commit 7119ca02436b98b2c6bc3e11ac73ec1c1a68a8b9
parent e482146b055647ce836e4756cdb0a716ac25e4ac
Author: Yuval Langer <yuval.langer@gmail.com>
Date: Mon, 20 Nov 2023 22:23:50 +0200
Add the new and improved chapter-1 exercise-9 (1.9) solution.
Diffstat:
2 files changed, 137 insertions(+), 94 deletions(-)
diff --git a/sicp/solutions/chapter-1/exercise-9.scm b/sicp/solutions/chapter-1/exercise-9.scm
@@ -1,103 +1,38 @@
-#!
+(define-library (sicp solutions chapter-1 exercise-9)
+ (import (scheme base))
+ (export inc
+ dec
+ first-+
+ second-+)
-*Exercise 1.9:* Each of the following two procedures defines a
-method for adding two positive integers in terms of the procedures
-`inc', which increments its argument by 1, and `dec', which
-decrements its argument by 1.
+ (begin
+ (define (inc x) (+ x 1))
+ (define (dec x) (- x 1))
-(define (+ a b)
- (if (= a 0)
- b
- (inc (+ (dec a) b))))
+ #!
-(define (+ a b)
- (if (= a 0)
- b
- (+ (dec a) (inc b))))
+ *Exercise 1.9:* Each of the following two procedures defines a
+ method for adding two positive integers in terms of the procedures
+ `inc', which increments its argument by 1, and `dec', which
+ decrements its argument by 1.
-Using the substitution model, illustrate the process generated by
-each procedure in evaluating `(+ 4 5)'. Are these processes
-iterative or recursive?
+ !#
-First:
+ (define (first-+ a b)
+ (if (= a 0)
+ b
+ (inc (first-+ (dec a) b))))
-(+ 4 5)
-(if (= 4 0)
- 5
- (inc (+ (dec 4) 5)))
-(if #f
- 5
- (inc (+ (dec 4) 5)))
-(inc (+ (dec 4) 5))
-(inc (+ 3 5))
-(inc (if (= 3 0)
- 5
- (inc (+ (dec 3) 5))))
-(inc (if #f
- 5
- (inc (+ (dec 3) 5))))
-(inc
- (inc (+ (dec 3) 5)))
-(inc
- (inc (+ 2 5)))
-(inc
- (inc (if (= 2 0)
- 5
- (inc (+ (dec 2) 5)))))
-(inc
- (inc (if #f
- 5
- (inc (+ (dec 2) 5)))))
-(inc
- (inc
- (inc (+ (dec 2) 5))))
-(inc
- (inc
- (inc (+ 1 5))))
-(inc
- (inc
- (inc (if (= 1 0)
- 5
- (inc (+ (dec 1) 5))))))
-(inc
- (inc
- (inc (if #f
- 5
- (inc (+ (dec 1) 5))))))
-(inc (inc (inc (inc (+ (dec 1) 5)))))
-(inc (inc (inc (inc (+ 0 5)))))
-(inc (inc (inc (inc (if (= 0 0) 5 (inc (+ (dec 0) 5)))))))
-(inc (inc (inc (inc (if #t 5 (inc (+ (dec 0) 5)))))))
-(inc (inc (inc (inc 5))))
-(inc (inc (inc 6)))
-(inc (inc 7))
-(inc 8)
-9
+ (define (second-+ a b)
+ (if (= a 0)
+ b
+ (second-+ (dec a) (inc b))))
-Recursive.
+ #!
-Second:
+ Using the substitution model, illustrate the process generated by
+ each procedure in evaluating `(+ 4 5)'. Are these processes
+ iterative or recursive?
-(+ 4 5)
-(if (= 4 0) 5 (+ (dec 4) (inc 5)))
-(if #f 5 (+ (dec 4) (inc 5)))
-(+ 3 6)
-(if (= 3 0) 6 (+ (dec 3) (inc 6)))
-(if #f 6 (+ (dec 3) (inc 6)))
-(+ (dec 3) (inc 6))
-(+ 2 7)
-(if (= 2 0) 7 (+ (dec 2) (inc 7)))
-(if #f 7 (+ (dec 2) (inc 7)))
-(+ (dec 2) (inc 7))
-(+ 1 8)
-(if (= 1 0) 8 (+ (dec 1) (inc 8)))
-(if #f 8 (+ (dec 1) (inc 8)))
-(+ (dec 1) (inc 8))
-(+ 0 9)
-(if (= 0 0) 9 (+ (dec 0) (inc 9)))
-(if #f 9 (+ (dec 0) (inc 9)))
-9
-
-Iterative.
