I'm trying to create a moving average window for temperature sensor data.
I've followed:
How to declare global variables for simulink containing matlab functions? and https://www.mathworks.com/help/simulink/ug/using-global-data-with-the-matlab-function-block.html
which explain how to do this for a single variable ie (A=5 and I can play around with A)
How do I do this with a global array/vector that I can edit the values of?
Simply doing
function y = fcn(u)
global A
A = [];
A(end+1)=u;
y = sum(A)/length(A)+10;
does not work.
You are reinitializing your global A variable each time you call the function. Do this instead:
function y = fcn(u)
global A
A(end+1)=u;
y = sum(A)/length(A)+10;
(Of course, if you are using this as part of a Simulink operation, you will need to add logic to keep the array from growing too far. I assume you generated a minimal example for purposes of the question.)
In many cases, an improvement is to use a persistent instead of a global. Persistent variables are kept between multiple call to a function, but are not available to any other function. This prevents unexpected changes to your variables. (Especially if your operational code uses a variable named "A".)
function y = fcn(u)
persistent A
A(end+1)=u;
y = sum(A)/length(A)+10;
A more general approach for using global or persistent variables is to is an isempty check to see if it needs to be initialized. Like this:
function y = fcn(u)
persistent A
if isempty(A)
A = 0; %Or whatever the appropriate initial value is. An empty array is the default.
end
A(end+1)=u;
y = sum(A)/length(A)+10;
Related
Is there a way to declare global variables in MATLAB?
Please don't respond with:
global x y z;
Because I can also read the help files.
I've declared a global variable, x, and then done something like this:
function[x] = test()
global x;
test1();
end
Where the function test1() is defined as:
function test1()
x = 5;
end
When I run test(), my output is x = []. Is there a way I can make it output the x=5, or whatever I define x to be in a separate function? In C, this would be an external variable, and I thought making it a global variable should accomplish just that.
You need to declare x as a global variable in every scope (i.e. function/workspace) that you want it to be shared across. So, you need to write test1 as:
function test1()
global x;
x = 5;
end
Referring to your comment towards gnovice using a global variable can be an approach to solve your issue, but it's not a commonly used.
First of all make sure that your .m files are functions and not scripts. Scripts share a common workspace, making it easy to unwillingly overwrite your variables. In contrast, functions have their own scope.
Use xUnit in order to generate repeatable unit test for your functions. By testing each function involved in your program you will track down the error source. Having your unit test in place, further code modifications, can be easily verified.
A possible way to get around the global mess is to assign the variable as appdata. You can use the functions setappdata and getappdata to assign and retrieve appdata from a MATLAB window. As long as a MATLAB session is active there exists a window denoted by 0.
>> setappdata(0,'x',10) % 0 indicates the root MATLAB window
Now the variable x is not visible to any script or function but can be accessed wherever needed by using getappdata.
function test
globalX = getappdata(0,'x');
disp(globalX);
end
x =
10
The good news is that you can assign any valid MATLAB object to appdata, just be cautious with the names, using unique names for appdata fields like ModelOptimizerOptions instead of a generic x,y would help. This works on compiled executables and code deployed on the MATLAB production server as well.
I would like to create a global array within a Matlab function block in Simulink that grows in size, each time the Matlab function block is called. The only approach to create global variables in a Matlab function block is to link it to a "Data Store Memeory" block but this does not allow me to create arrays which increase in size.
I am currently using a local array within the Matlab function block and writing the contents of the array to a file before exiting the function block and reading from it in the next iteration. This slows down the execution time though. Is there any better way of doing this?
You could use a persistent variable in a function to store the data. Place the following function in a file varsizeDataStore.m on your MATLAB path:
function y = varsizeDataStore(varargin)
%#codegen
% Update the bounds to fit your needs
coder.varsize('p',[1,2000], [0,1]);
persistent p;
if isempty(p)
p = zeros(1,0);
end
if nargin > 0
p = [p, varargin{1}];
end
y = p;
When you want to grow the persistent variable, pass a parameter to varsizeDataStore. To query the value call it with no parameters.
Obviously, you can change the logic invarsizeDataStore to suit your needs. Its purpose is to demonstrate the idea of using a persistent variable and a function as a way to store and query data without using a global variable.
In the workspace I make a matrix .
Now I can access the variable in script. Like doing Variable(2) will return 4.
But inside a function like
function y= getvariable(x)
y=Variable(x)
end
I get error
y=getvariable(2)
??? Undefined function or method 'Variable' for input
arguments of type 'double'.
Error in ==> getvariable at 3
y=Variable(x)
So how to make the Variable matrix global so that I can access it through any function?
Although you could use globals
>> global Variable = rand(50,12);
...
function y = getvariable(x)
% Always needed
global Variable;
% Here ya go
y = Variable;
end
the MUCH better alternative is to use
function x = getvariable(x)
% no body needed
end
which you call as
>> y = getvariable(Variable);
(Of course, for this contrived example, this would just be equal to
>> y = Variable;
)
Although there are some legitimate use cases for global variables, in general they tend to spaghettify your code and make it far more bug-prone and much harder to debug. Have a read on the subject.
As #rody suggested, pass the matrix and the x inside the function
I am just giving an example to make things clear.
Like you want to access the 10th element of Variable matrix, so make the function as
function y= getvariable(matrixname,no)
y=matrixname(no)
end
If you want to access 3rd element of Variable, so you type
y=getvariable(Variable,3)
you will get 3rd element
call global Variable before you define it in your workspace
call global Variable before you use it in your function
However I suggest you think of other ways to pass variables to your function, as globals might cause difficulties during debugging.
when I am doing a function in Matlab. Sometimes I have equations and every one of these have constants. Then, I have to declare these constants inside my function. I wonder if there is a way to call the values of that constants from outside of the function, if I have their values on the workspace.
