using global variables to avoid passing by value

It is often heard that using global variables is a bad practice.

However, personally, I often declare some variable as global to avoid passing its value to a function call.

My concern is, the function is to be called many many times, and if each time the variable which is a big chuck of data is to be passed by value to the function, it would take a lot of time for copying the data.

Therefore, is using global variable really a bad idea in this circumstance?

BTW, the global variable is not changed by the function in my case.

Answer 1

[John D'Errico]

Sorry. I'll call it a bad idea, generated because you erroneously think you are doing something good.

MATLAB does not actually copy the data if it is not changed when you pass it into a function. I know, you don't believe me.

global A
A = rand(20000);

timeit(@() test1(A))

ans = 0.0474

timeit(@() test2)

ans = 0.0477

timeit(@() test3(A))

ans = 1.7941

function y = test1(A)
  y = sum(A); % uses A, but does not change it
end
function y = test2()
  global A % global pass
  y = sum(A); 
end
function y = test3(A)
  A(1,1) = 2; % changing one element of A now forces a copy.
  y = sum(A); 
end

In this example, it seems passing in the array took slightly LESS time than by passing it as global. When I changed one element of A inside the function, now MATLAB was forced to copy the array. So as you can see, passing it in as global gained you nothing.

Skip the globals. They just make your code worse in many ways.

Answer 2

[alelap]

I agree with Jonh D'Errico's answer.

MATLAB uses a so called "copy-on-write" mechanism that is exactly designed to tackle such a circumstance:

https://www.mathworks.com/help/matlab/matlab_prog/avoid-unnecessary-copies-of-data.html