CSC 151 2019S, Class 21: Recursion basics lab

Overview

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

Preliminaries

News / Etc.

• Mentor sessions Wednesday 8-9 p.m., Thursday 8-9 p.m., Sunday 5-6 p.m.
  • No mentor session the Sunday at the end of break.
• Today we ground our understanding of recursion in practice.
• Today is Friday the 13th (it falls on a Wednesday this month). Be careful.

Upcoming work

• Reading for Friday: Helper Recursion.
• Epilogue due TONIGHT.
• Cover sheets due NOW.
  • There’s blank paper at the back of the room.
  • Your cover sheet is the only place “There’s more to life” belongs.
• No flashcards this week!
• Lab writeup: TBD.
• Quiz Friday: Match first names and faces. Those with multiple names will likely see both names appear.
  • If you won’t be in class on Friday, talk to me immediately after class about when you might take the quiz.

Extra Credit

I would certainly appreciate suggestions of other extra credit activities (preferably via email).

Extra credit (Academic/Artistic)

• Convocation, Thursday at 11 in JRC 101

Extra credit (Peer)

• Grinnell Singers, St. Paul Saturday, Rochester Sunday, …
• Women’s Golf, Amelia Island, last Sunday of spring break.
• Track and Field, Emory University, last weekend of spring break.

Extra credit (Wellness)

• Sub-free spring break

Extra credit (Wellness, Regular)

• 30 Minutes of Mindfulness at SHACS every Monday 4:15-4:45
• Any organized exercise. (See previous eboards for a list.)
• 60 minutes of some solitary self-care activities that are unrelated to academics or work. Your email reflection must explain how the activity contributed to your wellness.
• 60 minutes of some shared self-care activity with friends. Your email reflection must explain how the activity contributed to your wellness.

Extra credit (Misc)

Other good things

Questions

Quick survey

On a white index card, write your name and one of these three descriptions:

• Methodical - You find it best to slowly work through homework problems.
• Zoomer - You feel like you know the material so well that you can (and want to) zoom through problems.
• In between - Self descriptive.

I will do my best to partner people with similar classifications.

Lab

Some observations about Racket that may be helpful

> (sqr 3)
9
> (+)
0
> (*)
1

“I’m uncomfortable that the product of the empty list is 1.”

Then stop your recursion when the list length is 1.

Writeup: Exercise 7

Debrief

How to check whether a list has one element …

• The bad way (= (length lst) 1)
• The good way (and (not (null? lst)) (null? (cdr lst)))

(define largest
  (lambda (lst)
    (cond
      [(singleton? lst) ; has one element, not implemented yet
       (car lst)]
      [(< (car lst) (cadr lst))
       (largest (cdr lst))]
      [else
       (largest (cons (car lst) (cddr lst)))])))