net: Add (net ricketyspace sicp one thirtyone).

* net/ricketyspace/sicp/one/thirtyone.scm: New file.

1 files changed, 43 insertions, 0 deletions

diff --git a/net/ricketyspace/sicp/one/thirtyone.scm b/net/ricketyspace/sicp/one/thirtyone.scm
new file mode 100644
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--- /dev/null
+++ b/net/ricketyspace/sicp/one/thirtyone.scm
@@ -0,0 +1,43 @@
+;;;; Under Creative Commons Attribution-ShareAlike 4.0
+;;;; International. See
+;;;; <https://creativecommons.org/licenses/by-sa/4.0/>.
+
+(define-module (net ricketyspace sicp one thirtyone)
+  #:export (product factorial pi))
+
+(define (product term a next b)
+  (if (> a b)
+      1
+      (* (term a)
+         (product term (next a) next b))))
+
+(define (factorial n)
+  (let ((term (lambda (x) x))
+        (next (lambda (x) (1+ x))))
+    (product term 1 next n)))
+
+(define (pi precision)
+  (let* ((n (lambda (x) (* (+ (quotient x 2) 1) 2))) ; numerator of term.
+         (d (lambda (x) (+ (* (quotient (1+ x) 2) 2) 1))) ; denominator of term.
+         (term (lambda (x) (/ (* (n x) 1.0) (* (d x) 1.0))))
+         (next (lambda (x) (1+ x))))
+    (* (product term 1 next precision) 4)))
+
+;;; Guile REPL
+;;; scheme@(guile-user)> ,use (net ricketyspace sicp one thirtyone)
+;;; scheme@(guile-user)> (factorial 5)
+;;; $9 = 120
+;;; scheme@(guile-user)> (factorial 10)
+;;; $7 = 3628800
+;;; scheme@(guile-user)> (factorial 20)
+;;; $8 = 2432902008176640000
+;;; scheme@(guile-user)> (pi 1000)
+;;; $10 = 3.143160705532257
+;;; scheme@(guile-user)> (pi 10000)
+;;; $11 = 3.1417497057379635
+;;; scheme@(guile-user)> (pi 100000)
+;;; $12 = 3.1416083612780903
+;;; scheme@(guile-user)> (pi 1000000)
+;;; $13 = 3.1415942243828017
+
+