Scheme - Access Data Pairs in List

Question

I am brand-new to Scheme (using Dr. Racket), and this is a homework assignment, so please no outright answers. I've been trying to find a solution online, but have not found one that is satisfactory.

I am writing a procedure that will take one argument - a list of pairs. I then need to find the difference between each pair of two numbers using abs, then add up the differences for all pairs.

My code is below.

(define example2 '((2 -7) (-4 -20) (7 7) (-13 2)))

(define get-difference      
  (lambda (lyst)
    (apply plus (abs(list-ref lyst 0)) (abs(list-ref lyst 1)))))

(define total-error
  (lambda list-of-pairs
    (apply plus (map get-difference list-of-pairs))))

I am having trouble with two things (I think): Accessing each pair of data through the whole list Using abs appropriately for each pair in the list.

I can use get-difference if I pass it two numbers,

(define get-difference      
  (lambda (x y)
    (+ (abs x) (abs y))))

but I am trying to pass a list to it instead. From another assignment, I used a get-num function, and it worked correctly - however, this approach did not seem to work for my issue:

(define get-num
  (lambda (lyst)
    (list-ref lyst 1)))

(define average-grade
  (lambda (list-of-grades)
    (/ (apply plus (map get-num list-of-grades)) (length list-of-grades))))

Any hints or suggestions are appreciated. Thank you!

EDIT: I forgot to mention - I am not allowed to use recursion or looping.

Thanks for the suggestion! We haven't talked about folding in class, so I can't use it. I will, however, look into it for future use. — AbigailB
– AbigailB, Commented Feb 3, 2015 at 18:20

Óscar López · Accepted Answer · 2015-02-03 17:49:57Z

1

For starters, I think the get-difference function is incorrectly defined. Shouldn't it be like this instead?

(define get-difference      
  (lambda (x y)
    (abs (- x y))))

Back to the main problem. You're on the right track, using apply, map, list-ref will work, but you're combining them in the wrong way. Try this:

(define total-error
  (lambda (list-of-pairs)
    (apply + (map (lambda (pair) ; each element in the list is a pair
                    (get-difference (list-ref pair 0)
                                    (list-ref pair 1)))
                  list-of-pairs))))

Alternatively, we can do the difference-then-addition in a single step, using foldl:

(define total-error
  (lambda (list-of-pairs)
    (foldl (lambda (pair sum)
             (+ (get-difference (list-ref pair 0)
                                (list-ref pair 1))
                sum))
           0
           list-of-pairs)))

edited Feb 3, 2015 at 17:49

answered Feb 3, 2015 at 17:17

Óscar López

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2 Comments

AbigailB Over a year ago

Thank you! I have been confused about where to use apply and map correctly. So, just to make sure I'm clear, list-of-pairs is the argument being passed in to map, and the get difference function is called on each pair (which is passed to lambda as the parameter). I appreciate your help!

Óscar López Over a year ago

@AbigailB that's right. And after map has finished, apply + adds all the differences

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