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Adam
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Pass variable by reference to function

Asked by Adam
on 28 Sep 2012
Latest activity Commented on by Walter Roberson
on 16 Aug 2017
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?

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5 Answers

Answer by Daniel Shub
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

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.

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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.

  18 Comments

Hi Lucas,
the programming pattern that Image Analyst showed, namely
function Y = someFun(x, Y, z)
Y = x + Y;
involves no copying of Y at all, provided, that you
  • call it like var = someFun(x, var, y), i.e. both the function declaration and the calling function use the same variable names for input and output
  • Your MATLAB version is not too old: this optimization was introduced somewhere around 2010.
Give it a try with a large variable. You will see in the task manager no copy is done.
Titus
Hi, I tried your trick with very large matrix. By the jump in the memory consumption I can assure you it is not working by reference.
I think there is a typo in the comment by Titus Edelhofer, it should read
var = someFun(x, var, y),
rather than
var = someFun(x, Var, y),
@Royi, Which release are you running?

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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.

  2 Comments

I understand that only handles can be passed by reference. Is there a way of obtaining a handle for a variable?
No, not with documented Matlab. And not with http://undocumentedmatlab.com/ as far as I know.
Why do you want to pass by reference?

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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.

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Show 1 older comment
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)
Using pointers is not a hidden modification of a variable. It's a efficient practice widely used in any performing language.
Once you pass in a pointer, you lose control over whether there is a modification to the original data or not. You cannot tell from a C function declaration whether it modifies the data stored at any given pointer: at best you can hope that the person who wrote the function was diligent about putting in "const" where-ever possible.
Some languages require that every parameter to a function be marked as either input only, output only, or in/out . With such languages, there is never any surprise factor. If your goal is code cleanliness, reuse, and good practices, then you should prefer to avoid languages where the default is to permit modification of any item you have a pointer to.

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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.

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