On 10/4/10 3:54 PM, Bill Richter wrote:
My son's Racket Sudoku Solver extensively uses ormap to search a list,
and if it finds a match, return something good, else return false
http://schemecookbook.org/Cookbook/SudokuSolver. I'm interested in
know how HtDP recommends designing such programs.
I would first write a program like this:
;; [Maybe X] = X or false
;; [Listof String] -> [Maybe String]
(define (first-long-line los)
(cond [(empty? los) false]
[(> (string-length (first los)) 80)
(first los)]
[else
(first-long-line (rest los))]))
Later, I might write a program like:
;; [Listof Number] -> [Maybe Number]
(define (first-even los)
(cond [(empty? los) false]
[(even? (first los))
(first los)]
[else
(first-even (rest los))]))
Then I would observe the pattern and abstract following the recipe in
Section 21. The pattern captured in this abstraction is *not* what's
captured in HtDP's ormap, but it is what's captured in Racket's ormap.
There are no uses of ormap in HtDP, other than the Figure 57
definition, but andmap is claimed to be useful in
http://www.htdp.org/2003-09-26/Book/curriculum-Z-H-32.html#node_idx_1676
Exercise 25.1.2. Develop move-balls. The function consumes a list of
balls and moves each one until all of them have moved out of bounds.
Hint: It is best to write this function using filter, andmap, and
similar abstract functions from part IV.
I would avoid draw.ss and use universe instead (but ormap and the
abstraction of the above functions are both useful regardless).
David
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