CSC151 2015S, Class 35: Geometric Art Through Numeric Recursion
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* Same Partners!

_Overview_

* Preliminaries.
    * Admin.
    * Upcoming Work.
    * Extra Credit.
    * Questions.
* Quick notes on named let.
* Quiz!
* Lab.

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

### Admin

* Welcome to our junior visitors!
* Eboards remain available and linked.
* Friday PSA.
    * Remember that the Romans were paragons of moderate behavior.
    * Alternately: Remember that Rome fell.
    * Please take care of yourselves and your colleagues.

### Upcoming Work

* Lab writeup: Exercise 5.  
  <http://bit.ly/151-2015S-lab35>
* Reading for Monday: 
    [Pairs and Pair Structures](../readings/pairs-reading.html)
* HW7 due Tuesday.
  Choose your own partners.

### Extra Credit Opportunities

#### Academic 

* Scholars Convocation Wednesday: Alison Bechdel.

#### Peer Support (Afternoon Section)

* Wednesday, April 8, 4:30 Psych Job Talk Series, Senior Research
  Analyst from ACT
* April 16 BCA Dance Studio, Contra Dance Workshop.
* April 17 Salsa Showcase.
* April 18 Drag Show.
* April 24 Contra Dance in Main Hall

#### Miscellaneous

### Other Good Things (no extra credit)

### Questions

Quick notes on named let
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Common mechanism for defining recursive functions with helpers.
Example: Tally all the positive numbers in a list.

Example input: '(4 -1 2.3 -4)

First step, set up a table

        tally           remaining
        -----           ---------
        0               '(4 -1 2.3 -4) ; first thing is positive
          add 1            Throw away the first thing
        1               '(-1 2.3 -4) ; First thing is negatve
          do nothing       Throw away the first thing
        1               '(2.3 -4) ; First thing is postive
          add 1            Throw away the first thing
        2               '(-4) ; First thing is postive
        ...

Generalize.  Suppose our input is `lst`.

        tally           remaining
        -----           ---------
        0               lst

Using letrec

        (define tally-positive
          (lambda (lst)
            (letrec ([kernel (lambda (tally remaining)
                               (cond
                                 [(null? remaining)
                                  tally]
                                 [(positive? (car remaining))
                                  ...]
                                 )_))
                (kernel 0 lst))))

There's a huge gap between how we thought about it and how we have
to express it.  Solution?  Redesign the language.

        (define tally-positive
          (lambda (lst)
            (let kernel ([tally 0]
                         [remaining lst])
               (cond
                 [(null? remaining)
                  tally]
                 ...))))


        

Quiz
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Lab
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