I have a function and I want to each time change its input and integrate over a certain interval. For example, I first write an m file:
function y = myfun(x)
y = 1 ./ (x.^3 - 2*x - 5);
Now I want to integrate functions like myfun(x-2) or myfun(2*x). Does anyone know how I should pass them? Integral(myfun(x-2),a,b) creates an error.
Thanks
I suggest calling integral on a handle for your function as below:
h1 = #(x)myfun(x);
h2 = #(x)myfun(x-2);
h3 = #(x)myfun(x.^2);
integral(h1,a,b);
integral(h2,a,b);
integral(h3,a,b);
This should trick the integral function into thinking that you are just defining myfun as a function of x while allowing you to pass whatever expression you want to it. You could also pass additional parameters this way, such as:
h = #(x)myfun(x, params);
integral(h,a,b);
Where params could be a list of parameters that you use in your definition of myfun.
I hope that helps.
EDIT: I tested this on a server that I have access to which does have the integral function and it seemed to work. Hopefully this answers your question.
Related
I have N functions in MATLAB and I can define them using strcat, num2str and eval in a for loop. So without defining by hand I am able to define N functions. Let N=4 and let them be given as follows:
f1=#(x) a1*x+1;
f2=#(x) a2*x+1;
f3=#(x) a3*x+1;
f4=#(x) a4*x+1;
Now I add these four functions and I can do this by hand as follows:
f=#(x)(f1(x)+f2(x)+f3(x)+f4(x));
Here I can do it by hand because I know that N=4. However, in general I never know how many functions I will have. For all cases I cannot write a new function.
Is there any way to do this automatically? I mean if I give N=6 I am expecting to see MATLAB giving me this:
f=#(x)(f1(x)+f2(x)+f3(x)+f4(x)+f5(x)+f6(x));
Whenever I give N=2 then I must have the function f, defined as follows:
f=#(x)(f1(x)+f2(x));
How can we do this?
First of all, you should read this answer that gives a series of reasons to avoid the use of eval. There are very few occasions where eval is necessary, in all other cases it just complicates things. In this case, you use to dynamically generate variable names, which is considered a very bad practice. As detailed in the linked answer and in further writings linked in that answer, dynamic variable names make the code harder to read, harder to maintain, and slower to execute in MATLAB.
So, instead of defining functions f1, f2, f3, ... fN, what you do is define functions f{1}, f{2}, f{3}, ... f{N}. That is, f is a cell array where each element is an anonymous function (or any other function handle).
For example, instead of
f1=#(x) a1*x+1;
f2=#(x) a2*x+1;
f3=#(x) a3*x+1;
f4=#(x) a4*x+1;
you do
N = 4;
a = [4.5, 3.4, 7.1, 2.1];
f = cell(N,1);
for ii=1:N
f{ii} = #(x) a(ii) * x + 1;
end
With these changes, we can easily answer the question. We can now write a function that outputs the sum of the functions in f:
function y = sum_of_functions(f,x)
y = 0;
for ii=1:numel(f)
y = y + f{ii}(x);
end
end
You can put this in a file called sum_of_functions.m, or you can put it at the end of your function file or script file, it doesn't matter. Now, in your code, when you want to evaluate y = f1(x) + f2(x) + f3(x)..., what you write is y = sum_of_functions(f,x).
I'm trying to create Matlab code to calculate these following equations in Matlab
g(1,2)=g(1)+g(2)+g(1)g(2)
g(1,3)=g(1)+g(3)+g(1)g(3)
g(2,3)=g(2)+g(3)+g(2)g(3)
and values of g(1), g(2),g(3) are available.
Where should I start to write such Matlab code?
