Lisp program that duplicates an element of a list using a list of integers

Question

I'm working on a program that takes a list of elements and each individual element is duplicated based on an integer contained in a 2nd list of integers. for example if I had a list of

(A B C D)

being duplicated by:

(1 5 4 2)

I would have

(A B B B B B C C C C D D)

So far I have

(defun COPY (X Y) 
  (if (zerop Y) 
      nil 
      (cons S (COPY X (1 - Y)))))

Of course this is only duplicating a single element a single number of times. Does anybody have a good idea how to go about this?

`1` is not a function, so `(1 ...)` is an error. You want to call function `-` on arguments `1` and `Y`, like so: `(- 1 Y)` — coredump, Sep 28 '18 at 07:43
@RainerJoswig I see a space in OP's question, it might be a typo but the code as written has a problem. — coredump, Sep 28 '18 at 13:44
@coredump: no doubt, there is a space - but maybe it was a typo or similar. Sometimes newcomers might not understand that and write an extra space. — Rainer Joswig, Sep 28 '18 at 14:02
there's a typo there, it's `(cons X` and then `(COPY X (1- Y) ...`. :) — Will Ness, Oct 07 '18 at 02:03

sds · Accepted Answer · 2018-09-27T21:31:49.607

Use mapcan and make-list (which is the library version of your copy):

(mapcan (lambda (letter num)
          (make-list num :initial-element letter))
        '(A B C D) '(1 5 4 2))
==> (A B B B B B C C C C D D)

or just

(mapcan #'copy '(A B C D) '(1 5 4 2))

If you are required to use simple recursion, you can also write

(defun copy-list-elements (elements counts)
  (and elements counts
       (let ((count (pop counts)))
         (if (plusp count)
             (cons (car elements)
                   (copy-list-elements elements
                                       (cons (1- count) counts)))
             (copy-list-elements (cdr elements)
                                 counts)))))
(copy-list-elements '(A B C D E) '(1 5 4 0 2))
==> (A B B B B B C C C C E E)

Note that the recursive version is longer and probably slower.

score 3 · Answer 2 · answered Sep 27 '18 at 21:21

3

Loop over the two lists, call copy, and append the results.

(loop for letter in letters
      for count in counts
      nconcing (copy letter count))

answered Sep 27 '18 at 21:21

Barmar

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Lisp program that duplicates an element of a list using a list of integers

2 Answers2