University:
Boston University
Course:
QST SM 101 | Introduction to Management
Academic year:

2021
Views:

376

Pages:

24
Author:

Susan Shepherd

Simple Sorting Algorithms Review of Quick Sort Pick a pivot, arrange other elements based on if they’re greater or less than the pivot Repeat for each smaller group until the whole thing is sorted Usually O(nlogn), absolute worst case is O(n2) 2 Bubble Sort Compare each element (except the last one) with its neighbor to the right If they are out of order, swap them This puts the largest element at the very end The last element is now in the correct and final place Compare each element (except the last two) with its neighbor to the right If they are out of order, swap them This puts the second largest element next to last The last two elements are now in their correct and final places Compare each element (except the last three) with its neighbor to the right Continue as above until you have no unsorted elements on the left 3 Example of Bubble Sort 7 2 8 5 4 2 7 5 4 8 2 5 4 7 8 2 4 5 7 8 2 7 8 5 4 2 7 5 4 8 2 5 4 7 8 2 4 5 7 8 2 7 8 5 4 2 5 7 4 8 2 4 5 7 8 2 7 5 8 4 2 5 4 7 8 (done) 2 7 5 4 8 4 Can you guess its Big-O notation? 2 O(n ) 5 Selection sort Given a list of length n, Search elements 0 through n-1 and select the smallest Swap it with the element in location 0 Search elements 1 through n-1 and select the smallest Swap it with the element in location 1 Search elements 2 through n-1 and select the smallest Swap it with the element in location 2 Search elements 3 through n-1 and select the smallest Swap it with the element in location 3 Continue in this fashion until there’s nothing left to search 6 Analysis of Selection Sort 7 2 8 5 4 2 7 8 5 4 2 4 8 5 7 Analysis: The outer loop executes n-1 times The inner loop executes about n/2 times on average (from n to 2 times) 2 4 5 8 7 Work done in the inner loop is constant (swap two array elements) 2 4 5 7 8 Time required is roughly (n-1)*(n/2) You should recognize this as O(n2) 7 Invariants for Selection Sort For the inner loop: This loop searches through the array, incrementing inner from its initial value of outer+1 up to a.length-1 As the loop proceeds, min is set to the index of the smallest number found so far Our invariant is: for all i such that outer <= i <= inner, a[min] <= a[i] For the outer (enclosing) loop: The loop counts up from outer = 0 Each time through the loop, the minimum remaining value is put in a[outer] Our invariant is: for all i <= outer, if i < j then a[i] <= a[j] 8 Insertion sort From left to right, go through each element in the list. If it is smaller than the element to its left, check all the elements you’ve already done to see where it belongs. When you find the right place, insert it between the element that is bigger than it and the element that is smaller. 9 10 Analysis of insertion sort We have to check each of the n elements On average, there are n/2 elements already sorted The inner loop looks at (and moves) half of these This gives a second factor of n/4 Hence, the time required for an insertion sort of an array of n elements is proportional to n2/4 Discarding constants, we find that insertion sort is O(n2) 11 Mergesort Divide array into two halves. A A L L G G O O R R I T I H T M H S M S divide Mergesort Divide array into two halves. Recursively sort each half. A L G O R I T H M S A L G O R I T H M S divide A G L O R H I M S T sort Mergesort Divide array into two halves. Recursively sort each half. Merge two halves to make sorted whole. 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