pp: wide only using filter.

08-abstraction/pp01-wide-only.rkt

@@ -0,0 +1,32 @@
+#lang htdp/isl
+(require 2htdp/image)
+
+;
+; PROBLEM:
+;
+; Use the built in version of filter to design a function called wide-only
+; that consumes a list of images and produces a list containing only those
+; images that are wider than they are tall.
+;
+
+
+(define I1 (rectangle 20 10 "solid" "red")) ;; Wide.
+(define I2 (rectangle 10 20 "solid" "red")) ;; Not wide.
+(define I3 (rectangle 30 30 "solid" "green")) ;; Not wide.
+(define I4 (rectangle 45 44 "solid" "magenta")) ;; Wide.
+
+;; (listof Image) -> (listof Image)
+;; Produce list of images with those for which wide? produce #t.
+(check-expect (wide-only empty) empty)
+(check-expect (wide-only (list I2 I3)) empty)
+(check-expect (wide-only (list I1 I4)) (list I1 I4))
+(check-expect (wide-only (list I1 I2 I3 I4)) (list I1 I4))
+
+(define (wide-only loi)
+  (filter wide? loi))
+
+;; Image -> Boolean
+;; Produce #t if img w > img h; #f otherwise.
+(define (wide? img)
+  (> (image-width img) (image-height img)))
+