CODEWITHSUNDEEP
lispcommon-lisp
Alexander Hryk
Alexander Hryk

Reputation: 585

Unexpected result of the function (recursion)

I need to implement the function which creates a list from arguments that are passed to the function.

Here's my code:

(defun lstbuilder (&rest args)
  (if (eq (car args) NIL)
      NIL
      (cons (car args)
            (lstbuilder (cdr args)))))

This function doesn't work correctly. Results:

(lstbuilder 'a 'b 'c 'd 'e) ;expected (a b c d e)
(a (b c d e)) ;result

Upvotes: 3

Views: 121

Answers (2)

Rainer Joswig
Rainer Joswig

Reputation: 139411

Style

  • Please use standard Lisp formatting. Use an editor will help you to indent Lisp code.

  • Don't put a parenthesis alone on a line. This just wastes space without benefit.

  • Longer names get a - between the words: list-builder.

  • Don't use carand cdr for list operations. Use first and rest.

  • The end of list test is endp.

Example:

(defun list-builder (&rest args)
  (if (endp args)
      nil
    (cons (first args)
          (apply #'list-builder (rest args)))))

Since the args variable is already a list, we can just copy it:

(defun list-builder (&rest args)
  (copy-list args))

Or we can just reuse the list function, which already creates a list of its args:

(setf (symbol-function 'list-builder)
      #'list)

Upvotes: 5

C. K. Young
C. K. Young

Reputation: 223213

You need to use (apply #'lstbuilder (cdr args)) in order to "splat" the list's contents as the function call arguments.

Upvotes: 4

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