CODEWITHSUNDEEP
matlab
Andres
Andres

Reputation: 6200

How to find max value of variable function with 2 constant parameters?

I'm trying to find the maximum value of a function using fminbnd. I have used it with simple functions like f(x) = x^2+2*x so to find the minimum value I do fminbnd(f,-10,10). To find the maximum value I just have to do fminbnd(-f,-10,10).

Now I have another function with 3 variables but I want 2 of them to be constant:

f(Q,m,Fx) = (Fx^2*(m-1))/sqrt( (m*Fx^2-1)^2 + (Fx^2*(Fx^2-1)^2*(m-1)^2*Q^2) )

I'm currently plotting it using ezplot(f(q,m,Fx), [0 8 0 3]) being q and m constant values. If a try to get the maximum value with fminbnd(-f(q,m,Fx),0,8) it gives me an error.

How can I find the maximum value of this function?

Upvotes: 0

Views: 1563

Answers (1)

Matt
Matt

Reputation: 13923

You can define the function as function-handle and then use an anonymous function in the call for ezplot and fminbnd. This reduces your problem to only one variable, so it can be handled appropriately and generates no errors.

% define function-handle
f = @(Q,m,Fx) (Fx^2*(m-1))/sqrt( (m*Fx^2-1)^2 + (Fx^2*(Fx^2-1)^2*(m-1)^2*Q^2) );

% define constants
q = 1;
m = 10;

% plot the curve
ezplot(@(Fx) f(q,m,Fx), [0 8 0 3])

% compute the maximum
Fx = fminbnd(@(Fx) -f(q,m,Fx),0,8)
Fy = f(q,m,Fx)

% plot the maximum as point
hold on
plot(Fx,Fy,'*')
axis([0 8 0 1.2])

Gives the following result:

result

Upvotes: 1

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