How can I use attribute GLOBAL efficiently?

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?

Answer 1

[Jan]

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

[Igor]

This effects :)

But I have a question -- what is sense of double using global?

GLOBAL within function -- as I know, directive to parser to use global var (no create new local var.)

GLOBAL out function -- ?? Any variable emerged from command line have automatically grobal scope, haven't it?

[David Young]

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]

@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]

i suppose that at restricted sphere it's a plus!

but GLOBAL var. must be unique named

it's needed more believe in programmer!?

Answer 2

[Paulo Silva]

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)

[Igor]

I still want with global :)

globalexample

function [y]=y(x)

global a

a=3;

y=a*x.^2

end

----

>>a=5;globalexample(4), a

No effect from GLOBAL

[Paulo Silva]

you have to declare global a in the workspace before your code!

global a;a=5;globalexample(4), a

[Igor]

NO EFFECT

globalexample.m --

function [y]=y(x)

a=3;

y=a*x.^2

end

>> global a;a=5;globalexample(4), a

=

48

ans =

48

a =

5

[Paulo Silva]

NO EFFECT BECAUSE YOU DON'T READ, I SAID "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)"

YOU REMOVED global a FROM YOUR FUNCTION AND PUT IT IN THE WORKSPACE

[Jan]

@Paulo: You are right. Igor removed the GLOBAL statement from the function, which is a mistake. Such mistakes happen, especially if the author is not familiar with this method at all. I think, we have helped him to learn more.

[Paulo Silva]

I said it in just a few words what he should do and he ignored it, I should have used bold letters, bigger font or whatever. Mistakes happen but those darn NO EFFECTS shouldn't happen.

??? Error using ==> The Computer

Please replace the user of this computer and try again

[Jan]

@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==BeingImpolite

ans =

1

[Igor]

jokers...

MATLAB is a powerful THING!

Answer 3

[Walter Roberson]

Do not use a variable name which is the same as the name of your function: doing so may lead to unwanted recursion.

[Igor]

yes

Answer 4

[David Young]

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.

[Igor]

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]

@Igor: During debugging, GLOBAL is even more a COMEFROM than a GOTO.