CSC151.02 2014S, Class 25: Recursion Basics, Continued
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_Overview_

* Preliminaries.
    * Upcoming work.
    * Admin.
    * Questions on HW6.
* Reflection.
* Questions.
* Lab.

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

### Upcoming Work

* Reading for Tuesday: Recursion with Helper Procedures
* Today's lab writeup: Exercises 4 and 5
    * CSC 151.02 Writeup 16: Recursion Basics
* HW6

### Admin

* Kim will be teaching class Tuesday.  Thanks Kim!
* Mr. Stone will be teaching class Wednesday and Friday.
* Exam 2 will be distributed on Wednesday.  
* No quiz on Friday!
* Choose your own (new) partners tomorrow.  Strive to find someone you have
  not worked with before.
* Extra credit for volunteers to do eboards on Tuesday, Wednesday, and Friday.
    * KS, Tuesday
    * CS, Friday
* Extra credit: 
    * Convocation Wednesday, March 5.
    * Orchestra 
        * With additional Narration and Animation, Wednesday, March 5
        * 7:30 p.m. in Sebring-Lewis
    * Grinnell image presentations at noon on Thursday and Friday.
    * First Years: How to Use Your Summer, 8-9 pm on March 6 in JRC 101.
    * Balancing Acts
        * Friday, 7 March at 7:30 in Flanagan
        * Saturday, 8 March at 7:30 in Flanagan
        * Sunday, 9 March at 2:00 in Flanagan
    * Neverland Players this weekend
        * Friday, 7 March at 7:00 in the Wall
        * Saturday, 8 March at 2:00 in the Wall
        * Saturday, 8 March at 7:00 in the Wall
        * Sunday, 9 March at 2:00 in the Wall
    * Men's Tennis, Saturday, 8 March (8 am or 4pm)

### Questions on HW6

Should every procedure have documentation?

> Yes.  You can be casual on preconditions and postconditions

Will our notes suffice for 3 and 4?

> Yes, provided I remember to tell the graders.

What should the inputs to problem 4 look like?

> It's a program, so it doesn't really need inputs.  Just show me the sequence
  of procedure calls to generate the image.  E.g.,

        (define canvas (image-new 400 400))
        (render-polar-rose-on-image! canvas 50 23 "blue")

Reflection
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What did you learn on Friday?

* Recursion
* Using a function within a function so as to achieve repetition
* Purpose: Achieve repetition
* Technique: Implement a procedure by making that procedure call itself
  (some of the time)
    * If the condition holds, call itself
    * If the condition does not hold, stop
    * Or vice versa
* If we're working with a list, we can throw away an element of the list each
  time and stop when there's nothing left in the list.
* Why not just use map?
     * Can't have the condition part; always deals with every element in the list
     * map only produces a list; sometimes we want to produce values other than
       a list.
* General form of a recursive procedure

    (define fun
      (lambda (vals)
        (if (TEST)
            (fun  )
            BASE-CASE))) 

Questions
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Lab
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