Unused loop variables in Lisp

user_2560053 0
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Sometime, I need to iterate $n$ times where $n$ is the number of elements in a list. Of course, I could write something like:

(loop for i from 0 below (list-length l)
      do something)

But I like to write rather:

(loop for i from 0
      for u in l ; unused 'u' variable
      do something)

where the for u in l part is merely intended to stop the loop after $n$ iterations. But then I don't use the u variable, and the interpreter keeps complaining. Is there some way to handle this style of coding? Should I avoid it and use list-length instead?

unused-variables common-lisp lisp loops
3 Answers
(loop for i from 0
      for NIL in l
      do something)

NIL should do it.

The LOOP destructuring pattern can be empty and the rest list elements are ignored. Additionally: In a LOOP destructuring pattern, NIL indicates that the variable is not used.

Rainer_Joswig_695450

Try the repeat clause:

(loop repeat (length l) ...)

The argument expression of the repeat clause is required to be evaluated once.

user_12507720

One horrible hack to do this which does not involve any extra traversals and if you really need to use loop is:

(defun foo (l)
  (loop for i upfrom 0
        for u = l then (cdr u)
          while (consp u)
        collect i))

But probably Kaz's answer of (loop repeat (length l) ...) is fine in practice, if you must use loop.

user_170269340
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