From help-octave-request at bevo dot che dot wisc dot edu  Tue Jan 20 08:08:49 1998
Subject: Re: How to tell a function from a string 
From: Jim Van Zandt <jrv at mbunix dot mitre dot org>
To: "John W. Eaton" <jwe at bevo dot che dot wisc dot edu>
cc: "Lorenzo M. Catucci" <lorenzo at argon dot roma2 dot infn dot it>,         help-octave@bevo.che.wisc.edu
Date: Tue, 20 Jan 1998 09:08:38 -0500



On 17-Jan-1998, Lorenzo M. Catucci <lorenzo at argon dot roma2 dot infn dot it> wrote:

| In the comments after definition of newtroot, in Octave's manual, I read:
| 
| < any serious code would check
| < the number and type of all the arguments, ensure that the supplied
| < function really was a function, etc.
| 
| Now, since I didn't find an is_function() function, I'd like to know how
| must I act to make sure what was passed as a function name is really
| registered with the parser (or can be registered) as a function.

I also thought the documentation could have been more helpful.
Eventually I found this:

 - Built-in Function:  exist (NAME)
     Return 1 if the name exists as a variable, 2 if the name (after
     appending `.m') is a function file in the path, 3 if the name is a
     `.oct' file in the path, or 5 if the name is a built-in function.
     Otherwise, return 0.

and decided that "exist(NAME) > 1" was a reasonable test.  (Can it
ever return 4?)

I'm now surprised to read JWE's reply:

> Unfortunately, there isn't a good way to do this from Octave, though
> you could probably wrap is_valid function from src/variables.cc in a
> DEFUN easily enough.

What's wrong with the above test?

Also, please add a discussion to the above passage in the manual.
Maybe something like this:
   
      if (exist(fname)<2)
   	error("newtroot: expecting function argument");
      endif
      ...
      ensure that the supplied function really was a function, etc.   
      The test in the example is helpful, but fails if...

...or explain why this test is hopeless, or whatever.

                                - Jim Van Zandt