Correct use of tilde operator for input arguments

Question

Function:

My MATLAB function has one output and several input arguments, most of which are optional, i.e.:

output=MyFunction(arg1,arg2,opt1,opt2,...,optN)

What I want to do:

I'd like to give only arg1, arg2 and the last optional input argument optN to the function. I used the tilde operator as follows:

output=MyFunction(str1,str2,~,~,...,true)

Undesired result:

That gives the following error message:

Error: Expression or statement is incorrect--possibly unbalanced (, {, or [.

The error points to the comma after the first tilde, but I don't know what to make of it to be honest.

Problem identification:

I use MATLAB 2013b, which supports the tilde operator.
According to MATLAB's documentation the above function call should work:

You can ignore any number of function inputs, in any position in the argument list. Separate consecutive tildes with a comma...
I guess there are a few workarounds, such as using '' or [] as inputs, but I'd really like to understand how to correctly use '~' because actually leaving inputs out allows me to use exist() when checking the input arguments of a function.

If you need any further info from me, please let me know.

Thank you very much!

score 9 · Accepted Answer · answered Aug 08 '14 at 13:11

9

The tilde is only for function declaration. Matlab's mlint recommends to replace unused arguments by ~. The result is a function declared like this function output = MyFunction(a, b, ~, c). This is a very bad practice.

Since you have a function where the parameters are optional, you must call the function with empty arguments output=MyFunction(str1,str2,[],[],...,true).

A better way to do it is to declare the function with the varargin argument and prepare your function for the different inputs:

function output = MyFunction(varargin)

if nargin == 1
    % Do something for 1 input
elseif nargin == 2
    % Do something for 3 inputs
elseif nargin == 3
    % Do something for 3 inputs
else
    error('incorrect number of input arguments')
end

It is even possible to declare your function as follows:

function output = MyFunction(arg1, arg2, varargin)

The declaration above will tell Matlab that you are expecting at least two parameters.

See the documentation of nargin here.

... and the documentation of varargin here

answered Aug 08 '14 at 13:11

gire

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@lakesh Thanks for your replies, which are very useful. While I am using nargin in the function definition to make sure that users enter all required inputs, I decided against using varargin because it does not allow to assign descriptive variable names, such as strBenchmark. I've always thought that descriptive names make things easier for other users because they are shown in a small pop-up when you type the function name. I will think about using either [] or varargin. Thanks again! – DMueller Aug 08 '14 at 13:32
1

@DMueller True, `varargin` does not allow descriptive variable names. I always copy the inputs of `varargin` into variables with descriptive names. Having to copy variables is not good either (and wastes some memory) but is what I consider the best option given the limitations :) – gire Aug 08 '14 at 13:42
@lakesh I strongly disagree. When programming, being explicit is _very_ important. That includes variable names, function names, class names, etc. – gire Aug 08 '14 at 14:13

score 4 · Answer 2 · answered Aug 08 '14 at 13:20

To have variable number of inputs, use varargin. Use it together with nargin.

Example:

function varlist2(X,Y,varargin)
   fprintf('Total number of inputs = %d\n',nargin);

   nVarargs = length(varargin);
   fprintf('Inputs in varargin(%d):\n',nVarargs)
   for k = 1:nVarargs
      fprintf('   %d\n', varargin{k})
   end

Correct use of tilde operator for input arguments

2 Answers2