I would like to use MATLAB Coder to generate an executable (or a function in an object file) that accepts a pointer to an array as an input.
I used libpointer to create a pointer object and then tried to compile with the following codegen command:
codegen -config:lib foo -args {coder.typeof(pointer_object_name)}
The resulting error message reported that coder.typeof does not support the lipointer type.
My ultimate goal is to create something that can be called from another C function, with no MATLAB in sight, and receive a pointer to an array as an input. Can MATLAB Coder generate something like that?
#ryan-livingston asked for the signature of the function I would like MATLAB Coder to generate.
Suppose that samples is a pointer to an array of floats. I think I want MATLAB Coder to create a void foo(float *samples) that performs various computations on those floats and perhaps writes results to a file or socket.
Now that I have the attention of #ryan-livingston, I suppose I should ask the following.
Can Coder make functions such as resample work with pointers?
Are pointers already being used under the hood, making my concern unnecessary?
If you just generate code with a fixed-size array input, the generated code will be able to accept a pointer. For example:
function x = foo(x)
x = 2*x;
% You can use MATLAB fopen, fprintf, fwrite here to write x to a file
>> codegen foo -args zeros(10,20) -config:lib -report
produces the interface:
void foo(double x[200]);
which is the same as:
void foo(double *x);
because of array to pointer decay on calls in C.
Note that I've used the x = foo(x) syntax to have Coder pass x by reference to foo. Functions declared with the same variable as both input and output generally produce pass by reference when also called with the same variable as input and output at the callsite.
Related
I was having a doubt today :).
For
A=1;
is there any function f that does the same? like following
f(A,1);
It could help me in some cases like in cellfun or something like that.
You can do this easily if your variable A is a handle class object, thus giving it reference behavior. You could then create a method f for the class that accepts a class object A and a new value for it to store. See Object-Oriented Programming for more information.
For data types like double or cell there are no built-in functions that work this way. You could make your own function using assignin and inputname like so:
function f(var, value)
assignin('caller', inputname(1), value);
end
And call it as follows, with A already defined:
A = 0;
f(A, 1); % Changes the value of A to 1
However, this would generally be considered bad practice as it makes the code harder to follow, as call-by-value behavior is the expected norm.
In general no, MATLAB functions cannot change their input.
But, if you are brave, you can create a MEX-file that breaks that promise and does change the input. In a MEX-file you can write to the input array, but doing so carelessly causes havoc. For example,
B = A;
f(A,1); % <- modifies A
would cause B to also be modified, because MATLAB delays copying the data when you do B = A. That is, the two variables point to the same data until you modify one, at which point the data is copied. But in a MEX-file you can write to a matrix without doing this check, thereby modifying B also. The link I provided shows how to modify A carefully.
In my code, I have a line that looks like this:
f=#(test) bf{i}(5);
where bf is a cell array with functions from str2func() stored in it, i is a variable storing an integer, and the 5 is the argument to pass to the function. How can I get matlab to evaluate the line using the current value of i? Right now when I display f it outputs:
#(test)bf{i}(5)
Lets say i=1, I want it to output:
#(test)bf{1}(5)
Although technically the bf{1} should also be replaced with whatever function is stored in bf{1}. How can I force matlab to evaluate the variables in this statement?
When you create a function handle, the workspace variables are copied and the expression is evaluated when you call the function handle (Typically not a problem in memory consumption, matlab stores only changes).
Now the problem is, to tell Matlab when to evaluate what part of the expression.
If you are aiming for a better performance, pre-evaluate all constant parts of the function. Let's say your function is #(x)(g(3).*f(x)), in this case matlab would evaluate g(3) on every call.
Instead use:
f=#(x)(x.^2)
g_3=g(3)
h=#(x)(g_3.*f(x))
Now having the constant parts evaluated, you want to see the constants instead of the variabe name. I know two ways to achieve this.
You can use the symbolic toolbox, basically converting the function handle to a symbolic function, then to a function handle again. This not only displays the constants, but also substitutes f. This is not possible for all functions.
>> matlabFunction(h(sym('x')))
ans =
#(x)x.^2.*4.2e1
Another possibility is to use eval:
h=eval(['#(x)',sprintf('%e',g_3),'.*f(x)'])
Pre-evaluating constant parts of the expressions as I did in the first step is typically recommendable, but both solutions to get the constant visible in your function handle aren't really recommendable. The first solution using matlabFunction only applies to some functions, while the second comes with all the disadvantages of eval.
I have a function that goes something like this:
function [] = function1
-variable1= value1;
-variable2= value2;
-matrix1=[]
-matrix2=[]
-loops that fill in matrix1 and matrix2
-more declarations
-final function1 end
function2[]
takes values from function 1 and does stuff, creates new variables
funcion2 end
function3[] that uses values from function 2 and function4[] that uses function 3.
(Sorry for all of that by the way). Now, my question is, is there anyway to save variables, arrays, etc. in the workspace for later analysis? All 4 functions are on the same tab in the MATLAB editor.
