CSC207.01 2014S, Class 47: Heap Sort ==================================== _Overview_ * Preliminaries. * Admin. * Upcoming work. * Questions. * Heap sort. * Can we do better? * Lab. Preliminaries ------------- ### Admin * Sad Earnest. * Don't forget to register! * I need two graders for next semester who can do detailed comments. Yes, I will provide training. * I've heard concerns that the Eclipse formatter rearranges comments. I *think* the current style avoids that problem. ### Upcoming Work * Reading for Friday: No reading. * Today's writeup: Exercise 5. * Part 2 of the project is due Friday at 10:30 p.m. (No, I'm not updating the Web page.) ### Extra Credit * Today's Town Hall at noon and 7:30 on Work/Life balance. * One pride week activity. * Math extra Thursday: Sphere Packing. * CS Table Friday: Heartbleed. * Women's Track and Field Friday at Simpson. * Iowater project April 26 - Tag drains. Mail iowater@grinnell.edu for details. ### Questions Remaining Questions from Heaps ------------------------------ * Given the index of an element, what are the indices of its left child, right child, and parent? * Left child: 2*i + 1 * Right child: 2*(i+1) * Parent: (i-1)/2, round down * Why do we care about the indices of the children? * We can actually store it in the array. * For swapping down after removal * Why do we care about the index of the parent? * For swapping up after insertion Heap Sort --------- Repeatedly remove largest and shove at end. * Each time we grab the largest and shove at the end is O(logN) * The overall routine is O(NlogN) * Why is this better than other NlogN sorting algorithms? * Quicksort: Heap sort is predictable, Quicksort is not * Merge sort: This sorts in place! Merge sort does not. * Skip list sort: Heap sort is predictable. Skip list sort is not And uses lots of overhead. Can we have a better implementation of priority queues? ------------------------------------------------------- * No. Lab --- Writeup number 5!