Reorder function arguments in Lisp

Question

I'm interested in an operator, "swap-arg", that takes as input 1) a function f of n variables, and 2) index k, and then returns a the same function except with the first and kth input variables swapped. eg (in mathematical notation):

(swap-arg(f,2))(x,y,z,w) = f(z,y,x,w)

Now my first idea is to implement this using rotatef as follows,

(defun swap-args (f k) 
  (lambda (L) (f (rotatef (nth k L) (car L)))))

However, this seems inelegant since it uses rotatef on the input. Also, it's O(n), and could be O(n^2) in practice if applied repeatedly to reindex everything.

This seems like a common problem people would have already considered, but I haven't been able to find anything. What's a good way to swap inputs like this? Is there a standard method people use?

Your function f is not of n variables. It is called in your example as a single argument function. — Rainer Joswig
– Rainer Joswig, Commented Oct 23, 2010 at 10:21
The way I set it up i guess you're right, it takes a list as an argument. One would call it as follows, (f (x y z w)) I'm not stuck on calling it that way, something that swaps the literal arguments, eg (f x y z w) -> (f z y x w) would be good as well. — Nick Alger
– Nick Alger, Commented Oct 23, 2010 at 10:32
if X is a function, you can call it like that. If not, you get an error. — Rainer Joswig
– Rainer Joswig, Commented Oct 23, 2010 at 10:34

Rainer Joswig · Accepted Answer · 2010-10-23 10:30:02Z

4

Using APPLY:

(defun create-swapped-arg-function (f k)
  "Takes as input a function f of n variables and an index k.
Returns returns a new function with the first and kth input variables swapped,
which calls the function f."
  (lambda (&rest args)
    (apply f (progn
                (rotatef (nth k args) (first args))
                args))))

Example:

CL-USER 5 > (funcall (create-swapped-arg-function #'list 2) 0 1 2 3 4 5 6)
(2 1 0 3 4 5 6)

Another way to do it would be to build the source code for such a function, compile it at runtime and return it. That would be useful if these functions are not created often, but called often.

answered Oct 23, 2010 at 10:30

Rainer Joswig

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2 Comments

Nick Alger Over a year ago

That's basically what I was trying to write in the OP, except without the bugs. (: The problem still remains though - it is O(N) due to the call to n'th, and also uses rotatef on the inputs which would change them (if I understand correctly).

Rainer Joswig Over a year ago

@mazemaster255: It would not change the inputs when called with FUNCALL. It might when called with APPLY. If you want to avoid that use COPY-LIST to create a new one. Sure it would be slow at runtime. See the last paragraph of my answer for an alternative.

user110763 · Accepted Answer · 2010-10-23 11:26:55Z

2

Just for completeness, functions can also take keyword (named) arguments, using this the function can be called with any order of its keyword arguments.

answered Oct 23, 2010 at 11:26

user110763

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