問題1.33

ここまでくると、コードから意味を読み取るのは大変。馴れが重要なのかな？

filtered-accumulate

(define (filtered-accumulate filter combiner null-value term a next b)
  (if (> a b)
    null-value
    (if (filter a)
      (combiner (term a)
		(filtered-accumulate filter combiner null-value term (next a) next b)) 
      (filtered-accumulate filter combiner null-value term (next a) next b))))

; 階乗
(define (factorial n)
  (filtered-accumulate (lambda (x) #t) * 1 (lambda (x) x) 1 (lambda (x) (+ x 1)) n))

; 素数
(define (prime-square a b)
  (filtered-accumulate prime? + 0 square a (lambda (n) (+ n 1)) b))

; GCD
(define (gcd-product i n)
  (filtered-accumulate gcd-one? * 1 (lambda (x) x) i (lambda (n) (+ n 1)) n))