Scheme: Searching element in a list and sublist

debugcn Published at Dev

user2953423

I want to write a function that receives list and returns a list of each element. For example: get - (x 3 4 5 (x 4) 3 x (6))) and receive: (x (x) x ())

(define (lookForX lst)
  (cond
    ((null? lst) '())
    ((eq? (car lst) 'x) (cons (car lst) (lookForX (cdr lst))) )
    (else (lookForX (cdr lst)))))

my code result for:

(lookForX '(x 3 4 5 (x 4) 3 x (6))) 
-> (x x)

What am I doing wrong?

Sylwester

In you function you are only looking for x as element in the list and you are not doing sub lists:

(define (filter-x lst)
  (cond
    ((null? lst) '())
    ((eq? (car lst) 'x)
     (cons (car lst)
           (filter-x (cdr lst))))
    ((pair? (car lst))
     (cons (filter-x (car lst))
           (filter-x (cdr lst))))
    (else (filter-x (cdr lst)))))

(filter-x '(x 3 4 5 (x 4) 3 x (6)))
; ==> (x (x) x ())

Notice I renamed this to be more lisp like. Lisp code usually don't use camelCase but lisp-case. You can do it more general:

(define (filter-tree predicate? lst)
  (cond
    ((null? lst) '())
    ((predicate? (car lst))
     (cons (car lst)
           (filter-tree predicate? (cdr lst))))
    ((pair? (car lst))
     (cons (filter-tree predicate? (car lst))
           (filter-tree predicate? (cdr lst))))
    (else (filter-tree predicate? (cdr lst)))))

(define (filter-tree-x lst)
  (filter-tree (lambda (v) (eq? v 'x)) lst))

(filter-tree-x '(x 3 4 5 (x 4) 3 x (6)))
; ==> (x (x) x ())

(define (filter-tree-numbers lst)
  (filter-tree number? lst))

(filter-tree-numbers '(x 3 4 5 (x 4) 3 x (6)))
; ==> (3 4 5 (4) 3 (6))

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