[Scilab-users] How to convert the ~ input placeholder <= Re: convert matlab code to scilab

[Chin Luh Tan]

Hi Samuel, 



I think your assumption likely correct on the input and the output using ~



https://www.mathworks.com/matlabcentral/answers/288016-tilde-doesn-t-work-for-ignoring-my-inputs



"

You can use tilde to ignore input arguments when you define the function.

You cannot use tilde to ignore input arguments when you call the function.

You cannot use tilde to ignore output arguments when you define the function.

You can use tilde to ignore output arguments when you call the function.

If you want to be able to specify only the third argument, there must be some way for your function to disambiguate the one-input call from a call that specifies only the third input. Usually in MathWorks function this is done by specifying [] for the arguments for which the user doesn't want to specify. In that case the code would use the default value if either nargin is too small or if nargin is large enough and the specified input argument isempty.

"

>From what I could think off, mat2sci conversion to replace ~ at the output arguments should always be safe. As for the input, since the tilde could not be used as input when calling function, I think there should not be any cases that need to be implemented in conversion? 

Thanks.

Regards,

CL




---- On Tue, 26 Nov 2019 04:20:52 +0800 Samuel Gougeon <sgougeon at free.fr> wrote ----



Le 23/11/2019 à 15:35, Samuel Gougeon a
      écrit :

Hello Philipp, and to all (former ;-) matlabers,



Le
        16/10/2019 à 11:46, P M a écrit :

Dear experts,



trying to convert a matlab code to scilab I come across
            following line:

[~,~,Minstances_hat] = unique(B(:));


How to replace the "~" symbol?


B is the blue channel of a RGB image...hence a m x n matrix of integers (type(B) = 8 )







This issue is now reported as http://bugzilla.scilab.org/show_bug.cgi?id=16254.

Solving this issue would improve the converter.
 I may contribute to solve it. However, I do not use to use
        Matlab or Octave. So i don't know how the ~ placeholder set in
        the list of inputs is processed inside the called
        function. This message is a call for information about this
        topic.
 
 Ignoring an output is trivial. As suggested by Stéphane, the ans
        variable is a good candidate as replacement for ~,
 just as a fake recipient, as in "[~, ia] = unique(A)"   => 
        "[ans, ia] = unique(A)"

The meaning of ignoring an input is a priori completely
        different, at least from a Scilab point of view.

This leads to the fact that, on the "function ..." line, the
        converter will have to distinguish the list of inputs from the
        list of outputs, and then process the same ~ character in 2
        different ways.
 This processing would have to run also when the "function ..."
        line is split on several rows with the "..." continuation marks.
 
 First, i will need an actual example of function call with ~ in
        the input list, runnable in Octave.
 Any compact suggestion is welcome.
 At least 2 use cases are expected: one calling a function
        written in Octave/Matlab language (improperly called "macros" in
        Scilab), the other one calling a hard-coded function.

Let's consider the first case, with a "macro":

What does  the ~ sent placeholder pass to the function? How
          is it detected/detectable inside the macro ?

How is it processed by the function? Is there a generic
          default processing, as replacing it with the empty matrix [],
          or whatever else?



Where is the crowd of matlabers?

The page describing this feature:https://fr.mathworks.com/help/matlab/matlab_prog/ignore-function-inputs.html

 But i do not clearly understand it.

 Apparently, "canceling" an input is done (only?) at the function
    definition, not when calling it, unlike for the outputs.

 If so, then ans could also be used as a replacement, meaning
    that anyway this input won't be used inside the function.

 

A test of call with Octave yields an error:

>> function [r, s] = funtest(a,b,c)
 end
 >> funtest(1,~,3)
 parse error:
   invalid use of empty argument (~) in index expression
 >>> funtest(1,~,3)
                  ^
 >>

Any further insight is welcome.

 Samuel

 

 


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