CODEWITHSUNDEEP
schemelispracketevaluatefunction-call
BigPortions
BigPortions

Reputation: 7

Scheme help - how to call a function

I'm new to Scheme, and I am trying to manipulate a list and return the cdr of a pair in the list.

So far here is my code:

(define get-userid (lambda (ls)
  (if (pair? ls)
      (cdar ls)
      (get-userid (cdar ls)))))

The list I wish to manipulate is:

(define *rider-preferences4*
  '((userid . anna)
    (role . rider)
    (origin . "southampton")
    (destination . "glasgow")
    (time . "15:15")
    (capacity . 2)
    (smoking . no)))

Input and outputs:

> get-userid *rider-preferences4*
#<procedure:get-userid>
((userid . anna) (role . rider) (origin . "southampton") (destination . "glasgow") (time . "15:15") (capacity . 2) (smoking . no))
> get-userid (get-userid *rider-preferences4*)
#<procedure:get-userid>
**anna**

I want the result of just anna, and to be able to do it just by calling get-userid. How would this be possible? Even though I assume it's trivial.

Upvotes: 0

Views: 103

Answers (2)

uselpa
uselpa

Reputation: 18917

Since *rider-preferences4* is an association list you can use the specific procedures (assoc, assq, assv) to retrieve any key:

> (cdr (assq 'userid *rider-preferences4*))
'anna
> (cdr (assq 'destination *rider-preferences4*))
"glasgow"

so the code is

(define (get-userid alst)
  (cdr (assq 'userid alst)))

Besides being usable for any key, it is also independent of the position, so

> (get-userid '((role . rider) (userid . anna)))
'anna

will also work.

Upvotes: 1

&#211;scar L&#243;pez
&#211;scar L&#243;pez

Reputation: 236004

I think you're a bit confused with the way list operations work in Scheme, I suggest you take a look at the relevant section in the documentation. For this problem, the solution is as simple as this:

(define get-userid ; even simpler: (define get-userid cdar)
  (lambda (ls)
    (cdar ls)))

Explanation: the input list is a list of pairs, and we're interested in the cdr part of the first element (which holds the user id). So we take the cdr of the car, or abbreviated: the cdar. It works as expected:

(get-userid *rider-preferences4*)
=> 'anna

Upvotes: 1

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