Filter

(define (inside-unit-sphere? point)
  (<= (point.to-origin point) 1))

(define (filter test list)
  ;; FILTER: (X->Boolean, X*) -> X*
  (cond ((null? list) '())
        ((test (car list))
         (cons (car list)
               (filter test (cdr list))))
        (else (filter test (cdr list)))))

(define (how-many-in-unit-sphere points)
  (accumulate 0 +
    (map (lambda (pt) 1)
         (filter inside-unit-sphere? 
                 points))))

Jim Miller