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Creating a vector-filter function without the use of lists as an intermediate value.

Creating a vector-filter function without the use of lists as an intermediate value.

Date Created:Sunday October 26th, 2008 06:06 PM
Date Modified:Sunday October 26th, 2008 06:10 PM

(define (vector-filter pred vec)
    (let ((num (vector-filter-count pred vec 0 (- (vector-length vec) 1))))
      (vector-fill pred (make-vector num) vec 0 0)
    )
)
;; THIS DETERMINES THE NUMBER OF VALUES THAT RETURN TRUE FOR THE PREDICATE
(define (vector-filter-count pred vec count n)
    (if (= n -1)
        count
        (let ((val (vector-ref vec n)))
          (if (pred val)
           (vector-filter-count pred vec (+ count 1) (- n 1))
           (vector-filter-count pred vec count (- n 1))))))

;; THIS PLACES VALUES INTO THE nth POSITION OF THE NEW VECTOR WHEN
;; THE kth POSITION OF THE OLD VECTOR RETURNS TRUE FOR THE PREDICATE
;; THEREFORE, INCREMENT k each time, but not necessarily n
(define (vector-fill pred vec vec-old n k)
     (if (= n (vector-length vec))
         vec
         (if (pred (vector-ref vec-old k))
                   (begin
                      (vector-set! vec n (vector-ref vec-old k))
                      (vector-fill pred vec vec-old (+ n 1) (+ k 1)))
                   (vector-fill pred vec vec-old n (+ k 1)))))


(define v1 (vector 0 1 2 3 4 5 6 7 8))

(define pred (lambda (x) (= (modulo x 2) 0)))


;; stk> (vector-filter pred v1)
;; stk> #(0 2 4 6 8)

Vector Filter by Dan Lynch
is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 3.0 United States License
Based on a work at www.3daet.com
Permissions beyond the scope of this license may be available at http://www.3daet.com