To save any particular variable from workspace in Matlab you should execute the following
save('Name of the mat file','Name of the variable')
To save multiple variable the command shall be
save('name of the mat file','var1','var2'...)
variable can be array, matrix, structure, double anything. For any other use of the function save please docsearch it in the command prompt
As part of a group project we have a system of 2 non linear differential equations and we have to draw the S=S(t) , I=I(t) graphic using the midpoint method.
And I'm getting the following error when trying to insert the matrix with the corresponding differential equations:
"Error in inline expression ==> matrix([[-(IS)/1000], [(IS)/1000 - (3*I)/10]])
Undefined function 'matrix' for input arguments of type 'double'.
Error in inline/subsref (line 23)
INLINE_OUT_ = inlineeval(INLINE_INPUTS_, INLINE_OBJ_.inputExpr, INLINE_OBJ_.expr);"
The code I have done is the following:
syms I S
u=[S;I];
F=[-0.001*S*I;0.001*S*I-0.3*I];
F1=inline(char(F),'I','S');
h=100; %Valores aleatórios
T=100000;
ni=(T/h);
u0=[799;1];
f=zeros(1,2);
k=zeros(1,2);
i=1;
while i<=ni
f(1)=F1(u0(1));
f(2)=F1(u0(2));
dx=h*f;
k(1)=F1((u0(1)+h*(1/2)),(u0(2)+h*(1/2)));
k(2)=F1((u0(1)+h*(1/2)),(u0(2)+h*(1/2)));
u1=u0+h*k;
disp('i:'),disp(i)
disp('u= '),disp(u1)
u0=u1;
i=i+1;
end
I'm new to this so the algorithm it's very likely to be wrong but if someone could help me with that error I'd apreciate it. Thank you!
The problem that specifically creates the error is that you are putting two symbolic functions into a matrix and then calling char (which outputs matrix([[-(IS)/1000], [(IS)/1000 - (3*I)/10]]) rather than converting nicely to string).
The secondary problem is that you are trying to pass two functions simultaneously to inline. inline creates a single function from a string (and using anonymous functions instead of inline is preferred anyway). You cannot put multiple functions in it.
You don't need sym here. In fact, avoid it (more trouble than it's worth) if you don't need to manipulate the equations at all. A common method is to create a cell array:
F{1} = #(I,S) -0.001*S*I;
F{2} = #(I,S) 0.001*S*I-0.3*I;
You can then pass in I and S as so:
F{1}(500,500)
Note that both your functions include both I and S, so they are always necessary. Reconsider what you were expecting when passing only one variable like this: f(1)=F1(u0(1));, because that will also give an error.
I'm writing a program in MATLAB to solve integrals, and I have my function in a .M-file. Now I wonder how I can write a program in the .MAT-file that lets the user set a value that exists in the both files. The .M-file looks like this:
function fh = f(y)
fh = 62.5.*(b-y).*(40-20.*exp(-(0.01.*y).*(0.01.*y)));
and as you can see, the function depends on two variables, y and b. I want the user to set b. I tried putting b = input('Type in the value of b: ') in the .M-file but for some reason the user would then have to put in the same value four times.
Can I ask for the value of b in the .MAT-file?
Firstly, m-files store code (i.e. functions), while MAT-files store data (i.e. variables). You can save workspace variables to a MAT-file using the function SAVE and load them into a workspace from a file using the function LOAD. If you have a user choose a value for b, then save it to a MAT-file ('b_value.mat', for example), you can simply load the value from the MAT-file inside your m-file function like so:
function fh = f(y)
load('b_value.mat','b');
fh = 62.5.*(b-y).*(40-20.*exp(-(0.01.*y).*(0.01.*y)));
However, this is not a very good way to handle the larger problem I think you are having. It requires that you hardcode the name of the MAT-file in your function f, plus it will give you an error if the file doesn't exist or if b isn't present in the file.
Let's address what I think the larger underlying problem is, and how to better approach a solution...
You mention that you are solving integrals, and that probably means you are performing numerical integration using one or more of the various built-in integration functions, such as QUAD. As you've noticed, using these functions requires you to supply a function for the integrand which accepts a single vector argument and returns a single vector argument.
In your case, you have other additional parameters you want to pass to the function, which is complicated by the fact that the integration functions only accept integrand functions with a single input argument. There is actually a link in the documentation for QUAD (and the other integration functions) that shows you a couple of ways you can parameterize the integrand function without adding extra input arguments by using either nested functions or anonymous functions.
As an example, I'll show you how you can do this by writing f as an anonymous function instead of an m-file function. First you would have the user choose the parameter b, then you would construct your anonymous function as follows:
b = input('Type in the value of b: ');
f = #(y) 62.5.*(b-y).*(40-20.*exp(-(0.01.*y).^2));
Note that the value of b used by the anonymous function will be fixed at what it was at the time that the function was created. If b is later changed, you would need to re-make your anonymous function so that it uses the new value.
And here's an example of how to use f in a call to QUAD:
q = quad(f,lowerLimit,upperLimit);
In your m file declare b as a global
function fh = f(y)
global b
fh = 62.5.(b-y).(40-20.*exp(-(0.01.y).(0.01.*y)));
This allows the variable to be accessed from another file without having to create another function to set the value of b. You could also add b to the arguments of your fh function.