Passing by reference vs value

Hello !

Hope you are all doing fine.

What is better ?

clear; clc;

% let's consider a and b, some matrix

tic;
	multiply_tab(a, b);
toc;

tic;
	multiply(max(max(c)),min(min(d)));
toc;


function result = multiply(max_a, min_b)
	result = max_a * min_b;
end

function result = multiply_tab(a, b)
	result = max(max(a)) * min(min(b));
end

In fact, I would like to know if Matlab passes array parameters as a reference of the array in the memory or a new copy of the array.

Because I try to get my code the most efficient, finding the cleanest way between human readability and high computing speed.

Thanks !

Robin

Answer 1

[Stephen23]

"Passing by reference vs value"

Neither. MATLAB is more intelligent that either of those. MATLAB uses something called "copy on write", which essentially means that is passes by reference unitl the data is changed, at which point it makes a copy. This is explained in the blogs: "MATLAB makes copies of arrays it passes only when the data referred to changes"

source: https://blogs.mathworks.com/loren/2006/05/10/memory-management-for-functions-and-variables/

What does this mean for your code? In 99.99% of cases you should just use the simplest syntax, i.e. simply pass the variables and let MATLAB's memory management take care of how the variables are stored. In your example just pass a and b, no copies will be made.

In any case, TMW discourages writing code to take advantage of specific JIT features. The philosophy is that the JIT engine is written to optimize your code, not the other way around.

[Robin L.]

Thank a lot Stephen Cobeldick for your explanations !

I will use the simplest syntax : passing the variables as parameters and let Matlab manage how the variables are stored.

?

[Han Yoo]

How would you recursively call a function?

I need to make my own version of bwconncomp() using the flood fill algorithm.

[ioaImgConnComp] = setImgFloodFill(ioaImgConnComp, ivCoordX_pix, ...)

Without pass by reference, MATLAB keeps making a copy of the input image array (e.g., ioaImgConnComp), which is named the same as the output argument.

Pass by reference or point enables creating a recursive function easy and intelligent. What equivalent techniques are available on MATLAB?

Thanks.

[Stephen23]

@Han Yoo: recursive functions appear to create copies of their inputs (this might change depending on MATLAB version, etc). The simple solution to prevent this occuring is to write the recursive function as a nested function and define those variable/s in the "parent" workspace. Then all calls of the recursive function will access exactly the same variable/s in memory.