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Week 3 comments, questions

Course:  An Introduction to Common Lisp
Instructor:  jgw 
Book:
"COMMON LISP: A Gentle Introduction to Symbolic Computation"
by David S. Touretzky copyright 1990



ANSWERS to questions assigned
===================================
7.2 Let the global variable DAILY-PLANET containt the following table:
((olsen jimmy 123-76-4535 cub-reporter)
 (kent .... etc. etc.
 ... Use MAPCAR on this table to extract a list of social security 
numbers.
 
Answer: (MAPCAR #'third DAIly-PLANET)


7.7  Write a function that takes a list such as (UP DOWN UP UP) and 
"flips" each element, returning (DOWN UP DOWN DOWN).  Your function should 
include a lambda expression ...

Answer:
>(defun UP-DOWN (n) (OR (if (equal n 'UP) 'DOWN)(if equal n 'DOWN) 'UP)))
>UP-DOWN
>(mapcar #'UP-DOWN '(UP DOWN UP UP))

[I couldn't figure out how to use LAMBDA.  Tried (mapcar #'lambda UP-DOWN 
'(UP DOWN UP UP )), etc.]



7.14 Show how INTERSECTION and UNION can be written using REMOVE-IF and 
REMOVE-IF-NOT.

(defun my-intersection (x y) (remove-if-not #'(lambda (e) (member e y)) 
x))

(defun my-union (x y) (remove-if #'(lambda (e) (member e x)) y))) 
[???? doesn't work]
try
(defun my-union (x y) (append x (remove-if #'(lambda (e) y (member e x)) 
y)))

7.24  What is an applicatie operator?
Answer:  a FUNCALL

7.25  Why are LAMBDA expressions useful? 
Answer:  Because they hide a function which makes them invaluable where 
two or more values are called and one must be defined by a lambda 
function.  They're a pain-in-the-a#'#' though.





7.30  ... write an expression to return a trilingual dictionary ... 

Answer:  (mapcar #'(lambda (x y) (list x (last y))) 
'((one un)(two deux)(three trois)(four quatre)(five cinq))
'((one uno)(two dos)(three tres)(four quatro)(five cinco))







QUESTIONS about reading material
=================================

pg 210, last line:  
(remove-if-not #'oddp '(2 0 -4 6 -8 10)) [=>] NIL
   Does that mean 'remove-if-not' returns a NIL if no conditions are met?  
   So would:
 (remove-if-not #'oddp '(2 0 -4 6 3 8 10)) => 3 
 be right (be T)?
 
 
 on pg 211,  if the above output is 'NIL' should the following be NIL 
instead of 0:
 
 >(defun count-zeros (x)
   (length (remove-if-not #'zerop x)))

>(count-zeros '(1 2 3 4 5)) => 0

  It's a predicate, and there are no zeros and in the above example there 
are no odd numbers.  What's the difference?





COMMENTS
=================================
pg 206
"Throughout this book we will refer to the objects you get back from a 
#'(LAMBDA ...) expression as lexical closures."  [Pretty important, don't 
you think?]


This LAMBDA thing looks like one of the more important concepts covered in 
the book.  If anybody has brief sentence or two description to throw in 
I'd find it helpful.  I see a Lambda function as a predefined function 
without a handle.  Ahem!  A 'lexical closure' without a close.