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Re: Numeric argument defaulting

From: Kevin Rodgers
Subject: Re: Numeric argument defaulting
Date: Mon, 21 Jun 2004 09:27:38 -0600
User-agent: Mozilla/5.0 (X11; U; SunOS i86pc; en-US; rv: Gecko/20020406 Netscape6/6.2.2

Bill Brodie wrote:
> I have several utility functions that take optional numeric arguments and
> that I'd also like to make interactive.  There are two ways to do this:
> 1. Define the function to accept either a raw or a numeric argument.
> (defun f (&optional x)
>   (interactive "P")
>   (cond
>    ((numberp x))
>    ((null x) (setq x <default value>))
>    (t (setq x (prefix-numeric-value x))))
>   ...)
> -or-
> 2. Define an interactive wrapper around the function.
> (defun f (&optional x)
>   (if (null x) (setq x <default value>))
>   ...)
> (defun f-interactive (x)
>   (interactive "P")
>   (if x (f (prefix-numeric-value x)) (f)))
> Neither of these seems especially clean.  Is there a standard Emacs Lisp
> idiom for this?

The argument is numeric, so use (interactive "p").  It will default to 1
when the argument isn't specified interactively.  If you want to provide
a default when it's called programmatically, you have to do that

(defun f (&optional x)
  (interactive "p")
  (if (null x) (setq x 1))

Kevin Rodgers

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