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[challenge] Inverse Factorial

2 2021-06-03 11:57

Meh.

(defun infac (n)
  (loop for i from 0
	for f = (fac i)
	when (> f n) do (error "Not a factorial")
	when (= f n) return i))

4 2021-06-03 17:51 *

(define (inverse-factorial n)
  (letrec ((iter
            (lambda (i a)
              (cond
               ((> a n) (error "Not a factorial!"))
               ((= a n) (- i 1))
               (else (iter (+ i 1) (* a i)))))))
    (iter 1 1)))

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