---
title: Eboard 24  Patterns of list recursion
number: 24
section: eboards
held: 2019-04-03
link: true
---
CSC 151 2019S, Class 24:  Patterns of list recursion
====================================================

_Overview_

* Preliminaries
    * Notes and news
    * Upcoming work
    * Extra credit
    * Questions
* Looking for patterns
* Patterns of list recursion (extreme, reduce, etc.)
* List predicates

Preliminaries
-------------

### News / Etc.

* Oreos! (Up to 3)
* Mentor sessions Wednesday 8-9 p.m., Thursday 8-9 p.m., Sunday 5-6 p.m.

### Upcoming work

* Homework 7 due Tuesday.  Partners to be assigned this afternoon (sorry)
* Friday's quiz: Recursion
* Reading for Friday: See the schedule
* No flash cards this week.
* Lab writeup: Problem 5

### Extra Credit

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

#### Extra credit (Academic/Artistic)

* Convo Thursday: Microbes: The links between soil, gut, and health
* Noura Mint Seymali Concert tonight at 7:30 p.m. in Herrick
* Isabelle Demers concert, Saturday at 3:00 p.m. in Herrick

#### Extra credit (Peer)

* Grinnell Singers, Sunday the 7th at ???
* Women's golf, Saturday/Sunday somewhere in St. Louis
* Track and Field, Saturday at Cornell College (up the road, not in NY)
* Open mic night at Bob's.

#### Extra credit (Wellness)

#### 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)

* Wednesday the 10 at 4pm on Mac Field: Giant Laurel Leaf.  (Free t-shirt!)
* Scarlet and Give Back Day next Wednesday/Thursday (I think).  If you
  don't have money to donate, let me know and I will give you $5 to donate.

### Other good things 

### Questions

Lab
---

Ignore the zip code problems.  Sorry about that!

Lab writeup: Problem 5

Debrief
-------

Note: `and` and `or` stop as soon as they know their result.

* `(and ... #f .....)` stops at the #f.
* `(or ... #t ...)` stops at the #t.

Generalizing reduce

    (define REDUCE-PROC
      (lambda (lst)
        (if (null? (cdr lst))
            (car lst)
            (PROC (car lst) (REDUCE-PROC (cdr lst))))))

    (define REDUCE
      (lambda (proc lst)
        (if (null? (cdr lst))
            (car lst)
            (PROC (car lst) (REDUCE proc (cdr lst))))))

Extending `contains-needle?` for nested lists

    ; Using our pattern
    (define contains-needle?
      (lambda (lst)
        (and (not (null? lst))
             (or (eq? 'needle (car lst))
                 (contains-needle? (cdr lst))))))

    ; Rewritten less elegantly, but perhaps more helpfully
    (define contains-needle?
      (lambda (lst)
        (cond
          [(null? lst) 
           #f]
          [(eq? (car lst) 'needle) 
           #t]
          [else
           (contains-needle (cdr lst))])))

    ; Add: Check in nested lists
    (define contains-needle?
      (lambda (lst)
        (cond
          [(null? lst) 
           #f]
          [(eq? (car lst) 'needle) 
           #t]
          [(list? (car lst))
           (or
            (contains-needle (car lst))
            (contains-needle (cdr lst)))]
          [else
           (contains-needle (cdr lst))])))

What do you think about this solution for `smallest`?

    (define smallest
      (lambda (lst)
        (cond
          [(null? (cdr lst))
           (car lst)]
          [(< (car lst) (smallest (cdr lst)))
           (car lst)]
          [else
           (smallest (cdr lst))])))

We discover that `(smallest (range 100))` is really fast and
`(smallest (range 30))` is unbearably slow.

    > (time (smallest (reverse (range 25))))
    cpu time: 968 real time: 966 gc time: 8
    0
    > (time (smallest (reverse (range 26))))
    cpu time: 1908 real time: 1905 gc time: 8
    0
    > (time (smallest (reverse (range 27))))
    cpu time: 3780 real time: 3781 gc time: 8
    0
    > (time (smallest (reverse (range 28))))
    cpu time: 8240 real time: 8240 gc time: 24
    0