CSC151.02 2013F, Class 36: Vectors
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_Overview_

* Admin.
* Rethinking lists: Good aspects, bad aspects.
* An Alernative: Vectors.
* Yay!  A new data type!
* Behind the scenes: How Scheme implements vectors.
* Important vector procedures.

_Admin_

* Today's writeup: Exercise 6
* New assignment: Mostly building color trees.
* Review session needed.  Yes!
* Upcoming extra credit opportunities:
    * Any one Grinnell prize event this week.
    * Thursday extras this week: Reports from internships (I think)
    * Grinnell Town Hall Meeting, Nov 13 noon or 7:30, I believe
    * CS Table, Friday: TBD (it's been a bad week)
    * Orchestra, Saturday, 7:30 in SL
    * 24-hour theatre and dance production, 7:30 Saturday
    * Drag Show
    * Some other love your body week event
    * Alumni Basketball game 3pm Saturday at Bear 
* Cool talk on supernovas 7:30 in ARH 302

Rethinking lists
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* They collect values and let you do things to each value!
* Extensible!
* And shrinkable
* ANd reversable
* But ...
    * Slow to access the nth element
    * Hard to change the middle

A solution: Vectors
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* Collects values!
* Fast to access the nth element!  
* Can change the nth element!
* But not easily growable or shrinkable.
    * "Not easily" == "Computational expensive."
* About has hard to reverse as a list
    * Although semantics issues - Does vector-reverse create a new
      vector or just mutate a vector?
        * Mutates!  Mutation is good.  It's the point.
    * Depends on what you name it
        * vector-reverse
        * vector-reverse!

Thinking about a new type!
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* What's its name?  Vectors
* What do we use it for?  Store collections of values, using indices
* What are situations in which we use them?
    * To keep track of all the images we've created
    * To keep track of students by id number
* How do we express them? / How are they created?
    * (vector val val val)
    * #(val val val)
    * (make-vector n val)
* What procedures can we use?
    * (vector-ref vec pos) - gets a value at a position
    * (vector-set! vec pos newval) - sets a value
    * Is there a non-mutation vector-set?  No.  You can write it.
* How does Racket present them to us?
    * '#(...)
* How are they different from other types?
* How do we do recursion with them?
    * Recurse over position
* How do we break them?

Specific to vectors
* What kinds of values can we put in them?  Any!

Behind the scenes: How Scheme implements vectors
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