How do I make an “empty” anonymous function in MATLAB?

Question

I use anonymous functions for diagnostic printing when debugging in MATLAB. E.g.,

debug_disp = @(str) disp(str);
debug_disp('Something is up.')
...
debug_disp = @(str) disp([]);
% diagnostics are now hidden

Using disp([]) as a "gobble" seems a bit dirty to me; is there a better option? The obvious (?) method doesn't work:

debug_disp = @(str) ;

This could, I think, be useful for other functional language applications, not just diagnostic printing.

Answer 1

You could add a regular do-nothing function to your codebase.

function NOP(varargin)
%NOP Do nothing
%
% NOP( ... )
%
% A do-nothing function for use as a placeholder when working with callbacks
% or function handles.

% Intentionally does nothing

Then you can use a function handle to it instead of to an anonymous function wherever you want to no-op something out.

debug_disp = @NOP;

Now it's somewhat self-documenting, making it explicit that you intended to do nothing, instead of grabbed the wrong input for disp(). It will be apparent in the source code, plus, when you're in the debugger and examining variables holding function handles, it'll show up as "@NOP", which may be more readable than an anonymous handle. And you can get a list of all nopped-out operations in the "profile report" output by looking at a list of callers to NOP.

You could also use Matlab's built-in @deal, which in the degenerate case does nothing and returns nothing.

Answer 2

I think disp([]) or disp('') is perfectly acceptable. It doesn't return anything and it has no side effects.

Answer 3

If you're simply looking for a "do-nothing" command to replace the body of the anonymous function, I'd probably go with DRAWNOW:

debug_disp = @(str) drawnow;

This will simply flush the event queue and update the graphics instead of displaying any text.

Answer 4

Here's a do-nothing anonymous function. It does nothing, and returns an empty array, which you can just ignore. You'll need to suppress disp by putting a semicolon after it.

debug_disp = @(str) [];

The disp([]) should work fine too. Whichever style you prefer.

Answer 5

try debug_disp = @(str)(1);