CSC207.01 2014S, Class 35: Priority Queues and their Basic Implementation
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_Overview_

* Preliminaries.
    * Admin.
    * Questions.
    * HW6.
* A few notes on priorities.
* Lab.

Preliminaries
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### Upcoming work.

* No writeup today!
* No reading for Friday.
* Exam due Friday!

### Admin

* Pop Tarts!
* Consider running for the SEPC.
* Extra credit:
    * Convocation, today, noon: Elizabeth Kolbert (the Sixth Extinction)
    * Technology in the liberal arts symposium on Thursday.
    * CS table Friday: The Performance Contract, which is *highly relevant
      to this class*.
    * Dance in Chicago this weekend.

### Questions on the exam

_What do you mean by "You can treat the height of the skip list as constant"_

> If your algorithm is O(height), it's O(1).  (If you think carefully, 
  height is about log_2(n).

_How do we call the order comparator in problem 1?_

   int order = order.compare(val1, val2);
   if (order < 0) 
     {
       // Code for "val1 precedes val2"
     } // (order < 0)
   else if (order == 0)
     {
       // Code for "the values are the same"
     } // (order == 0)
   else // (order > 0)
     {
       // Code for "val1 follows val2"
     } // (order > 0)

_How do we get the last problem to work if we adding/removing elements cause
iterators to fail?_

> Use `set` rather than `add` or `remove`.

### A Summary of HW6

Implement queues with arrays, with wrapping.

Implement the "pretty output" parenthesis matching algorithm.

Implement an RPN calculator.

* Reverse of Scheme.  Operands appear before the operations
     * To add 3 4, "3 4 +"
* Why?
     * Complete unambiguous; no order of operations
     * Incredibly easy to implement with stacks


A few notes on priorities
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* For priority queues, we use comparators to determine what the highest
  priority element is.
* Hidden assumption that we almost never write as a precondition
     * Transitivity: if a < b and b < c then a < c
     * Reflexitivity: if a < b then b > a
     * Complete: Any two elements are comparable

Lab
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