learning-sicp

commit 13dba597cc44a16ce58771aa698f3f8d3a16b598
parent 6dc652e101ad8cc62c38d77f3de9a63a92fe1d69
Author: Yuval Langer <yuval.langer@gmail.com>
Date:   Sun, 16 Apr 2023 15:47:55 +0300

Add solutions to exercise 3.18 and exercise 3.19.

Diffstat:
A
sicp/solutions/3_18.scm
 | 
17
+++++++++++++++++
A
sicp/solutions/3_19.scm
 | 
21
+++++++++++++++++++++
A
sicp/tests/3_18.scm
 | 
25
+++++++++++++++++++++++++
A
sicp/tests/3_19.scm
 | 
25
+++++++++++++++++++++++++

4 files changed, 88 insertions(+), 0 deletions(-)
diff --git a/sicp/solutions/3_18.scm b/sicp/solutions/3_18.scm
@@ -0,0 +1,17 @@
+(define-library (sicp solutions 3_18)
+  (import (scheme base))
+  (export cyclic?)
+
+  (begin
+    (define (cyclic? l)
+      (define (cyclic?' l known-pairs)
+        (cond
+         ((null? l) #f)
+         ((pair? l)
+          (if (memq l known-pairs)
+              #t
+              (cyclic?' (cdr l)
+                        (cons l known-pairs))))
+         (else #f)))
+
+      (cyclic?' l '()))))
diff --git a/sicp/solutions/3_19.scm b/sicp/solutions/3_19.scm
@@ -0,0 +1,21 @@
+(define-library (sicp solutions 3_18)
+  (import (scheme base))
+  (export cyclic?)
+
+  (begin
+    (define (cyclic? l)
+
+      (define (cyclic?' tortoise hare)
+        (cond
+         ((or (null? hare)
+              (null? (cdr hare))
+              (null? (cddr hare)))
+          #f)
+         (if (memq tortoise hare)
+             #t
+             (cyclic?' (cdr tortoise)
+                       (cddr hare)))))
+
+      (if (null? l)
+          #f
+          (cyclic? l (cdr l))))))
diff --git a/sicp/tests/3_18.scm b/sicp/tests/3_18.scm
@@ -0,0 +1,25 @@
+(define-library (sicp tests 3_18)
+  (import (scheme base))
+  (import (srfi :1))
+  (import (srfi :64))
+  (import (only (sicp solutions 3_18) cyclic?))
+
+  (begin
+    (test-begin "3.18")
+
+    (define a (list 1 2 3))
+
+    (define b
+      (let ([a (list 1 2 3)])
+        (set-cdr! (last-pair a) a)
+        a))
+
+    (test-equal
+        #f
+      (cyclic? a))
+
+    (test-equal
+        #t
+      (cyclic? b))
+
+    (test-end "3.18")))
diff --git a/sicp/tests/3_19.scm b/sicp/tests/3_19.scm
@@ -0,0 +1,25 @@
+(define-library (sicp tests 3_18)
+  (import (scheme base))
+  (import (srfi :1))
+  (import (srfi :64))
+  (import (only (sicp solutions 3_18) cyclic?))
+
+  (begin
+    (test-begin "3.18")
+
+    (define a (list 1 2 3))
+
+    (define b
+      (let ([a (list 1 2 3)])
+        (set-cdr! (last-pair a) a)
+        a))
+
+    (test-equal
+        #f
+      (cyclic? a))
+
+    (test-equal
+        #t
+      (cyclic? b))
+
+    (test-end "3.18")))