How to skip a parameter in a function? ex: A = fread(obj,​size,'prec​ision') without size.

Hello.

Is here any syntax to not enter the second input to a function while entering the third?

I like to use A = fread(s, 'uint16');

But matlab complains that 'uint16' is not valid for size...

Answer 1

[Guillaume]

In general, there is no way to skip parameters that are defined in the function signature. If it's not your code, you have no choice but to provide all the arguments before the one you want to supply.

If you're writing your own code and you want to make your function flexible so that the user only has to provide the arguments that he needs, you have two options:

- The matlab way:

Use name - value pair arguments in combination with varargout. There are many ways to write this, for example:

function out = myflexiblefunction(required, varargout)
   par1 = defaultvalueforpar1;
   par2 = defaultvalueforpar2;
   for argidx = 2:2:nargin
      switch varargout{argidx}
         case 'Parameter1'
             par1 = varargout{argidx+1};
         case 'Parameter2'
             par2 = varargout{argidx+2};
      end
   end
   out = required + par1 - par2;
end

Usage:

out = myflexiblefunction(somevalue, 'Parameter2', someothervalue);

I greatly dislike this method as a) it involves a lot of parsing that the programming language should do for you, b) the user gets no hint as to the name of optional arguments (no tab completion) and has to look them up from the documentation or code.

- The function object way:

Use a class with properties for each optional argument:

classdef myflexiblefunction
   properties
      Parameter1 = defaultvalueforpar1
      Parameter2 = defaultvalueforpar2
   end

     methods
        function out = run(this, required)
           out = required + this.par1 - this.par2;
        end
     end
  end

Usage:

fn = myflexiblefunction; 
fn.Parameter2 = someothervalue;
out = fn.run(somevalue);

You get tab completion, the argument name parsing is done for you and you can reuse the function object so you don't need to supply the optional arguments several times. The downside is that the object syntax may be unusual to some people.

[Stephen23]

Option Three: using a struct to pass the desired options. This is used in some MATLAB functions (e.g. ode solvers), but requires a similar amount of internal processing to the name-value options. The advantages are:

• a clear linking of fields and values.
• keeping all the options within one saveable/passable/adaptable variable.

Answer 2

[Thorsten]

Supply an empty argument; the function should use the default instead:

 fread(fid, [], 'uint8')

(for fread you can just drop the second argument)

[David]

 EDU>> fread(s, [], '*uint16');
 Error using serial/fread (line 171)
 Invalid PRECISION specified. Type 'help serial/fread' for more information.

[Thorsten]

Too bad it does not work for fread. But for other functions I've used this syntax. It works for those functions where the default value is used if the number of arguments is to small OR the argument is empty.

Answer 3

[Greg]

The syntax for fread is for the second input to be the number of elements to read. If you're asking the question for this case specifically, why not just include that as an input? If you want to read the whole file and don't know how many elements it has, I believe you can just use a 'very large number' and it will read the whole file.

e.g. A = fread(s,10^100,'uint16');

[David]

I meant in general, I have a few times found my self not needing all inputs and often feel that it would be much easier for me to simply skip some inputs, in this case I am using fread in serial communications and it is nether the first nor the last part of the communication so I don't want to set some very high value. Is it possible to just skip some inputs or do I have to enter a second in order to use the third, in general?

[Greg]

Not that I know of. Usually I edit the order of the inputs in the function code if I need to do this, but I see that's not possible for fread. I agree it would be nice to have a simple solution to this.

[David]

Oh, I did not think about that. Then I know fread is no need to try but I will certainly try to change the order in others. Thanks.