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-rw-r--r--net/ricketyspace/sicp/one/thirtyone.scm43
1 files changed, 43 insertions, 0 deletions
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+;;;; 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
+
+