CSC 151.01, Class 11: Reading lists and tables from files

Overview

• Preliminaries
  • Notes and news
  • Upcoming work
  • Extra credit
  • Questions
• Leftover topics from last class.
• Lab
• Debrief

Preliminaries

News / Etc.

• New partners!
• Happy Valentine’s day.
• Yesterday, someone said “I didn’t realize that I could ask you questions about the exam.” So, please know that it is okay to wander by my office and ask questions or to email questions (and to re-email them if I don’t respond).

Upcoming work

• Flash cards due TODAY at 5pm.
  • I do hope to get them summarized this week.
• Exam 1
  • Cover sheet due in class TODAY
  • Epilogue due TONIGHT
• Lab writeup for class 11, due before class Friday.
  • Exercise 6
  • “CSC 151.01 Writeup for Class 11 (Your Name)”
  • Mail to csc151-01-grader@grinnell.edu
• Readings (due before class Friday)
  • Boolean values and predicate procedures
  • Filtering lists
• Quiz Friday
  • Topics: Testing, tables, heterogeneous lists
  • Review sessions Thursday at 7pm and 8pm in Science 3821
• Homework 4 due next Tuesday
  • Today’s partners are your HW partners.

Extra credit (Academic/Artistic)

• Visit the two exhibits at the Faulconer Gallery.
• TODAY, Feb. 14 at 4:15 p.m. in Noyce 2021: Cool CS Alumni talk about inclusion, life, and career after Grinnell.
• TONIGHT, Feb. 14 at 8:00 p.m. Women and Non-Gender-Conforming People in CS (with special guests) in the CS Commons.
• TOMORROW, Feb. 15 at 7:00 p.m. in Harris. Deej the Film.

Extra credit (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.)
• Math SEPC study break 7pm today. Crafts and candy.
• Wednesday (February 14th), the Langan CAs will be co-hosting an event with SHIC about sex positivity and SHIC resources. The event will most likely take place 8:30-9:30pm in Langan first lounge. There will definitely be food.
• Peer editing with SS. Talk to SS about the details. Make your English Lit more literate.
• Midwest Swim and Dive Conference. 10am Friday to 5pm Sunday.
  • 30 minutes counts.

Extra credit (Misc)

• Participate in Islamic awareness week.
  • MAP Presentation today at 4:15 in JRC 209
  • Tomorrow at 4pm: Councilwoman at Drake
  • Friday at 4pm in CRSSJ: Turkish Delights, Henna
  • Friday at 2:15pm in CRSSJ open prayer
• Lunar New Year Festival, 6:30 p.m. Harris Gym.

Other good things

• Art House movie at 7:30 pm TONIGHT. Call me by your name.

Questions

Will you explain how to do problem 6? How about the most general case?

Yes. I plan to go over the exam on Monday. I also plan to distribute an answer key.

Do you know that exams 2, 3, and 4 are online?

Yes. Those are last semester’s. All of the old exams for this class are online somewhere. The hope is that none of the solutions are online. (But if you find the solutions online, use them, and cite them, you do get full credit.)

Can I write csv files on my Mac?

Yes. But make sure to save as a text file not as an RTF file (which is the default).

Do I have to resubmit if I included the incredibly accurate statement There’s more to life than CS in the file?

No.

There seems to be a line missing in the reading.

Yup.

> (define courses (read-csv-file "/home/rebelsky/share/csv/courses.csv"))

Can it hurt me to write “There’s more to life?”

Only if you are worried about writers cramp or exploding pens. It will not negatively affect your grade on the exam or my opinion of you.

Leftover observations from last class

sort and tally (and, soon filter), like map and reduce, do something to a list by repeatedly using another procedure.

• map applies the procedure to individual elements and gives a new list.
• reduce repeatedly applies the procedure to neighboring pairs of elements and gives a single value
• sort rearranges the list by applying the procedure to pairs of value and swapping the position of pairs that are out of order.
• tally counts elements in the list by applying a procedure that determines whether or not a value counts.

We will find that it becomes useful to focus on the procedures that map, reduce, filter, and sort use. (“First solve it for an element or a pair. Then generalize to a list with map, reduce, or sort.” Or “Solve for one, then for many.”)

• In the case of sorting, we tend to focus on comparing two values.

Make it a habit to write the four P’s for every procedure you write.

Lab

Writeup: Exercise 6

Debrief

Most of this is left for the next class.

Observation: Some of you did not have your prior lab. You will find prior labs useful for all sorts of purposes. Please ask your partner to send you the lab at the end of each class.

• We asked you to use that code in part to remind you of those issues.
  • The code you’ve written might again be useful.
  • You should therefore make sure you have a copy
• We also asked you to use that code to help you practice citing things.

Observation: Many of you found the length and then used that number in your future calculations. E.g., (drop (- 45751 5) lst). You’d be better off using the call to length. E.g., (drop (- length 5) lst).

Observation: Some of you tried to skip exercises 4 and 5 and jump straight to exercise 6. But exercise 6 requires exercises 4 and 5. If you skip exercises, you should read over them first.

;;; Procedure:
;;;   zip->city
;;; Parameters:
;;;   zip, one of those silly zip-code lists
;;; Purpose:
;;;   Exract the city
;;; Produces:
;;;   city, a string
(define zip->city
  (section list-ref <> 3))

;;; Procedure:
;;;   prepend-city
;;; Parameters:
;;;   entry, one of those silly zip lists
;;; Purpose:
;;;   Shove a copy of the city name at the front of the list
;;; Produces:
;;;   citified, a list
;;; Preconditions:
;;;   entry has the form ...
;;; Postconditions:
;;;   (cdr citified) = entry
;;;   (car citified) is a string that names the city
(define prepend-city
  (lambda (entry)
    (cons (zip->city entry) entry)))

> (cdr (assoc "Grinnell" (map prepend-city zips)))
'("50112" 41.685324 -92.630258 "Grinnell" "IA" "Poweshiek")