Comments on: Mental Model for feval

By: Loren

Loren — Mon, 01 Dec 2008 22:29:35 +0000

Ryan-

Yes, I was limiting my thoughts to M-files, not mex-files. For mex-files, I believe you have to call feval with function handles currently.

–Loren

By: Ryan Ollos

Ryan Ollos — Mon, 01 Dec 2008 22:26:05 +0000

Loren,

I found it was necessary to use feval in order to evaluate a function handle in the MATLAB workspace from within a mex function, using mexCallMATLAB.

In this case, I couldn’t find a way to evaluate the function handle directly. This is the only time I can recall ever having to use feval.

It appears that mexCallMATLAB builds a string from the input arguments and evals it in the caller workspace, which I think it the reason why I had to all feval working with a function handle.

The relevant portions of my code are:

% call mexFEval on function handle, returning 1 arg
% fh is a handle to a nested function
r = mexFEval(fh);

—

% the mex function has a single line (aside from error checking)

mexCallMATLAB(nlhs, plhs, nrhs, prhs, “feval”);

- Ryan

By: Matthew Simoneau

Matthew Simoneau — Tue, 30 May 2006 13:40:39 +0000

FYI, the page I linked to above has moved to a new location.

By: Matthew Simoneau

Matthew Simoneau — Mon, 06 Feb 2006 15:32:47 +0000

I’m constantly surprised to find relatively sophisticated MATLAB users who are unfamiliar with this “command/function duality”. A section in the MATLAB documentation describes it clearly, but many people never see it:

By: Ben

Ben — Tue, 31 Jan 2006 19:57:39 +0000

I’ll agree with Per here: I certainly fall victim to “transparency to documentation” occasionally. But sometimes that’s more of a problem with the user (including myself) that MATLAB shouldn’t have to fix; certainly big things like “Notes For C Programmers” are completely necessary and put up appropriately large red flags for common mental model errors when jumping between C and MATLAB.

However an advantage with MATLAB is that you can browse the source of many MATLAB functions to get a direct view (and a good complement to documentation) of what’s going on under the hood, unless they are builtin, like ‘dir’ :-(.

By: loren

loren — Tue, 31 Jan 2006 16:53:06 +0000

Krisna- Welcome to the MATLAB community! I recommend you read Getting Started with MATLAB, run demos, read the Usenet MATLAB newsgroup comp.soft-sys.matlab, and watch this blog for helpful topics.

By: krishna kr tiwari

krishna kr tiwari — Tue, 31 Jan 2006 15:31:28 +0000

I am a post graguate student.Studing in master of eectronics science.I currently worked on MATLAB but nobody know about the MATLAB.So please you give me a proper guiedens.

krisna kr tiwari
IPS ACADEMY.Indore(M.P.)
INDIA

By: per isakson

per isakson — Thu, 26 Jan 2006 15:13:55 +0000

RTFM. The problem (to some of us) is that we already have mental models of *commands* like DIR, SAVE, and PRINT when we try Matlab for the first time. Our models work well with Matlab when we start using it *interactively*. Our models are so well established that *looking through* the documentation of the commands does not change our models.

To me a problem with the Matlab documentation is that I tend to overlook powerful (added) features of functions because they are “hidden” in the body text of the descriptions and not addressed by the examples.

By: loren

loren — Mon, 16 Jan 2006 12:31:18 +0000

Is the doc for dir not clear perhaps? You can get this structure of output by using dir directly also: fstr = dir('*.gif') in the help description it says:

files = dir('directory') returns the list of files in the specified directory (or the current directory, if dirname is not specified) to an m-by-1 structure with the fields: name, date, bytes, isdir.

By: Yang

Yang — Sun, 15 Jan 2006 07:27:44 +0000

Yeah, I think the benefit of usage of the “feval” function is to return a structure argument like the ways described in “More on eval”.