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How can I use attribute GLOBAL efficiently?

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Igor
Igor on 31 Mar 2011
MATLAB Help describes this topic (word "global") very shortly.
I ask it from such question:
M-file as function:
function [y]=y(x) a=2; y=a*x.^2 end
how to force the procedure to modify global variables? that is, after proc. work, some results will be saved in globals and be available for other procedures.
For example, assigning a=2 I wish to keep. But I don't want use simple ways: 1) text file 2) write [y,a] in header, or y=[y; a] in body
If I write global a; a=2
this haven't effect...
and may anybody give good practical code example with and without GLOBAL?

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Accepted Answer

Jan
Jan on 31 Mar 2011
Working with GLOBALs is fragile in all programming languages: It is very hard to find out, where and why the value was set the last time.
Therefore I'd use Paulo's approach ever: "function [y,a]=y(x)" and forward of [a] to all functions, which need it.
Another approach is using a dedicated function to store the value persistently:
function a = Store(Command, a)
persistent a_
switch Command
case 'get', a = a_;
case 'set', a_ = a;
otherwise error('unknown command');
end
Although this has the same drawback as GLOBAL ([a] can be influenced from anywhere), you can at least use the debugger to track changes and usage of [a].
But it is at least possible to use GLOBALs (I avoid using "y" as variable and function name at the same time):
function yValue = y(x)
global a
a = 2;
yValue = a * x .^ 2;
end
Then from the command line or inside another function:
global a
disp(a) % >> nothing
k = y(1:10);
disp(a) % >> 2

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David Young
David Young on 1 Apr 2011
Igor: you have to declare the variable as global in every workspace in which it is used, including the command-line workspace. Variables in the command-line workspace are not global unless you declare them to be. (You can see this with a simple experiment: make a function that declares a global and prints its value, give a variable with the same name a value at the command line and call your function.)
Jan
Jan on 1 Apr 2011
@Igor: Imagine declaring a variable once as GLOBAL would be enough. Then I insert "global sin; sin=0;" in any M-file. Afterwards no Matlab function would be able to calculate a SIN anymore, because the variable would shadow the function SIN. The same problem would appear for all variables: if "y" is such a brute GLOBAL concerning all functions, any function using this would overwrite the value - e.g. Matlab's MEAN function!
Igor
Igor on 1 Apr 2011
i suppose that at restricted sphere it's a plus!
but GLOBAL var. must be unique named
it's needed more believe in programmer!?

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More Answers (3)

Paulo Silva
Paulo Silva on 31 Mar 2011
The way you should always work with is:
function [y,a]=y(x)
if you still want to use global variables you must declare the variable as global everywhere you want to use it (other functions and workspace)

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Jan
Jan on 31 Mar 2011
@Paulo: Let me cite Matt Fig: "... the rest is just fluff".
Igor might be 8 or 88 years old, working with Matlab since 6 hours only, very tired or confused by the the impressive power of Matlab. Anyway, I do not see a reason to be inpolite.
Paulo Silva
Paulo Silva on 31 Mar 2011
Paulo==BeingImpolite
ans =
1
Igor
Igor on 1 Apr 2011
jokers...
MATLAB is a powerful THING!

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Walter Roberson
Walter Roberson on 31 Mar 2011
Do not use a variable name which is the same as the name of your function: doing so may lead to unwanted recursion.

David Young
David Young on 31 Mar 2011
If you find a real need to use global variables to share data between functions, consider adopting an object-based approach instead. The functions that need to share become methods in a class, and the shared variables become properties of that class.

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Igor
Igor on 1 Apr 2011
From theoretical aspect, you are right.
As I suppose, using GLOBAL instead of object incapsulation is like as a using GOTO for procedure oriented programmers.. :)
Jan
Jan on 1 Apr 2011
@Igor: During debugging, GLOBAL is even more a COMEFROM than a GOTO.

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