Racket: Write a function to access a component of a structure

Question

I want to abstract the following functions:

(define (road_length1 n)
  (road-length (node-road1 n)))
(define (road_length2 n)
  (road-length (node-road2 n)))
(define (road_length3 n)
  (road-length (node-road3 n)))

My structures for a node and road are like this:

  (define-struct node [Letter road1 road2 road3])
  (define-struct road [name length])

Hence I want to be able to access the length of a given road from a node, like this:

 (define (road_length n road_number)
    (road-length (node-(road_number) n)))

However this doesn't work

Note that, as @Leif demonstrates below, proper Racket naming conventions use hyphenation, not underscores, so your function should be called road-length, not road_length. — Alexis King
– Alexis King, Commented Dec 17, 2015 at 18:26

Leif Andersen · Accepted Answer · 2015-12-17 18:24:42Z

2

Probably the easiest way is for you to store your nodes in either a vector or list

To do this, I would just store it in your struct like this:

(struct node (letter roads))

(As a side note, struct is a better (or at least newer) way to create a struct, than define-struct.)

Once you do that, you can pass in the index of the road you want as a paramter to the function.

(define (road-length node road)
  (list-ref (node-roads node) road))

(I should note that there is 'technically' a way to do exactly what you want here with macros, but I'm almost certain that it would be overkill in this situation, when functions work just fine.)

answered Dec 17, 2015 at 18:24

Leif Andersen

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Comments

Sylwester · Accepted Answer · 2015-12-18 10:43:34Z

I personally feel the fact the roads are numbered is smelling pretty bad already. I would have changed it to this:

(define-struct node [letter roads] #:constructor-name make-raw-node)
(define (make-node letter . roads)
  (make-raw-node letter (list->vector roads)))

(define (node-road-length node road-index)
  (road-length (vector-ref (node-roads node) road-index)))

Note that in sane programming languages we start indexing elements from 0 so road-number is in the range [0,2].

If you really want to have the indexes in the names, then you make a map between the indexes and their accessors:

(define node-road-length
  (let ((lookup (vector node-road1 
                        node-road2 
                        node-road3)))
    (lambda (node road-index)
      (road-length ((lookup road-index) node)))))

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