CODEWITHSUNDEEP
functionschemeconstants
John Friedrich
John Friedrich

Reputation: 343

Scheme Maintaining Value throughout program

In the following code the values for avg and avg2 change as the code progresses. How can I make it so that the values are constant throughout the function without defining them outside the function? Should I use a helper function?

(define (covariance-list x y)
  (let ((avg (average x)))
  (let ((avg2 (average y)))
  (if (null? x) 
      '()
      (cons (* (- (car x) avg)(- (car y) avg2)) 
            (covariance-list (cdr x) (cdr y)))))))

Upvotes: 0

Views: 84

Answers (2)

Adam Burry
Adam Burry

Reputation: 1902

One way would be with an auxiliary function:

(define (covariance-list x y)
  (define (covariance-list-aux x y avg-x avg-y)
    (if (null? x)
      '()
      (cons (* (- (car x) avg-x) (- (car y) avg-y))
            (covariance-list-aux (cdr x) (cdr y) avg-x avg-y))))
  (covariance-list-aux x y (average x) (average y)))

Upvotes: 1

Ben
Ben

Reputation: 693

I don't think you want them constant, since they depend on the parameters of the function. You probably just want them to not be recomputed during each recursive call.

(define (covariance-list x y)
 (let ((avg (average x))
       (avg2 (average y)))
     (let loop ((x x)
                (y y))
       (if (null? x)
         '()
          (cons (* (- (car x) avg)
                   (- (car y) avg2))
                (loop (cdr x) (cdr y)))))))

Upvotes: 3

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