Unused loop variables in Lisp

Question

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?

Your code is more efficient, but unless your lists are unusually long or this is a critical path, then it's probably premature optimization. The `list-length` version is definitely more clear. To answer the actual question, can you specify which interpreter you're using / care about? Disabling interpreter warnings is likely to be an interpreter-specific technique — Silvio Mayolo, Jul 14 '22 at 15:44
@MartinPůda It is not the same thing; I really need all the power of the `loop` macro for computing sophisticated independent sequences; the whole thing is that I want exactly as many terms as in l. — Thomas Baruchel, Jul 14 '22 at 16:40

Rainer Joswig · Accepted Answer · 2022-07-16T14:54:26.083

5

(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.

edited Jul 16 '22 at 14:54

answered Jul 16 '22 at 14:41

Rainer Joswig

136,269
10
221
346

2

Thank you for that; this is really what I was asking for! I could have explained my question a little better by referring to the "throwable variable" in other languages (underscore in Python), but you perfectly understood the point. – Thomas Baruchel Jul 16 '22 at 18:57
This is just a really useful thing to know! Thanks! – ignis volens Jul 18 '22 at 14:28

Kaz · Answer 2 · 2022-07-16T03:11:27.217

1

Try the repeat clause:

(loop repeat (length l) ...)

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

edited Jul 16 '22 at 03:11

answered Jul 16 '22 at 02:21

Kaz

55,781
9
100
149

score 0 · Answer 3 · answered Jul 16 '22 at 10:17

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.

Unused loop variables in Lisp

3 Answers3