From a9a239985213d21dac744ca362c46d4eb7b2419c Mon Sep 17 00:00:00 2001
From: peterlee <yankeeguyu@gmail.com>
Date: Sun, 8 Nov 2020 20:19:58 +0800
Subject: [PATCH] solutions

---
 Exercise_246.rkt | 19 +++++++++++++++++++
 Exercise_247.rkt | 16 ++++++++++++++++
 Exercise_248.rkt |  8 ++++++++
 Exercise_249.rkt |  9 +++++++++
 Exercise_250.rkt | 40 ++++++++++++++++++++++++++++++++++++++++
 Exercise_251.rkt | 23 +++++++++++++++++++++++
 Exercise_252.rkt | 32 ++++++++++++++++++++++++++++++++
 Exercise_253.rkt | 11 +++++++++++
 Exercise_256.rkt |  4 ++++
 9 files changed, 162 insertions(+)
 create mode 100644 Exercise_246.rkt
 create mode 100644 Exercise_247.rkt
 create mode 100644 Exercise_248.rkt
 create mode 100644 Exercise_249.rkt
 create mode 100644 Exercise_250.rkt
 create mode 100644 Exercise_251.rkt
 create mode 100644 Exercise_252.rkt
 create mode 100644 Exercise_253.rkt
 create mode 100644 Exercise_256.rkt

diff --git a/Exercise_246.rkt b/Exercise_246.rkt
new file mode 100644
index 0000000..307e43f
--- /dev/null
+++ b/Exercise_246.rkt
@@ -0,0 +1,19 @@
+#lang racket
+
+(check-expect (extract < (cons 6 (cons 4 '())) 5) (list 4))
+(check-expect (extract > (cons 6 (cons 4 '())) 5) (list 6))
+
+(define (extract R l t)
+  (cond
+    [(empty? l) '()]
+    [else (cond
+            [(R (first l) t)
+             (cons (first l)
+                   (extract R (rest l) t))]
+            [else
+             (extract R (rest l) t)])]))
+
+
+(extract < (cons 6 (cons 4 '())) 5)
+
+(extract < (cons 4 '()) 5)
\ No newline at end of file
diff --git a/Exercise_247.rkt b/Exercise_247.rkt
new file mode 100644
index 0000000..29a6bc8
--- /dev/null
+++ b/Exercise_247.rkt
@@ -0,0 +1,16 @@
+#lang racket
+
+(check-expect (extract < (cons 6 (cons 4 '())) 5) (list 4))
+(check-expect (extract > (cons 6 (cons 4 '())) 5) (list 6))
+
+(define (extract R l t)
+  (cond
+    [(empty? l) '()]
+    [else (cond
+            [(R (first l) t)
+             (cons (first l)
+                   (extract R (rest l) t))]
+            [else
+             (extract R (rest l) t)])]))
+
+(extract < (cons 8 (cons 4 '())) 5)
\ No newline at end of file
diff --git a/Exercise_248.rkt b/Exercise_248.rkt
new file mode 100644
index 0000000..8fa7299
--- /dev/null
+++ b/Exercise_248.rkt
@@ -0,0 +1,8 @@
+#lang racket
+(define (squared>? x c)
+  (> (* x x) c))
+
+
+(squared>? 3 10)
+
+(squared>? 4 10)
\ No newline at end of file
diff --git a/Exercise_249.rkt b/Exercise_249.rkt
new file mode 100644
index 0000000..f9fbdd5
--- /dev/null
+++ b/Exercise_249.rkt
@@ -0,0 +1,9 @@
+#lang racket
+
+(define (f x) x)
+
+(cons f '())
+
+(f f)
+
+(cons f (cons 10 (cons (f 10) '())))
\ No newline at end of file
diff --git a/Exercise_250.rkt b/Exercise_250.rkt
new file mode 100644
index 0000000..8e6e3e2
--- /dev/null
+++ b/Exercise_250.rkt
@@ -0,0 +1,40 @@
+#lang racket
