How can I check for semicolon termination of my own function

My function does something and then writes information to the screen by using

function a = say_hi(b)
  a = b;
  fprintf(1, 'Hello World');
end

I would like to check if the line in the script that called this function was terminated by a semicolon, such that merely does a = b in that case. Is that possible?

Kind regards,

Jan

[Doug Hull]

OP said in answers instead of comments:

---

Thanks Walter,

I understand what you say. Let me rephrase the question. I could do what I want by defining the function as follows:

function a = say_hi(b, print)

a = b;

if (print == 1)

fprintf(1, 'Hello World');

end

end

But then I would have an additional input parameter, which is undesired. I am looking for a keyword such as 'nargin' that is available inside the function space to check for its termination i.e. '\n' or ';\n'.

Answer 1

[Walter Roberson]

Semi-colon does not suppress anything explicitly printed: it only affects whether the value of assignments are automatically printed.

[Jan]

Thanks Walter,

I understand what you say. Let me rephrase the question. I could do what I want by defining the function as follows:

function a = say_hi(b, print)

a = b;

if (print == 1)

fprintf(1, 'Hello World');

end

end

But then I would have an additional input parameter, which is undesired. I am looking for a keyword such as 'nargin' that is available inside the function space to check for its termination i.e. '\n' or ';\n'.

[Walter Roberson]

I am not _aware_ of any way to check that. I'm not sure, but I think if it did exist, I could formulate a "Liar's Paradox" around it.

[Jan]

Hi Walter,

I am not sure that I understand the paradox, the semicolon is either there or it is not. I was wondering if I could check it just as I can check the input and output arguments. See my answer below.

[Walter Roberson]

With eval, I could construct a clause that had a semi-colon if and only if the test said that there was no semi-colon. Remember Matlab is interpreted and what is to be executed can be constructed on the fly.

Answer 2

[Jan]

I just realized that I just want the function to output text if I use it without assigning the output argument. So it would look like this:

function a = say_hi(b)
  a = b;
  if (nargout == 0)
    fprintf(1, 'Hello World');
  end
end

The reason I did not want the 'print' argument in the list, was that maybe I want to increase the number of arguments in the future. I would not want the 'print' argument somewhere in the middle of the list, but at the end. As that would cause backwards compatibility issues, I could use an optional input argument 'DisplayOn' to solve it (rather lengthy):

function a = say_hi(b, varargin)
  print = 1;
  if nargout == 1
    print = 0;
  end
  if size(varargin,2) >= 2
    for j = 1:2:length(varargin)
      param = varargin(j);
      value = varargin(j+1);
      if strcmp(param, 'DisplayOn')
        if strcmpi(value, 'true')
          print = 1;
        else
          print = 0;
        end
      end
    end
  end

   a = b;
   if (print == 1)
      fprintf(1, 'Hello World');
   end
end

[Walter Roberson]

Your first function can possibly be rewritten as:

function a = say_hi(b)

if nargout

a = b;

else

fprintf(1, 'Hello World');

end

In your second function, you should use strcmpi(param, 'DisplayOn') and for the value, instead of looking for the string 'true', you should look for non-zero values, to be consistent with how parameters are defined for the Matlab libraries.

I believe there are utility routines that can do the parameter parsing for you; you can find examples in nearly any of the Mathworks-provided routines.