CODEWITHSUNDEEP
schemeracketlet
Chase
Chase

Reputation: 93

Declaring local variable in function from another list

I'm trying to define an element according to given values from another list

  (define foo   ; not working but this is general idea
    (λ (x ys)
      (for/list ([y ys])
        (if (eq? x (car y))
            (let ('x (cdr y))
              (boolean? x))  ; x not passed, returns #f
            '()))))   ; or use x to do something like (eval ('and x #t))

Desired output:

>(foo 'a '((a . true) (b . false)))
#t

to show the boolean of (cdr y) actually passes onto x (just an example that I'm trying)

Algorithm - search through ys for which (car y) of each element in ys to be equal to a and set the boolean (cdr y) to a. So the above x should have boolean #t.

Not quite understanding why it doesn't work.

Upvotes: 1

Views: 40

Answers (1)

Mulan
Mulan

Reputation: 135197

I'm doing my best to understand your problem, but it's not totally clear what your objective is.

It looks like you're trying to return a single #t/#f value, but if you look at the documentation for for/list, you'll see that it will always return a list.

Let's try something like this instead

(define (foo x ys)
  (let loop ([ys ys])
    (cond [(empty? ys) '()]
          [(eq? x (caar ys)) (cdar ys)]
          [else (loop (cdr ys))])))

(foo 'a '((a . true) (b . false))) ;=> 'true
(foo 'b '((a . true) (b . false))) ;=> 'false

However, if you want to convert 'true to #t and 'false to #f, you could add a helper function like so

(define (bool x)
  (eq? 'true x))

(define (foo x ys)
  (let loop ([ys ys])
    (cond [(empty? ys) '()]
          [(eq? x (caar ys)) (bool (cdar ys))]
          [else (loop (cdr ys))])))

(foo 'a '((a . true) (b . false))) ;=> #t
(foo 'b '((a . true) (b . false))) ;=> #f

Upvotes: 2

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