In how many functions using the global variable, can a global variable be declared?

Hi!:)

I have a question: In my software that I am trying to make in Matlab, I have about two hundred functions and I have done a basic theoretical mistake which requires me taking in the temperature variable in all functions but instead of changing all functions in this way because then I have to change many things in many places in app designer, I want just to declare the variable temperatur to be be global in those functions where it is required. I have already declared temperatur as a global variable in two functions of each type of functions I have so I am wondering whether it can be done by the third , meaning can the variable be used as a global variable by three functions at the same time..?

Thanks

[Stephen23]

https://blogs.mathworks.com/videos/2010/03/08/top-10-matlab-code-practices-that-make-me-cry/

Answer 1

[John D'Errico]

There is no limit on how many functions can use a global variable. At the same time, I will plead, for your own sanity, to reconsider this path. Passing data around via global variables can make your debugging life pure hell. Did one of those functions mistakenly change the value of that variable? Which one? Where did something silly happen?

[Muazma Ali]

Hi one question related to this : as far as I remember, in one of the functions where the global variable is declared, the value of the variable has to be set and the other function which are just using this value, there you also have to declare it as global..isnt it like that? I just want to be sure

[Muazma Ali]

And it can be an output variable in a function but not an input..?

[Walter Roberson]

@Muazma Ali

It is not strictly necessary to assign the global variable in at least one function -- but if you never assign it then everywhere it is declared it will have the value [] (empty array).

For example, you might have

global debug_flag
if ~isempty(debug_flag)
    fprintf('got to position #17342\n');
end

and it would be perfectly valid for debug_flag not to be set anywhere.

Likewise, it is valid for more than one function to assign to the global variable. Common even:

function set_debug
  global debug_flag
  debug_flag = 1;
end
function clear_debug
  global debug_flag
  debug_flag = [];
end

Answer 2

[Steven Lord]

A variable in the global workspace is accessible from any workspace where it is declared as global. You may think this is good, as all you have to do is define it as global in the functions where you use it. But this can also be a very bad thing, as any workspace that declares it global (not just your functions) can see and modify the variable. This can be especially problematic if you used a short variable name (temp or t) as some other function can clobber the global variable's value without realizing it is going to break your function (or even knowing your function exists!)

I strongly recommend you either bite the bullet and change your functions' signatures (and call sites) to pass the temperature in and out of those functions, or (since you mention App Designer) store the temperature as a property of the app and modify the functions to use that property instead of a global variable. This still does have some of the problems around access (anyone with the app handle may be able to access it, depending on its Access, GetAccess, and/or SetAccess property attributes) but it does make it more difficult for someone to "accidentally" break your code (they'd have to "accidentally" get access to the app handle rather than just using the same variable name you used).