Average Running Time

The best case running time of insertion sorting is O(n) but the worst-case running time is . Therefore, we might suspect that the average running time falls somewhere in between. In order to determine it, we must define more precisely what we mean by the average running time. A simple definition of average running time is to say that it is the running time needed to sort the average sequence. But what is the average sequence?

The usual way to determine the average running time of a sorting algorithm is to consider only sequences that contain no duplicates. Since every sorted sequence of length n is simply a permutation of an unsorted one, we can represent every such sequence by a permutation of the sequence . When computing the average running time, we assume that every permutation is equally likely. Therefore, the average running time of a sorting algorithm is the running time averaged over all permutations of the sequence S.

Consider a permutation of the sequence S. An inversion  in P consists of two elements, say and , such that but i<j. That is, an inversion in P is a pair of elements that are in the wrong order. For example, the permutation contains three inversions--(4,3), (4,2), and (3,2). The following theorem tells us how many inversions we can expect in the average sequence:

Theorem  The average number of inversions in a permutation of n distinct elements is n(n-1)/4.

extbfProof Let S be an arbitrary sequence of n distinct elements and let be the same sequence, but in reverse.

For example, if , then .

Consider any pair of distinct elements in S, say and where . There are two distinct possibilities: Either , in which case is an inversion is ; or , in which case is an inversion is S. Therefore, every pair contributes exactly one inversion either to S or to .

The total number of pairs in S is . Since every such pair contributes an inversion either to S or to , we expect on average that half of the inversions will appear in S. Therefore, the average number of inversions in a sequence of n distinct elements is n(n-1)/4.

What do inversions have to do with sorting? As a list is sorted, inversions are removed. In fact, since the inner loop of the insertion sort method swaps adjacent array elements, inversions are removed one at a time! Since a swap takes constant time, and since the average number of inversions is n(n-1)/4, the average running time for the insertion sort method is .

Copyright © 1998 by Bruno R. Preiss, P.Eng. All rights reserved.