One problem here appears to be that you have functions of one and two variables, but you're calling them the same thing (g). Let's call the function of one variable g and the function of two variables (that you want to create) f. Assuming the function g already exists, you can create f using an anonymous function like so:
f = #(x1, x2) g(x1)+g(x2)+g(x1)*g(x2);
And calling it like this:
result = f(1, 2);
is equivalent to:
result = g(1)+g(2)+g(1)*g(2);
I am trying to implement following in MATLAB,
n1 = 6;
n2 = 1;
n3=0.1;
global ps
ps=zeros(3,15);
[R,t,d]=model(n1,n2,n3);
ps=R;
[x, fval] = fmincon(#Obj,[1/3,1/3, 1/3],[],[],[],[],[],[],#cons);
function f = Obj(x)
f = x(1)^2+x(2)^3+x(3)^4;
function [c, ceq] = cons(x)
c=[];
ceq(1) = sum(ps(1,:))*x(1)+sum(ps(2,:))*x(2)+sum(ps(3,:))*x(3) - (sum(d(1,:)));
ceq(2) = sum(x) - 4;
I'm getting following error, what is wrong here?
variable or function 'ps' undefined
You don't need to and definitelty should not use global variables for this. It is a very bad habit and inefficient to boot. Any time you think about using global you should ask yourself if there is another way and search for it. It is only in the very very rare case that globals are needed/helpful (usually in large codebases such as toolboxes).
In your case, you should pass your ps variable in as a parameter by creating an anonymous function. First define your cons function like this so that it takes in a parameter argument:
function [c, ceq] = cons(x,ps)
Then create the anonymous function with one input (x) and one captured parameter (the variable ps, which needs to be defined before this):
[x, fval] = fmincon(#Obj,[1/3,1/3, 1/3],[],[],[],[],[],[],#(x)cons(x,ps));
Alternatively you can save a handle to the anonymous function and pass that in:
cfun = #(x)cons(x,ps);
[x, fval] = fmincon(#Obj,[1/3,1/3, 1/3],[],[],[],[],[],[],cfun);
Here's a blog post from The MathWorks with other bad habits.
I cannot run your lines of code, but I think you have to put
global ps
in your function cons to inform Matlab that you are referring to the global ps. More information on global variables can be found here: http://www.mathworks.de/de/help/matlab/ref/global.html
EDIT: For a cleaner design, you should take into account horchler's advice: https://stackoverflow.com/a/20721808/3060323
Hi I have a question about how to achieve the following behaviour in Matlab.
A.x=pi
A.sin=#()sin(A.x)
A.sin() % Returns 1.2246e-16, essentially 0 so all good so far.
% Now for the problem
A.x = pi/2
A.sin() % Returns 1.2246e-16, meaning the new A.x is not used. It should return 1.
Does anyone have any ideas how to achieve this. I know I could define A.sin as #(x) sin(x) then provide A.x but would rather find an alternative.
Thanks.
Create this class:
classdef mytrig
properties
x
end
methods
function out = sin(obj)
out = sin(obj.x);
end
end
end
Then at the command line:
>> A = mytrig;
>> A.x = pi;
>> A.sin
ans =
1.2246e-016
>> A.x = pi/2;
>> A.sin
ans =
1
The way you're doing it at the moment won't work, because when you create the function handle A.sin=#()sin(A.x), the function handle captures a copy of the current workspace, including x, which then remains fixed afterwards, even if you subsequently change x. If you want to be able to change x afterwards yourself, these best way would be to be implement a class as above.
Hope that helps!
Once you assign a value to a variable in Matlab, it is fixed.
If you want to have something that is updated automatically, please look into classes.
If you don't like classes, you could also define a function, for example
myAsin = #()sin(A.x)
Can't test it now, but as it is a function you should be getting the updated value when you call it after A.x is updated.
I want to define a function like this:
function f = f1(fun,a,b,c)
f = c*fun(a+b);
Here fun is some function that I will pass when I use the function f.
How can I do this in Matlab?
Have you tried it? The best way to learn a tool like matlab is to try things!
In fact, you don't even need to create an m-file function. I'll do it using a function handle here.
fun = #(x) sin(x);
f1 = #(f,a,b,c) c*f(a+b);
f1(fun,2,3,4)
ans =
-3.8357
I could have defined f1 as an m-file function too, but that would have required I save a file. Why bother?
What you are looking for is the function handle.
You can obtain the function handle of a function (in the following case, sqrt) by placing the "at" symbol ('#') in front of the function name:
a = 1;
b = 2;
c = 3;
fun = #sqrt; % obtain the function handle of sqrt()
f = f1(fun, a,b,c); % pass the function handle of sqrt() into your function f1().
When you use fun, it will be as if you are using the sqrt function.
For more details, you may also refer to another Stackoverflow question: function handle in MATLAB