;; The first three lines of this file were inserted by DrScheme. They record metadata
;; about the language level of this file in a form that our tools can easily process.
#reader(lib "htdp-intermediate-lambda-reader.ss" "lang")((modname lect07c) (read-case-sensitive #t) (teachpacks ((lib "universe.ss" "teachpack" "2htdp"))) (htdp-settings #(#t constructor repeating-decimal #f #t none #f ((lib "universe.ss" "teachpack" "2htdp")))))
;; lect07c -- Oct.16


; What we've learned about loops:
; - a special case of recursion
; - common types of loops: map,filter (and fold, which won't be on exam).
;   (and how we can actually make each of those official loops (functions)
;    in languages with higher-order functions).

; We've seen 'lambda', to create anonymous functions.
; We've seen that '(define (foo x) ...)' is syntactic sugar
; for '(define foo (lambda (x) ...))'
;  (and thus 'define' and functions are entirely orthogonal).

;----
; Practice with 'lambda':

; Using `sort` to sort...
;  - sort a list of trucks `alot` by x-coord:
(sort alot (λ (t1 t2) (< (truck-x t1) (truck-x t2))))
      

;  - sort a strings, ignoring "The"
(sort (list "The Cars" "The Who" "Two Skinny J's" "Two Nice Girls")
      (lambda (s1 s2) ;; Our anonymous lambda:
        (let* {[remove-pref-if
                (lambda (str prf)
                  (let* {[len (string-length prf)]}
                    (if (and (>= (string-length str) len) 
                             (string=? (substring str 0 len) prf))
                        (substring str len)
                        str)))]
               }
          (string<=? (remove-pref-if s1 "The ")
                     (remove-pref-if s2 "The "))))

;
; How is passing-a-function done in Java?
; We want to pass a function, but Java doesn't allow that,
; so here's the contortion we'll go through:
; We'll pass an object which contains exactly one method
; of a pre-defined type.  That class is 'Comparator'.
;  [Why is 'compareTo' not good enough?]
; And we'll create that method on the fly,
;  subclassing Comparator using an 'anonymous inner class'.
; For an example, see:
;   http://www.radford.edu/itec380/2009fall-ibarland/Lectures/lect07-anon-lambda/Anon.java
;

; Write quicksort:

(define (quicksort nums)
  (if (<= (length nums) 1)
      nums
      (let* {[pivot (list-ref (random (length nums)) nums)]
             [smalls (filter (λ (n) (< n pivot)) nums)]
             [eqls   (filter (λ (n) (= n pivot)) nums)]
             [bigs   (filter (λ (n) (> n pivot)) nums)]
             }
        (append (quicksort smalls)
                eqls
                (quicksort bigs)))))
      




; If time:
; Ancestor trees, continued.