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Pass variable by reference to function

Asked by Adam on 28 Sep 2012
Latest activity Commented on by Vit on 12 Mar 2014

I have read that the only way to pass by reference with a function is if you pass a handle. Is there a matlab call to get the "handle" of a variable or structure?

0 Comments

Adam

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5 Answers

Answer by Daniel on 1 Oct 2012
Accepted answer

While I think IA's answer is the better way to do it. If you really want to do what you are talking about you can create a subclass of handle.

classdef myClass < handle
	properties
		data
	end
	methods
		function h = myClass(data)
		  h.data = data	;
		end
	end
end

Then defining a function (which could be a method) like

function myFunction(h)
	h.data = h.data+2;
end

lets you do something like

h = myClass(0);
myFunction(h);
h.data
ans =
   2

of course things like h+2 will not work unless you implement a plus method.

1 Comment

Alessandro on 27 Feb 2013

Nice post ! Matlab should have something like this as a standard object ! The problem with this class is that it still copy the value from workspace to the class but is a good workaround.

Daniel
Answer by Image Analyst on 29 Sep 2012

For all practical purposes, you can get the same effect by passing the input argument back out. For example, in the function definition:

function myVariable = ChangeMyVariable(myVariable)
  myVariable = 2 * myVariable;

Then to call it, do this:

myVariable = ChangeMyVariable(myVariable);

And, obviously, myVariable could be anything, such as a structure like you mentioned. For all intents and purposes, this is the same as the code you'd use if it were pass by reference:

ChangeMyVariable(myVariable);

The net effect is you get your variable changed, it's just slightly different syntax.

11 Comments

Daniel on 4 Oct 2012

To me it is not that IA expressed his opinion, but that the response was not as friendly as it could have been.

@Walter, it seems like you are for deleting the flag and leaving the comment untouched.

Walter Roberson on 4 Oct 2012

I would copy the flag content into a Comment for the record, and then delete the flag.

Vit on 12 Mar 2014

I'm sorry to say that but original reply is wrong! It is not same! For my matlab 2011b on Win calling function with variable and edit that variable do duplicate used memory for that variable (even when stored to same variable). If variable has 1GB then it became a problem when you want just pass reference and modify original variable (Which I do). If I use class <handle it works well and does not duplicate memory consumption.

Image Analyst
Answer by per isakson on 28 Sep 2012
Edited by per isakson on 28 Sep 2012

No, there is no way to pass a variable by reference. See Memory Management for Functions and Variables.

Handle objects, http://www.mathworks.se/help/matlab/matlab_oop/user-defined-classes.html, are passed by "reference".

2 Comments

Adam on 28 Sep 2012

I understand that only handles can be passed by reference. Is there a way of obtaining a handle for a variable?

per isakson on 28 Sep 2012

No, not with documented Matlab. And not with http://undocumentedmatlab.com/ as far as I know.

Why do you want to pass by reference?

per isakson
Answer by James Tursa on 29 Sep 2012

I will ask again what others have already asked. Why do you want the "handle" to the variable? What advice we give you will depend on the answer to that question. FYI, by default MATLAB passes a shared data copy of the arguments to the function ... i.e., there is no data copy involved. So it is already efficient in that sense. But we really need to know what you are doing with the variable inside the function in order to best answer your question, and whether passing by "reference" makes sense for your application.

2 Comments

Adam on 1 Oct 2012

This isn't a memory question as much a code cleanliness, reuse, and good practices question as well as my own edification. I'm not looking for coding advice, but an answer to the question of if it is fundamentally possible to get the "handle" of a variable or structure?

Walter Roberson on 3 Oct 2012

If it is a question about code cleanliness, reuse, and good practices, then you are looking for coding advice, and IA's recommendation to use a return value is not "imposed" is an appropriate "good practices" response. Hidden modification of variables is not good practice; it is, for example, emphatically disallowed in Yourdon & Constantine's "Structured Design" (1979)

James Tursa
Answer by Alessandro on 16 Apr 2013

Hello I would say a possible solution is the following (without using classes):

%Reference to some matlab Data
  function reference = globaldata()
      data = [];
        reference = struct('GET',@GET,'SET',@SET);
        function dataout = GET()
            dataout = data;
        end
        function SET(datain)
            data =datain;
        end
    end

I saw something like this for making linked lists in matlab on stack overflow.

0 Comments

Alessandro

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