CODEWITHSUNDEEP
listschemeracketassociative-array
dd123
dd123

Reputation: 47

How do I get the keys from an association list? scheme/ racket

how do I get the keys from an association list? I.E (

define lst1 (list(list 2 "three")(list 2 "two")(list 1 "one")))
       (get-keys lst1) => (list 2 2 1)

the below is what I wrote:

(define(get-keys alst)
  (cond
    [(empty? alst) empty]
    [(number?(first(first alst)))(cons(first(first alst))(get-keys (rest alst)))]
    [else(get-keys (rest alst))]))

however this is highly inneficient when I try and use get-keys as a helper function..What I am really trying to do is write a program that consumes two association lists and returns only the keys that do not repeat.

Upvotes: 0

Views: 2063

Answers (1)

Óscar López
Óscar López

Reputation: 236004

Here's a really simple solution, using built-in higher-order procedures (don't reinvent the wheel!):

(define (get-keys alst)
  (map first alst))

The above procedure maps the first procedure over the input list, that is: it takes the first element from each association. If at some point you need to delete duplicate keys, then use remove-duplicates, for example:

(define lst1 (list (list 2 "three") (list 2 "two") (list 1 "one")))
(remove-duplicates (get-keys lst1))
=> '(2 1)

Alternatively, if you're note allowed to use map we can always write a procedure from scratch using only primitive operations, like this:

(define (get-keys alst)
  (cond
    [(empty? alst) empty]
    [else (cons (first (first alst))
                (get-keys (rest alst)))]))

Upvotes: 4

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