;;; =======================================================
;;;  CMPU-101, Spring 2013
;;;  Assignment 4
;;;  DUE DATE:  March 8th, by 8 pm 
;;; =======================================================

(require 2htdp/image)

(display "\n         CS101 Assignment 4, Spring 2013")
(display "\n          PLEASE WRITE YOUR NAME HERE\n\n")


;; Fill in the function definitions within the boundaries 
;; provided. Use the design recipe and follow the grading
;; criteria posted along with this assignment.

; 
; Problem 1:
; 
;  A list of anything (LOX) is either:
;  1. empty, or
;  2. it's a (cons x lix), where x is anything and lix     
;     is an LOX one element smaller than (cons x lix).
; 
; 
; Write a function called LAST that consumes an LOX and 
; returns the last element in the list. 
; 
; If the input list is empty, the function should return 
; an error message: 
; 
;       "Error in last: No elements in list."
;       
; A note of caution: If the recursion goes all the way
; to the empty list, the same string will be returned 
; for every list. Therefore, this function may require 
; more than one base case so that it stops before becoming
; empty.
;          

(display "\n\n----------------------------\n")
(display "Problem 1: LAST")
(display "\n----------------------------\n")

;; Contract: 
;; Header:          
;; Purpose:  

;;; Pre-function tests (given to help you understand problem):
;(check-expect (last (list "Tom" "Dick" "Harry")) "Harry")
;(check-expect (last (list 2 "four" 5 "six" 7 1)) 1)
;(check-expect (last empty) "Error in last: No elements in list.")
;
;;; Function definition:
;(define last 
;   
;   
;  

; Post-function printfs:

(newline)
(newline)

; 
; Problem 2:
; 
;  A list of numbers (LON) is either:
;  1. empty, or
;  2. it's a (cons n lin), where n is a number and lin     
;     is an LON one element smaller than (cons n lin).
; 
; 
; Write a function called GENLIST-N that takes in a
; non-negative (i.e., 0 included) natural number n and 
; returns a LON that contains all the numbers from 1 to n, 
; in that order, from left to right.
; 
; Write this function using a local accumulator function 
; inside the GENLIST-N function so the initial value of 
; the accumulator is hidden from the caller of the function. 
; 
;          

(display "\n\n----------------------------\n")
(display "Problem 2: GENLIST-N")
(display "\n----------------------------\n")

;; Contract: 
;; Header:        
;; Purpose:  

;;; Pre-function tests (given to help you understand problem):
;(check-expect (genlist-n 3) (list 1 2 3))
;(check-expect (genlist-n 6) (list 1 2 3 4 5 6))  
;(check-expect (genlist-n 0) empty)
;
;;; Function:
;(define genlist-n 



;;; Post-function printfs:

(newline)
(newline)

; Problem 3:
; 
;  Write a function called filter-nums that consumes   
;  a LOX and produces a LON containing only the numbers   
;  in the input list.
;  
;  See data definitions for LOX and LON in problems
;  1 and 2. You do not need to repeat those definitions     
;  for this problem.
; 

(display "\n\n----------------------------\n")
(display "Problem 3: FILTER-NUMS")
(display "\n----------------------------\n")

;; Contract: 
;; Header:          
;; Purpose:  

;; Pre-function tests (given to help you understand problem):
;(check-expect (filter-nums (list 42 "45" "joe" 0)) (list 42 0))
;(check-expect (filter-nums (list "78" 'jack 'joe 'zero)) empty)
;(check-expect (filter-nums (list 0 -23 90 'hi)) (list 0 -23 90))
;(check-expect (filter-nums empty) empty)

;; Function:
;(define filter-nums 


;; Post-function printfs:


(newline)
(newline)

; 
; Problem 4:
; 
; Write a function called CONCENTRIC-RINGS that consumes 
; a LON called lor.  The list lor contains positive numbers 
; that represent the radii of circles. The function should
; produce purple circles in outline mode with the radius 
; of each circle equal to the number specified by (first lor).   
; The circles should be displayed on an empty scene that is 
; 400 X 400 pixels in area.
; 
; Each circle should be centered in the same location,
; at (200,200). Use the place-image function to bind together 
; all the images on one empty scene.
; 
; See the data definition for list of numbers, LON, in
; problem 2.  You do not need to repeat this definition for
; this problem.
; 


(display "\n\n----------------------------\n")
(display "Problem 4: CONCENTRIC-RINGS")
(display "\n----------------------------\n")

;For your convenience, the following constants are given:

(define TENS (list 1 10 20 30 40 50 60 70 80 90 100 110 120
                   130 140 150 160 170 180 190 200))
;(define NUMS (genlist-n 200)) 
;;; NOTE: NUMS is constructed using function from problem 2,
;;;       so be sure that function is written before uncommenting
;;;       the line defining NUMS above.

;; Constant definitions (use whichever ones you choose or make 
;; your own):
;(define HEIGHT 400)  ;; Height of scene
;(define WIDTH 400)   ;; Width of scene
;(define MODE "outline") ;; Mode of circles
;(define COLOR "purple")  ;; Color of circles
;(define MT (empty-scene WIDTH HEIGHT)) ;; empty scene
;(define CENTER-W (/ WIDTH 2))   ;; x coordinate of MT center   
;(define CENTER-H (/ HEIGHT 2))  ;; y coordinate of MT center

;; Contract: 
;; Header:         
;; Purpose:  
;; Pre-function tests:  None possible because function produces
;;                      an image.  Tests are by visual inspection.

;;; Function:  
;(define concentric-rings 
;
;
;
;;; Post-function printfs:

(newline)
(newline)