MATLAB: Passing by reference an object in a function

passing by reference

I have a simple question about passing a variable by reference in Matlab. from the forums I have seen, there is no clear answer about that. From what I have understood( but may be I am wrong), let say that you have a function that take a table A and want to change in this table only the h line. if I do

function arg=myFunction(A,h)
  A(h,:) = A(h,:)+2;
end

and call myFunction(A,3), A will not be changed. When specifying myFunction like this

function A=myFunction(A,h)
  A(h,:) = A(h,:)+2;
end

and calling myFunction(A,3), A is changed. But from what I understood of what matlab does, matlab is copying A, then change in the copy the h lines of A, then provide output the copy. First question: Is this true or not? and if yes, is there a way to pass it by reference, which means directly modifying A. I know that by doing directly A(h,:) = A(h,:)+2; in the command line, I can obtain the same result. but on some applications, when A is very large, like Gibbs sampler, where i want to sample sequentially each line of A conditionnally to the other ones, it looks to me that copying for every call the large dataset A is inefficient. I am not an expert of matlab, please excuse if the question is stupid.

Best Answer

For inplace operations on data using functions, see this Blog by Loren:

http://blogs.mathworks.com/loren/2007/03/22/in-place-operations-on-data/

Basically, there are rules you must follow for the function signature, and you must call the function from within another function.

Regarding your question about what MATLAB normally does for an arbitrary function argument, the basic answer is this:

If the function is a mex routine, MATLAB passes the address of the original variable (i.e., not a shared data copy). The mex routine is expected to treat all input arguments as read-only.

If the function is a m-file and the variable is not an OOP object derived from handle, MATLAB passes a shared data copy of the variable (i.e., a new variable but no data copy yet). If the function subsequently changes the passed variable, the variable is first unshared (i.e., a data copy takes place) and then the changes are made to this newly unshared variable.

If the function is a m-file and the variable is an OOP object derived from handle, MATLAB basically allows the function to make changes to the variable inplace (i.e., data is allowed to be altered inplace with no data copy).

Note that struct and cell array references (e.g., mystruct.field1 or mycell{3}) always create a temporary shared data copy of the field or cell element. So if you have this expression as an agrument a temporary shared data copy will always be passed, even to a mex routine.;

Loren's Blog talks about how to alter some of this behavior to get MATLAB to operate inplace in some circumstances.

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MATLAB: Can MATLAB pass by reference

If you are attempting to use pass-by-reference to modify the input argument passed into a function, the answer to the question depends on whether the input is a handle object or a value object. The two types of objects are described in the Object-Oriented Programming

http://www.mathworks.com/access/helpdesk/help/techdoc/matlab_oop/brfylq3.html and Programming Fundamentals http://www.mathworks.com/access/helpdesk/help/techdoc/matlab_prog/brtm8si.html sections of the documentation. By default, objects (including matrices, arrays, etc. of the built-in data types) are value objects.

Handle objects do exhibit reference behavior when passed as function arguments; value objects do not. When you pass a handle object to a function, MATLAB still copies the value of the argument to the parameter variable in the function (with one bit of subtlety; see below.) However, all copies of a handle object refer to the same underlying object.

If a function modifies a handle object passed as an input argument, the modification affects the object referenced by both the original and copied handles. In this case, the function does not need to return the result to be reassigned.

If instead you are attempting to use pass-by-reference to avoid unnecessary copying of data into the workspace of the function you're calling, you should be aware that MATLAB uses a system commonly called "copy-on-write" to avoid making a copy of the input argument inside the function workspace until or unless you modify the input argument. If you do not modify the input argument, MATLAB will avoid making a copy. For instance, in this code:

function y = functionOfLargeMatrix(x)
y = x(1);

MATLAB will not make a copy of the input in the workspace of functionOfLargeMatrix, as x is not being changed in that function. If on the other hand, you called this function:

function y = functionOfLargeMatrix2(x)
x(2) = 2;
y = x(1);

then x is being modified inside the workspace of functionOfLargeMatrix2, and so a copy must be made.

For more information on this behavior, read this posting http://blogs.mathworks.com/loren/2006/05/10/memory-management-for-functions-and-variables/ on Loren Shure's blog.

[From the MATLAB FAQ of Ancient Times]

MATLAB: Oop: how to switch between passing by reference and value

The documentation is pretty sparse, but also there: http://www.mathworks.co.uk/help/matlab/matlab_prog/copying-objects.html#brultwv

Copy Method for Handle Classes

Handle classes can derive copy functionality from the matlab.mixin.Copyable class. Class designers should investigate the use of this class in their class hierarchy design

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MATLAB: Can MATLAB pass by reference

MATLAB: Oop: how to switch between passing by reference and value

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