Problem with fminsearch Error in fminsearch : (line 200) fv(:,1) = funfcn(x,varargin{:});

hello every one

Here's my code :

    % Variables
    eta = 20 ;
    beta = 100 ;
    mensualites5 = 8.974646657515070*10^3 ;
    k5 = 120 ;
    portion_alpha = linspace(0,1,100);
    k_alpha = k5.*portion_alpha ; 

    % Equations
    reinvestissement = (1-portion_alpha)*mensualites5 ;
    benefice = eta*reinvestissement.*exp(-k_alpha/beta) ; 
    profit = (benefice-portion_alpha.*mensualites5).*k_alpha ;
    minus_profit = -(benefice-portion_alpha.*mensualites5).*k_alpha ;

    % Function + fminsearch
    f = @(portion_alpha) minus_profit ;
    x0 = 0.35 ;
    [portion_alpha, fmin] = fminsearch(f,x0);
    disp(fmin)
    disp(portion_alpha)

I always have this error message :

    Unable to perform assignment because the size of the left side is
    1-by-1 and the size of the right side is 1-by-100.

    Error in fminsearch (line 200)
    fv(:,1) = funfcn(x,varargin{:});

    Error in Exercice1_2 (line 130)
    [portion_alpha, fmin] = fminsearch(f,x0);

I don't understand why. Could somebody help me pleaaaase ? Thx

Answer 1

[Zenin Easa Panthakkalakath]

Hello Amandine,

It seems that the function f is not defined in the right way. Here f is a function that takes 'portion_alpha' as an argument and returns 'minus_profit'. 'minus_profit' isn't even defined in the function and hence the value of the same is taken from the defenition before.

If you try to run the following command, it would result in a logical array of 1s. This means that the function is invariant to the input argument; or in other words, function is constant.

>> f(1) == f(2)

I think you should try to define the function in the right way and see if f(x) for different values of x are giving you the answers as expected.

Please read through the documentation for more infromation: https://www.mathworks.com/help/matlab/ref/fminsearch.html

Regards,

Zenin