13.6. The Cost of Exchange Sorting¶
13.6.1. The Cost of Exchange Sorting¶
Rewrite along these lines: Here are two measures of "out of order": inversions and min-swaps. Selection sort (especially w/ optimization) meets min-swaps, but that's not a useful measure in general. Insertion sort tracks inversions, it is I + n. Now, if we had an exchange sort, what would cost be? Go on to the proof.
Here is a summary for the cost of Insertion Sort, Bubble Sort, and Selection Sort in terms of their required number of comparisons and swaps in the best, average, and worst cases. The running time for each of these sorts is \(\Theta(n^2)\) in the average and worst cases.
The remaining sorting algorithms presented in this tutorial are significantly better than these three under typical conditions. But before continuing on, it is instructive to investigate what makes these three sorts so slow. The crucial bottleneck is that only adjacent records are compared. Thus, comparisons and moves (for Insertion and Bubble Sort) are by single steps. Swapping adjacent records is called an exchange. Thus, these sorts are sometimes referred to as an exchange sort. The cost of any exchange sort can be at best the total number of steps that the records in the array must move to reach their "correct" location. Recall that this is at least the number of inversions for the record, where an inversion occurs when a record with key value greater than the current record's key value appears before it.