CSC 151.01, Class 37: Insertion Sort

Overview

• Preliminaries
  • Notes and news
  • Upcoming work
  • Extra credit
  • Questions
• Lab
• Debrief

Preliminaries

News / Etc.

• New partners.

Upcoming work

• Project due tomorrow night!
• Lab writeup for class 37: Exercise 5. Due Wednesday before class.
• Flash Cards due Wednesday at 9pm.
  • Optional.
  • Grade is percent of eight flashcard assignments you complete (capped at 100%).
• Reading for Wednesday: Merge sort

Extra credit (Academic/Artistic)

• CS Table Tuesday (topic tbd)
• PBK Scholars’ Convocation Thursday
  • Warning! PBK Convos generally involve twenty minutes of induction ceremonies.
• CLS Public Engagement in Science Symposium
  • “Engaging the Public in Science: Evolution in Action in the Classroom”. Dr. Louise Mead, Education Director of BEACON Center for the Study of Evolution in Action May 3, 4 p.m., JRC 101
  • “Using Human Examples to Teach Evolution: Recent Research in U. S. High School Classrooms” Dr. Briana Pobiner, Smithsonian’s National Museum of Natural History May 3, 7:30 p.m., JRC 101
  • Up Goer Five Challenge Poster Display May 4, 10 a.m. – 5 p.m., Bucksbaum Rotunda
  • Poster Reception May 4, 3 p.m., Bucksbaum Rotunda
  • “Our Journeys: From (Star Wars) Grinnell to Minute Physics & Minute Earth” Alex ’11 and Henry ’09 Reich May 4, 4 p.m., Faulconer Gallery

Extra credit (Peer)

• Grinnellian this Saturday, May 5 on Commencement Stage. No plays around 5pm.
• Latin-American Ensemble, Thursday at some time.

Extra credit (Recurring peer)

• Listen to KDIC Wednesdays at 6pm - Witty banter with other personalities and/or co-host. Also Indian, Arabic, and Farsi music.
  (Up to two units of extra credit.)
• Listen to KDIC Thursday at 7pm - Classic Rock. (60’s and 70’s)
• Peer editing with SS. Talk to SS about the details. Make your English Lit more literate.

Extra credit (Misc)

Other good things

• Vocal recitals Friday.
• (Lots of other music stuff, too.)

Questions

Can we debrief on the last two procedures in the reading?

• Conceptually, insertion sort divides the vector into two parts: the things that are sorted and the things that are not yet visited.
• In class, we said something like
  • Pull out the first unprocessed value
  • Find the appropriate location in the sorted section
  • Shift
  • Put it there.
• In practice, as long as we’re going to shift, we may just as well shift every element that is larger as soon as we notice it is larger.
• The insert! procedure does this shifting.
• Once we have insert!, insertion-sort! just involves calling insert! starting at every position from 1 to length-1.

What does vector-insert! return?

• Nothing; called for the side effect.

Do we use binary search?

• No; the shifting is expensive enough that it’s not helpful.

Why did we talk about vector-swap!?

• It’s useful for selection sort, another sorting routine.
• It’s also generally useful.

Why does vector-insert! take four parameters?

• vector into which we are inserting the value
• value we are inserting
• the position we just emptied (see below)
• comparator

What does it mean to have “no value” in a position in a vector?

• The “no value” is conceptual, rather than actual. There’s still a value there. We just pretend that there isn’t.

Lab

Writeup: Exercise 5.

How do I write a procedure that returns the same value it got as input?

• (lambda (x) ...)
• You should be able to figure out the ellipses.

Debrief

What do you see as the big differences between the list-based insertion sort and the vector-based insertion sort?

What ordering of elements in a list makes insertion sort fast?

What ordering of elements in a vector makes insertion sort fast?

What if we sort a “random” list or vector of elements. Will the number of steps be closer to to the fast ordering or the slow ordering? Can you estimate the number of steps?