CSC151 2007S, Class 05: Symbols and Lists

Admin:
* Are there questions on HW3?
  * Yes, it is the case that the final does not count in (approximately)
    part c.

Overview:
* Reminder: Key parts of an algorithm.
* Symbolic values.
* Lists.
* Lab.

/What might we see or care about in an algorithm?/

* Variables - Named values
  (define NAME VALUE)
* The basic operations on those values
  * Generic
    * Untie a twisty-tie
    * Add two numbers
  * In Scheme
    * Basic numeric operations
	  (+ NUM NUM)
	* Other numeric operations
	  expt, log, max, min
	* Operations on strings:
	  string-append
	* Lists and their operations
	* Symbols and their operations
* Sequences of operations
  * For one operation: (OPERATION OPERAND OPERAND ...)
  * For more than one operation: Just put them in order.
  * When applying a procedure to other expressions, the inner
    expressions are computed first
	(+ (* 2 3) (* 4 5))
* Conditionals to decide between sequences of operations
* Loops to repeat operations
* Subroutines to group and parameterize operations

/What might we expect to see in a Scheme program?/
* Parentheses
* Lists

/Symbols/
* Look a lot like names
* All meaning is external
* Write with 'cat
  * cat and 'cat are different
* What can you do with symbols?
  * Check if they are equal.

/Lists/
* Value of a list is "a sequenced bunch of values"
* Core operations:
  * (cons VALUE LIST) - shove VALUE on the front of LIST, making a new list
    (cons 'a (list 'b 'c)) => (a b c)
  * (car LIST) - get the first element
  * (cdr LIST) - get all but the first element
    * Detour mnemonic - a way to represent something that's easy to remember
  * null - the name of the empty list
  * (null? LIST) - is list empty?
* Advanced operations
  * (list VAL0 VAL1 ... VALn) - build a list more easily
  * (reverse LIST)
  * (list-ref LIST POS) - extract the thing at the given position
  * (length LIST) - how many elements are in the list?
* How do we make a list of values, where the values are computed?
  (list (+ 23 b) (- 156 a))

/Reflection/

* Reminder: Reading for tomorrow (numbers!)
* Scheme has another mechanisms for sequencing
  In (OP (OP OPERAND) (OP OPERAND)), the inner operations are
    performed first.
* How do you build (a (b c) (d (e)))