CSC151, Class 21: Recursion Revisited Revisited

Admin:
* Ignore this line, it is a test
* EC Talk, Monday, 4:15, Civil War Mourning, ARH 102
* EC Talk, Today, 4:30, Science 3821, DrFu
* Other good things for you to do at 11 am
  * See Gene Gaub play piano
  * See David Harrison talk about International Studies

Goals for Today:
* Finish Tuesday's lab
* Finish Monday's lab

Things you should think about when writing recursive procedures
* Think about the return value in the base case
  * In particular, make sure that it's the right type of thing
  * If you are writing: contains-a-color?
    * The base case should probably be #t or #f
  * If you are writing: (count-colors lst), that counts the number
    of colors in a list
    * The base case should probably be a number
    * For the empty list, that number should probably be 0
  * If you are writing: (darkest-color lst), that finds the darkest
    color in a list of ONLY colors
    * Return a color (not a list, not a boolean, not a number, but a color)\
    * When base case?  When there's only one element in the list
    * What base case?  That color
    * What precondition:
      The list must have colors (ooh, we can use contains-a-color?)
* Sam's reference page is wrong, or so someone claims
  * cddr vs caddr
  * What should (cddr (list 1 2 3 4 5)) be? (3 4 5)
  * What should (cddr (list 1)) be? "There's only one thing, so ..." ERROR
    (or, "Success")
  * What should (cddr (list 1 2)) be?  The empty list
  * Nope, it works JUST AS SAM CLAIMED!  
  * But what the nice friendly successful students wanted was the
    third element of the list.
    * THe cddr is a list, not an elemnt
    * The car of the cddr is the third element of the lists: caddr