1.33 (过滤累积和 求区间内所有素数之和)

(define (filtered-accumulate filter? combiner null-value term a next b)
  (define (iter a result)
    (cond ((> a b) result)
          ((filter? a) (iter (next a)
                         (combiner (term a) result)))
          (else (iter (next a)
                      result))))
  (iter a null-value))

(define (smallest-divisor n)
  (find-divisor n 2))
 
(define (find-divisor n test-divisor)
  (cond ((> (square test-divisor) n) n)
        ((divides? n test-divisor) test-divisor)
        (else (find-divisor n (+ test-divisor 1)))))
 
(define (divides? a b)
  (= (remainder a b) 0))
 
(define (prime? n)
  (if (< n 2)
      #f
      (= n (smallest-divisor n))))
 
(define (square x)
  (* x x))

(define (primes-sum a b)
  (filtered-accumulate prime?
            +
            0
            (lambda (x) x)
            a
            (lambda (x) (+ x 1))
            b))

(primes-sum 1 10)