-
-!#
+ !#
+ ))
diff --git a/sicp/tests/chapter-1/exercise-9.scm b/sicp/tests/chapter-1/exercise-9.scm
@@ -0,0 +1,108 @@
+(define-library (sicp tests chapter-1 exercise-9)
+ (import (scheme base)
+ (scheme eval)
+ (srfi :64)
+ (sicp solutions chapter-1 exercise-9))
+
+ (begin
+ (test-begin "chapter-1-exercise-9")
+
+ (for-each
+ (lambda (test-expr)
+ (test-equal (+ 4 5)
+ (eval test-expr
+ (environment '(scheme base)
+ '(sicp solutions chapter-1 exercise-9)))))
+ '((first-+ 4 5)
+ (if (= 4 0)
+ 5
+ (inc (first-+ (dec 4) 5)))
+ (if #f
+ 5
+ (inc (first-+ (dec 4) 5)))
+ (inc (first-+ (dec 4) 5))
+ (inc (first-+ 3 5))
+ (inc (if (= 3 0)
+ 5
+ (inc (first-+ (dec 3) 5))))
+ (inc (if #f
+ 5
+ (inc (first-+ (dec 3) 5))))
+ (inc
+ (inc (first-+ (dec 3) 5)))
+ (inc
+ (inc (first-+ 2 5)))
+ (inc
+ (inc (if (= 2 0)
+ 5
+ (inc (first-+ (dec 2) 5)))))
+ (inc
+ (inc (if #f
+ 5
+ (inc (first-+ (dec 2) 5)))))
+ (inc
+ (inc
+ (inc (first-+ (dec 2) 5))))
+ (inc
+ (inc
+ (inc (first-+ 1 5))))
+ (inc
+ (inc
+ (inc (if (= 1 0)
+ 5
+ (inc (first-+ (dec 1) 5))))))
+ (inc
+ (inc
+ (inc (if #f
+ 5
+ (inc (first-+ (dec 1) 5))))))
+ (inc (inc (inc (inc (first-+ (dec 1) 5)))))
+ (inc (inc (inc (inc (first-+ 0 5)))))
+ (inc (inc (inc (inc (if (= 0 0) 5 (inc (first-+ (dec 0) 5)))))))
+ (inc (inc (inc (inc (if #t 5 (inc (first-+ (dec 0) 5)))))))
+ (inc (inc (inc (inc 5))))
+ (inc (inc (inc 6)))
+ (inc (inc 7))
+ (inc 8)
+ 9))
+
+ ;; Recursive.
+
+ (for-each
+ (lambda (test-expr)
+ (test-equal (+ 4 5)
+ (eval test-expr
+ (environment '(scheme base)
+ '(sicp solutions chapter-1 exercise-9)))))
+ '((second-+ 4 5)
+ (if (= 4 0) 5 (second-+ (dec 4) (inc 5)))
+ (if #f 5 (second-+ (dec 4) (inc 5)))
+ (second-+ 3 6)
+ (if (= 3 0) 6 (second-+ (dec 3) (inc 6)))
+ (if #f 6 (second-+ (dec 3) (inc 6)))
+ (second-+ (dec 3) (inc 6))
+ (second-+ 2 7)
+ (if (= 2 0) 7 (second-+ (dec 2) (inc 7)))
+ (if #f 7 (second-+ (dec 2) (inc 7)))
+ (second-+ (dec 2) (inc 7))
+ (second-+ 1 8)
+ (if (= 1 0) 8 (second-+ (dec 1) (inc 8)))
+ (if #f 8 (second-+ (dec 1) (inc 8)))
+ (second-+ (dec 1) (inc 8))
+ (second-+ 0 9)
+ (if (= 0 0) 9 (second-+ (dec 0) (inc 9)))
+ ;; XXX: Last time I wrote this I had an error that caused an
+ ;; infinite loop here and the only awy I found it was by
+ ;; adapting it into this SRFI-64 test system. The error is
+ ;; that (= 0 0) is #t, but instead I evaluated it into #f,
+ ;; probably because I killed and yanked it all in some way or
+ ;; another, I think, but I am not sure. The result is that it
+ ;; never gotten 9. In other words: Testing is important!
+ #; (if #f 9 (second-+ (dec 0) (inc 9)))
+ ;; The right solution:
+ (if #t 9 (second-+ (dec 0) (inc 9)))
+ 9))
+
+ ;; Iterative.
+
+ ))
