CSC 151.01, Class 34: Binary search

Overview

• Preliminaries
  • Notes and news
  • Upcoming work
  • Extra credit
  • Questions
• Tutor/mentor evaluations
• Lab

Preliminaries

News / Etc.

• New partners!
• Note: You do not get EC if you participate in an event. The EC is for people who come to support you.
• I apologize for missing Friday’s PSA. I hope that you all got sufficient sunshine on Sunday!
• We have tutor/mentor evaluations today. Please take them seriously; they help us improve the program.
• SamR will, on occasion, write helper code for your projects.

Upcoming work

• Project Proposal due TONIGHT at 10:30 p.m.
• Lab writeup for class 34: Exercise 4. Due Wednesday.
• No reading for Wednesday.
• Flash Cards due next Wednesday at 9pm.
  • Optional.
  • Grade is percent of eight flashcard assignments you complete (capped at 100%).

Extra credit (Academic/Artistic)

• Diversity in Filmmaking, Monday, 4:15 p.m., Bucksbaum 152
• Squire Lecture in Physics, Tuesday at 11:45: STEM Eqwuality and Inclusion, a Female Astronomer’s View. Noyce 1023.
• CS Table Tuesday noon in Day PDR: One Laptop Per Child.
• The Cypher Paradigm: Closing Cypher, Tuesday, 8:30 p.m., ARH 120.
• Vance Byrd Book Talk Thursday at 4:15 pm, Burlilng 1st Floor Lounge

Extra credit (Peer)

• Escape Room, 7-9pm TONIGHT, Gates
• Thursday, April 26th, Noyce 2022: Mellon Mays Project introductions, 4:15 to 6:30 p.m.
• Friday, 8pm, Main: Contra Dance
• Dance Ensemble Thursday 7:30pm, Friday 7:30pm, Saturday 7:30pm, Sunday 2pm. Roberts. Tickets go “on sale” today.
• Open Mike Thursday 9-11pm in Bob’s.

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)

• Any Sexual Assault Awareness Month event.

Other good things

Questions

How should I have approached problem 1 on the quiz?

• Problem 1 on the quiz was a “file recursion” problem. How do you process all of the data in a file?
• You need to do all of the following
  • Open the file for reading. open-input-file, parameter is the name of the file.
    • The name of the file is a string. If you are giving a particular file, put it in quotation marks. If not, just use the variable name.
  • Read data again and again and again with a recursive kernel. There are three procedures
    • peek-char to look at the next character but not “consume” it. (You will still have to read or you will recurse forever.)
    • read-char, to read the next character
    • read, to read a Scheme value. (number, string, symbol, list)
  • Stop when you reach the end of the file. The thing you read is an eof-object.
  • Close the port.
  • Return the value you computed (potentially with a little post-processing)

(define file->nums
  (lambda (fname)
    (let ([inport (open-input-file fname)])
      (let kernel ([numbers null])
        (let ([next (read inport)])
          (cond
            [(eof-object? next)
             (close-input-port inport)
             (reverse numbers)]
            [(number? next)
             (kernel (cons next numbers))]
            [else
             (kernel numbers)]))))))

Can you explain this get-key thing?

• When we are searching a list of objects, there is some characteristic of each object that we are looking for.
• get-key is the procedure that extracts that piece of information from each object.
• If we are searching a list of students for someone with a particular name. get-key extract the name.
• If we are searching a list of students for someone with a particular major. get-key extract the major.

Tutor/Mentor Evaluations

Thank you for taking the time to fill these out.

Lab

Writeup: Exercise 4.

Hint: Try running the algorithm by hand on a three-element (or four-element vector).

• Vector: 0: Sally, 1: Sam, 2: Shelby
• Search for Ella
  • lb: 0, ub: 2. midpoint is 1. That’s Sam. Sam comes after Ella. So the new ub is midpoint-1, or 0.
  • lb: 0, ub: 0. midopint is 0. That’s Sally. Sally comes after Ella. So the new ub is midpoint-1, or -1
  • lb: 0, ub: -1. Stop. We want to return -1/2.
• Search for Titus.
  • lb: 0, ub: 2. midpoint is 1. That’s Sam. Sam comes before Titus. So the new lb is midpoint+1, or 2.
  • lb: 2, ub: 2. midpoint is 2. That’s Shelby. Shelby comes before Titus. So the new lb is midpoint+1, or 3.
  • lb: 3, ub: 2. Stop. We want to return 2.5 (or 2 1/2)
• Search for Sarah
  • lb: 0, ub: 2. midpoint is 1. That’s Sam. Sam comes before Sarah. So the new lb is midpoint+1, or 2.
  • lb: 2, ub: 2. midpoint is 2. That’s Shelby. Shelby comes after Sarah So the new ub is midpoint-1, or 1.
  • lb: 2, ub: 1. Stop. We want to return 1.5 (or 1 1/2)
• Do you see a pattern?