While this algorithm may not have to make as many swaps, it does increase the amount of comparing required to sort a single element. Implementation Selection sort works by splitting the array into two parts: a sorted array and an unsorted array. If we are given an array Step-by-step process for of the numbers 5, 1, 6, 2, 4, and 3 and we wanted to sort it selection sort using selection sort, our pseudocode might look something like this: 5 1 6 2 4 3 repeat for the amount of elements in the array find the smallest unsorted value swap that value with the first unsorted value When this is implemented on the example array, the pro- 1 5 6 2 4 3 gram would start at array[0] (which is 5). We would then compare every number to its right (1, 6, 2, 4, and 3), to find the smallest element. Finding that 1 is the smallest, it gets swapped with the element at the current position. Now 1 is 1 2 6 5 4 3 in the sorted part of the array and 5, 6, 2, 4, and 3 are still unsorted. -
Sorting Partnership Unless You Sign Up! Brian Curless • Homework #5 Will Be Ready After Class, Spring 2008 Due in a Week
Announcements (5/9/08) • Project 3 is now assigned. CSE 326: Data Structures • Partnerships due by 3pm – We will not assume you are in a Sorting partnership unless you sign up! Brian Curless • Homework #5 will be ready after class, Spring 2008 due in a week. • Reading for this lecture: Chapter 7. 2 Sorting Consistent Ordering • Input – an array A of data records • The comparison function must provide a – a key value in each data record consistent ordering on the set of possible keys – You can compare any two keys and get back an – a comparison function which imposes a indication of a < b, a > b, or a = b (trichotomy) consistent ordering on the keys – The comparison functions must be consistent • Output • If compare(a,b) says a<b, then compare(b,a) must say b>a • If says a=b, then must say b=a – reorganize the elements of A such that compare(a,b) compare(b,a) • If compare(a,b) says a=b, then equals(a,b) and equals(b,a) • For any i and j, if i < j then A[i] ≤ A[j] must say a=b 3 4 Why Sort? Space • How much space does the sorting • Allows binary search of an N-element algorithm require in order to sort the array in O(log N) time collection of items? • Allows O(1) time access to kth largest – Is copying needed? element in the array for any k • In-place sorting algorithms: no copying or • Sorting algorithms are among the most at most O(1) additional temp space. -
Binary Search
UNIT 5B Binary Search 15110 Principles of Computing, 1 Carnegie Mellon University - CORTINA Course Announcements • Sunday’s review sessions at 5‐7pm and 7‐9 pm moved to GHC 4307 • Sample exam available at the SCHEDULE & EXAMS page http://www.cs.cmu.edu/~15110‐f12/schedule.html 15110 Principles of Computing, 2 Carnegie Mellon University - CORTINA 1 This Lecture • A new search technique for arrays called binary search • Application of recursion to binary search • Logarithmic worst‐case complexity 15110 Principles of Computing, 3 Carnegie Mellon University - CORTINA Binary Search • Input: Array A of n unique elements. – The elements are sorted in increasing order. • Result: The index of a specific element called the key or nil if the key is not found. • Algorithm uses two variables lower and upper to indicate the range in the array where the search is being performed. – lower is always one less than the start of the range – upper is always one more than the end of the range 15110 Principles of Computing, 4 Carnegie Mellon University - CORTINA 2 Algorithm 1. Set lower = ‐1. 2. Set upper = the length of the array a 3. Return BinarySearch(list, key, lower, upper). BinSearch(list, key, lower, upper): 1. Return nil if the range is empty. 2. Set mid = the midpoint between lower and upper 3. Return mid if a[mid] is the key you’re looking for. 4. If the key is less than a[mid], return BinarySearch(list,key,lower,mid) Otherwise, return BinarySearch(list,key,mid,upper). 15110 Principles of Computing, 5 Carnegie Mellon University - CORTINA Example