+
+(check-within (tabulate sin 5) (tab-sin 5) 0.000001)
+(check-within (tabulate sqrt 5) (tab-sqrt 5) 0.000001)
+
+(define (tabulate g n)
+  (cond
+    [(= n 0) (list (g 0))]
+    [else
+     (cons
+      (g n)
+      (tabulate g (sub1 n)))]))
+
+
+(check-expect (tab-sqr 3) (list 9 4 1 0))
+
+(define (tab-sqr n)
+  (tabulate sqr n))
+
+(check-within (tab-tan 3) (list (tan 3) (tan 2) (tan 1) (tan 0)) 0.000001)
+
+(define (tab-tan n)
+  (tabulate tan n))
+
+
+(define (tab-sin n)
+  (cond
+    [(= n 0) (list (sin 0))]
+    [else
+     (cons
+      (sin n)
+      (tab-sin (sub1 n)))]))
+
+(define (tab-sqrt n)
+  (cond
+    [(= n 0) (list (sqrt 0))]
+    [else
+     (cons
+      (sqrt n)
+      (tab-sqrt (sub1 n)))]))
\ No newline at end of file
diff --git a/Exercise_251.rkt b/Exercise_251.rkt
new file mode 100644
index 0000000..77ddf6b
--- /dev/null
+++ b/Exercise_251.rkt
@@ -0,0 +1,23 @@
+#lang racket
+
+(check-expect (fold1 '(1 2 3 4 5) 0 +) (sum '(1 2 3 4 5)))
+(check-expect (fold1 '(1 2 3 4 5) 1 *) (product '(1 2 3 4 5)))
+
+(define (fold1 l i g)
+  (cond
+    [(empty? l) i]
+    [else
+     (g (first l)
+        (fold1 (rest l) i g))]))
+
+
+(check-expect (fold1-sum '(1 2 3 4 5)) (+ 1 2 3 4 5))
+
+(define (fold1-sum l)
+  (fold1 l 0 +))
+
+
+(check-expect (fold1-product '(1 2 3 4 5)) (* 1 2 3 4 5))
+
+(define (fold1-product l)
+  (fold1 l 1 *))
diff --git a/Exercise_252.rkt b/Exercise_252.rkt
new file mode 100644
index 0000000..40d4821
--- /dev/null
+++ b/Exercise_252.rkt
@@ -0,0 +1,32 @@
+#lang racket
+
+(define LO-N1 (list 1 2 3 4 5 6))
+(define LO-POSN1 (list (make-posn 20 30) (make-posn 40 70) (make-posn 50 50)))
+  
+(define emt
+  (empty-scene 100 100))
+(define dot
+  (circle 3 "solid" "red"))
+
+
+(check-expect (fold2 LO-N1 1 *) (product LO-N1))
+(check-expect (fold2 LO-POSN1 emt place-dot) (image* LO-POSN1))
+
+(define (fold2 l i g)
+  (cond
+    [(empty? l) i]
+    [else
+     (g (first l)
+        (fold2 (rest l) i g))]))
+
+
+(check-expect (fold2-product LO-N1) (product LO-N1))
+
+(define (fold2-product l)
+  (fold2 l 1 *))
+
+
+(check-expect (fold2-image* LO-POSN1) (image* LO-POSN1))
+
+(define (fold2-image* l)
+  (fold2 l emt place-dot))
diff --git a/Exercise_253.rkt b/Exercise_253.rkt
new file mode 100644
index 0000000..7a692c8
--- /dev/null
+++ b/Exercise_253.rkt
@@ -0,0 +1,11 @@
+#lang racket
+
+(check-expect (number? 5) #true)
+
+(check-expect (equal? #true "abde") #false)
+
+(check-expect (+ 1 2 3) 6)
+
+(check-expect (list 5) (list 5))
+
+(check-expect (list? '(1 2 3 4)) #true)
\ No newline at end of file
diff --git a/Exercise_256.rkt b/Exercise_256.rkt
new file mode 100644
index 0000000..8d7000f
--- /dev/null
+++ b/Exercise_256.rkt
@@ -0,0 +1,4 @@
+#lang racket
+
+(check-expect (argmax second (list (list "a" 4) (list "b" 2) (list "c" 9) (list "d" 5)))
+              (list "c" 9))