vd: quick lambda expression/function examples.

09b-lambda-exprs/vd01-lambda-exprs.rkt

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+#lang htdp/isl+
+(require 2htdp/image)
+
+;
+; NOTE: isl+ is intermediate student language with lambda
+;
+
+;
+; PROBLEM:
+;
+; Rewite each of the following function definitions using lambda.
+;
+
+;; Number (listof Number) -> (listof Number)
+;; produce only elements of lon > threshold
+(check-expect (only-bigger 2 empty) empty)
+(check-expect (only-bigger 3 (list 2 4 5)) (list 4 5))
+
+#;
+(define (only-bigger threshold lon)
+  (local [(define (pred n)
+            (> n threshold))]
+    (filter pred lon)))
+
+(define (only-bigger threshold lon)
+  (filter (lambda (n) (> n threshold)) lon))
+
+;; (listof Image) -> (listof Natural)
+;; produce list of areas of images
+(check-expect (all-areas (list (rectangle 2 3 "solid" "blue")
+                               (square 10 "solid" "white")))
+              (list 6 100))
+
+#;
+(define (all-areas loi)
+  (local [(define (area i)
+            (* (image-width i)
+               (image-height i)))]
+    (map area loi)))
+
+(define (all-areas loi)
+  (map (lambda (i)
+         (* (image-width i)
+            (image-height i))) loi))
+
+;; (listof Number)  ->  (listof Number)
+;; produce list of numbers sorted in ASCENDING order
+;; ASSUMPTION: lon contains no duplicates
+(check-expect (qsort empty)                empty)
+(check-expect (qsort (list 8))             (list 8))
+(check-expect (qsort (list 7 8 9))         (list 7 8 9))
+(check-expect (qsort (list  4 3 2 1))      (list 1 2 3 4))
+(check-expect (qsort (list 6 8 1 9 3 7 2)) (list 1 2 3 6 7 8 9))
+
+#;
+(define (qsort lon)
+  (if (empty? lon)
+      empty
+      (local [(define p (first lon))
+              (define (<p? n) (< n p))
+              (define (>p? n) (> n p))]
+        (append (qsort (filter <p? lon))
+                (list p)
+                (qsort (filter >p? lon))))))
+
+(define (qsort lon)
+  (if (empty? lon)
+      empty
+      (local [(define p (first lon))]
+        (append (qsort (filter (λ (n) (< n p)) lon))
+                (list p)
+                (qsort (filter (λ (n) (> n p)) lon))))))
+