Unused loop variables in Lisp-CodePudding

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?

CodePudding user response：

(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.

CodePudding user response：

Try the repeat clause:

(loop repeat (length l) ...)

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

CodePudding user response：

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.