repeating elements in common lisp [closed]

问题:

I am try to create a function with two arguments x and y which creates a list of y times repeated elements X but im getting confused on how to do it which or which method to use i think list compression can do but i want a shorter and simple method for example i want my simple code to be like this

if y = 4
 and x = 7
 result is list of elements (7, 7, 7, 7)

how can i go about it any ideas?? books links or anything that will give me a clue i tried searching but i have not been lucky

回答1:

Try this, it's in Scheme but the general idea should be easy enough to translate to Common Lisp:

(define (repeat x y)
  (if (zero? y)
      null
      (cons x
            (repeat x (sub1 y)))))

EDIT:

Now, in Common Lisp:

(defun repeat (x y)
  (if (zerop y)
      nil
      (cons x
            (repeat x (1- y)))))

回答2:

You can use make-list with the initial-element key:

CL-USER> (make-list 10 :initial-element 8)
   (8 8 8 8 8 8 8 8 8 8)

While a good example of how you can code such a function by yourself is provided by Óscar answer.