Test existence of global variable within function?

Hi, I want to access the status of a variable every where within different function. I created the global variable:

global debugFlag
debugFlag = true;

I create a function idebugging the goal will be to use them when developping other function as:

function output = myawesomefunction(input)
  ...
  doing stuff
  ...
  if isdebugging
    fprintf(stuff)
  end
  ...
end

The function I created is this one:

function existing = isdebugging()
    %%Check if debugFlag global variable exist and is true
    if exist('debugFlag', 'var')
        existing = debugFlag;
    else
        existing = false;
    end
end

But this function always return false.

I checked the answer to this question and tried with

W = evalin('caller','whos');
existing = ismember('debugFlag',[W(:).name])

But this always return true. How can I achieve the function I want? I am sure other people are achieving this for debugging, what is the proper method to have some debugging information?

Thank you.

[Gabor]

Before going into all this nonsense:

ismember('variable_of_interest', who('global'))

credit to:Walter Roberson

Answer 1

[Stephen23]

You need to also explicitly declare that global variable inside the functions where you want to use it:

function existing = isdebugging()
global debugFlag % <- your need this!
existing = ~isempty(debugFlag) && debugFlag;
end

Note that there is absolutely no point in testing for the existence of that global variable because, as the global documentation clearly describes, "If the global variable does not exist the first time you issue the global statement, it is initialized to an empty 0x0 matrix." So it will always exist, but it can be empty, true, or false, depending on the state of your code. So those are the variable states that you need to check for in your function.

Personally I would recommend that you avoid using a global variable for this: given that you are happy to have a special function isdebugging, then you could easily use a persistent variable, which neatly encapsulates the functionality into one function and prevent unexpected interference with the global variable:

function otp = isdebugging(inp)
persistent state
state = (~isempty(state) && state) || (nargin>0 && inp);
otp = state;
end

Simply call it to set the state, e.g. isdebugging(true).

"I am sure other people are achieving this for debugging, what is the proper method to have some debugging information?"

Most experienced programmers use the inbuilt debugging tools:

https://www.mathworks.com/help/matlab/matlab_prog/debugging-process-and-features.html

[Gabor]

"Note that there is absolutely no point in testing for the existence of that global variable because, as the global documentation clearly describes"

The problem with this, if the Global Variable accidently not defined before the use in the function, but we "define it empty in the function" than it will delete the original variable after we get back to the original state after running the function. This created unwanted empty variables for me numerous times.

Basically it is almost dangerouse to use global variable just because the fact it will start deleting variables if it is not declared where it suppose to. For me it just do not make any sense to defined it empty if it is not defined, thats why I would like to know if it is defined or not so I dont create accedental empty global variables.

Or I just miss the logic... why do we have to define it twice?

"If the global variable does not exist the first time you issue the global statement, it is initialized to an empty 0x0 matrix"

[Gabor]

I guess my workaround will be that I create it anyway and check it if it empty and if it is than I delete it right away.

[Stephen23]

"Basically it is almost dangerouse to use global variable just because the fact it will start deleting variables if it is not declared where it suppose to."

It is certainly "dangerous" to use global variables. They are best avoided.

"Or I just miss the logic... why do we have to define it twice?"

You don't need to define global variables twice. However if you keep redefining it (including via the default value of a GLOBAL call if the variable does not exist in the global workspace) then it will have whatever value you gave it last. That the the whole point of global variables.

Of course you do need to declare it in every workspace where you want to refer to it:

funOne()
funTwo()

    3.1416

funThree()

Warning: The value of local variables may have been changed to match the globals. Future versions of MATLAB will require that you declare a variable to be global before you use that variable.

funFour()

    3.1416

function funOne()
global someval % this redefines the variable!
someval = pi;
end

function funTwo()
global someval
disp(someval)
end

function funThree()
someval = sqrt(2);
global someval
end

function funFour()
global someval
disp(someval)
end

The warning is there for a reason.

I agree that TMW should forbid usage before the variable is declared. That would resolve your accidental creating of empty variables (as would defining them as global at the top of the code).

Answer 2

[Walter Roberson]

ismember('variable_of_interest', who('global'))

[Gabor]

This should be on the top as the one and only normal answer to the question!

Thank you

[Stephen23]

"This should be on the top as the one and only normal answer to the question!"

Lets try it right now and see how it fails:

global debugFlag
debugFlag = false; % debug state is FALSE!

YourApproachFails()

If this displays then your approach does NOT work

MyAnswerWorks()

Oh, my answer works. Debugging is disabled!

function YourApproachFails()
if ismember('debugFlag',who('global'))
    disp('If this displays then your approach does NOT work')
else
    disp('Hurrah! Debugging is disabled!')
end
end

function MyAnswerWorks()
global debugFlag % <- you need this!
if ~isempty(debugFlag) && debugFlag
    disp('If this displays then my approach does NOT work')
else
    disp('Oh, my answer works. Debugging is disabled!')
end
end

Have a think about how you would need to modify your approach to make it work correctly.

[Walter Roberson]

ismember('variable_of_interest', who('global'))

detects whether the global variable exists at all, and does not have the side effect of creating the global variable

[Stephen23]

"detects whether the global variable exists at all, and does not have the side effect of creating the global variable"

Yes, I am quite aware of that.

The OP makes it clear that they also want to check the value of that global variable. Which this code does not do.