this was confusing

3 files changed, 59 insertions, 1 deletions

diff --git a/coding-exercises/2/14.rkt b/coding-exercises/2/14.rkt
index 50d28fc..c2a0ca9 100644
--- a/coding-exercises/2/14.rkt
+++ b/coding-exercises/2/14.rkt
@@ -49,9 +49,12 @@
     (print-interval-percent (mul-interval i1 i1))
     (print-interval-percent (mul-interval i1 i2))))
   
-;; There are at least some problems as I understand:
+;; There are at least some problems as I understand from 2.14 and 2.15:
 ;; 1. Repeated intervals in an equation are dependent on each other
 ;; 2. Multiplicative identity is undefined and should not be uncertain
+;;
+;; In 2.16 I would try to rewrite the code to try to make the inverse under multiplication certain
+;; and would try to keep track of dependent intervals and maybe only introduce uncertainty once.
 (define (print-lem)
   (let ((i1 (make-center-percent 100.0 2.0))
         (i2 (make-center-percent 200.0 3.0)))
diff --git a/coding-exercises/2/17.rkt b/coding-exercises/2/17.rkt
new file mode 100644
index 0000000..e4b1735
--- /dev/null
+++ b/coding-exercises/2/17.rkt
@@ -0,0 +1,36 @@
+#lang racket
+(require sicp)
+
+(define (append list1 list2)
+  (if (null? list1)
+    list2
+    (cons (car list1) (append (cdr list1) list2))))
+
+(define (length items)
+  (define (length-iter a c)
+    (if (null? a)
+      c
+      (length-iter (cdr a) (inc c))))
+  (length-iter items 0))
+
+(define (list-ref items n)
+  (if (= n 0)
+    (car items)
+    (list-ref 
+      (cdr items) 
+      (dec n))))
+
+(define (last-pair l)
+  (if (null? (cdr l))
+    l
+    (last-pair (cdr l))))
+
+(define (print-list)
+  (define odds (list 1 3 5 7 9 11))
+  (println odds)
+  (println (list-ref odds 2))
+  (println (length odds))
+  (println (append odds (list 22)))
+  (println (last-pair odds)))
+
+(print-list)
diff --git a/coding-exercises/2/18.rkt b/coding-exercises/2/18.rkt
new file mode 100644
index 0000000..5fab5f8
--- /dev/null
+++ b/coding-exercises/2/18.rkt
@@ -0,0 +1,19 @@
+#lang racket
+(require "../../shared/lists.rkt")
+
+;; grow in reverse method
+;; or pointer reversal method in other languages
+(define (reverse-rec l) 
+  (define (iter l p)
+   (cond 
+     ((null? l) p)
+     (else
+      (iter 
+        (cdr l) 
+        (cons (car l) p)))))
+  (iter l (list)))
+
+(define (print)
+    (println (list 1 4 9 16 25))
+    (reverse (list 1 4 9 16 25)))
+(print)