I don't want to write this values as inputs of my function in the function declaration.
In addition to the solutions provided by Iterator, which are all great, I think you have some other options.
First of all, I would like to warn you about global variables (as Iterator also did): these introduce hidden dependencies and make it much more cumbersome to reuse and debug your code. If your only concern is ease of use when calling the functions, I would suggest you pass along a struct containing those constants. That has the advantage that you can easily save those constants together. Unless you know what you're doing, do yourself a favor and stay away from global variables (and functions such as eval, evalin and assignin).
Next to global, evalin and passing structs, there is another mechanism for global state: preferences. These are to be used when it concerns a nearly immutable setting of your code. These are unfit for passing around actual raw data.
If all you want is a more or less clean syntax for calling a certain function, this can be achieved in a few different ways:
You could use a variable number of parameters. This is the best option when your constants have a default value. I will explain by means of an example, e.g. a regular sine wave y = A*sin(2*pi*t/T) (A is the amplitude, T the period). In MATLAB one would implement this as:
function y = sinewave(t,A,T)
y = A*sin(2*pi*t/T);
When calling this function, we need to provide all parameters. If we extend this function to something like the following, we can omit the A and T parameters:
function y = sinewave(t,A,T)
if nargin < 3
T = 1; % default period is 1
if nargin < 2
A = 1; % default amplitude 1
end
end
y = A*sin(2*pi*t/T);
This uses the construct nargin, if you want to know more, it is worthwhile to consult the MATLAB help for nargin, varargin, varargout and nargout. However, do note that you have to provide a value for A when you want to provide the value of T. There is a more convenient way to get even better behavior:
function y = sinewave(t,A,T)
if ~exists('T','var') || isempty(T)
T = 1; % default period is 1
end
if ~exists('A','var') || isempty(A)
A = 1; % default amplitude 1
end
y = A*sin(2*pi*t/T);
This has the benefits that it is more clear what is happening and you could omit A but still specify T (the same can be done for the previous example, but that gets complicated quite easily when you have a lot of parameters). You can do such things by calling sinewave(1:10,[],4) where A will retain it's default value. If an empty input should be valid, you should use another invalid input (e.g. NaN, inf or a negative value for a parameter that is known to be positive, ...).
Using the function above, all the following calls are equivalent:
t = rand(1,10);
y1 = sinewave(t,1,1);
y2 = sinewave(t,1);
y3 = sinewave(t);
If the parameters don't have default values, you could wrap the function into a function handle which fills in those parameters. This is something you might need to do when you are using some toolboxes that impose constraints onto the functions that are to be used. This is the case in the Optimization Toolbox.
I will consider the sinewave function again, but this time I use the first definition (i.e. without a variable number of parameters). Then you could work with a function handle:
f = #(x)(sinewave(x,1,1));
You can work with f as you would with an other function:
e.g. f(10) will evaluate sinewave(10,1,1).
That way you can write a general function (i.e. sinewave that is as general and simple as possible) but you create a function (handle) on the fly with the constants substituted. This allows you to work with that function, but also prevents global storage of data.
You can of course combine different solutions: e.g. create function handle to a function with a variable number of parameters that sets a certain global variable.
The easiest way to address this is via global variable:
http://www.mathworks.com/help/techdoc/ref/global.html
You can also get the values in other workspaces, including the base or parent workspace, but this is ill-advised, as you do not necessarily know what wraps a given function.
If you want to go that route, take a look at the evalin function:
http://www.mathworks.com/help/techdoc/ref/evalin.html
Still, the standard method is to pass all of the variables you need. You can put these into a struct, if you wish, and only pass the one struct.
Is there a way to declare global variables in MATLAB?
Please don't respond with:
global x y z;
Because I can also read the help files.
I've declared a global variable, x, and then done something like this:
function[x] = test()
global x;
test1();
end
Where the function test1() is defined as:
function test1()
x = 5;
end
When I run test(), my output is x = []. Is there a way I can make it output the x=5, or whatever I define x to be in a separate function? In C, this would be an external variable, and I thought making it a global variable should accomplish just that.
You need to declare x as a global variable in every scope (i.e. function/workspace) that you want it to be shared across. So, you need to write test1 as:
function test1()
global x;
x = 5;
end
Referring to your comment towards gnovice using a global variable can be an approach to solve your issue, but it's not a commonly used.
First of all make sure that your .m files are functions and not scripts. Scripts share a common workspace, making it easy to unwillingly overwrite your variables. In contrast, functions have their own scope.
Use xUnit in order to generate repeatable unit test for your functions. By testing each function involved in your program you will track down the error source. Having your unit test in place, further code modifications, can be easily verified.
A possible way to get around the global mess is to assign the variable as appdata. You can use the functions setappdata and getappdata to assign and retrieve appdata from a MATLAB window. As long as a MATLAB session is active there exists a window denoted by 0.
>> setappdata(0,'x',10) % 0 indicates the root MATLAB window
Now the variable x is not visible to any script or function but can be accessed wherever needed by using getappdata.
function test
globalX = getappdata(0,'x');
disp(globalX);
end
x =
10
The good news is that you can assign any valid MATLAB object to appdata, just be cautious with the names, using unique names for appdata fields like ModelOptimizerOptions instead of a generic x,y would help. This works on compiled executables and code deployed on the MATLAB production server as well.