Minimizing a multivariable function

Question

Physiker192 2014-8-24

0
链接

此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/152168-minimizing-a-multivariable-function

评论： Star Strider 2014-8-24

Hello, i am trying to find the minimum of a multivariable function (the number of variables may vary also but there is a pattern to construct the function) the code i am running is the following:

function F = jac(x)
n=input('n=');
x0=random('unid',90,1,n,1);
x0=pi./x0;
E=input('E=');
h=3;
F=0;
for i=1:n
    D= 0.5;
%     V=0;
%     b=0;
    for j=1:n
        if (mod(j,2)==0)
           D = D + cos(h*x(j));
        else
           D = D - cos(h*x(j)); 
        end 
    end
    D=(2*E*sqrt(2)/(pi*h))*abs(D);
    F = F + 1/h*(D^2);
    h=h+2;
end
M=fminsearch(@(x)jac,x0);

the error i am getting is that the function x is undefined can anyone help me please? thanks in advance

2 个评论
显示无隐藏无

John D'Errico 2014-8-24

Please learn to format your code to make it readable. I did it for you this time. (Look for the code button.)

John D'Errico 2014-8-24

编辑：John D'Errico 2014-8-24

Now that I can read your code, I see the call to random. random functions are quite difficult to minimize. However, it looks like you think that is a call to create starting values.

Worse, you have several input statements in the beginning of the function you are trying to minimize? WHY????

My guess from all these things (and others in your code) is you don't have a clue how an optimizer works or how to call one. Or maybe you don't understand how to write functions. It looks like you are confused about what the objective function called by an optimizer should be doing.

Time to start reading the help. Look at the examples. Then and only then, try solving a SMALL optimization problem where you know the answer. Work up to real world problems only when you understand how to solve basic ones.

请先登录，再进行评论。

请先登录，再回答此问题。

Answer 1

Star Strider 2014-8-24

0
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/152168-minimizing-a-multivariable-function#answer_149691

编辑：Star Strider 2014-8-24

在 MATLAB Online 中打开

You are going to encounter the recursion error when you run your code because you seem to be calling fminsearch inside your function.

I didn’t run your code however certain parts of it need to be corrected.

Define your ‘n’ and ‘E’ variables outside the function in your workspace and pass them as arguments. Redefine your function statement to accommodate them.

For example, the function statement becomes:

function F = jac(x,n,E)

then eliminate the input statements inside ‘jac’. Your script then becomes:

x0=random('unid',90,1,n,1);
x0=pi./x0;
n = ...    % <— Define ‘n’
E = ...    % <— Define ‘E’
M=fminsearch(@(x)jac(x,n,E),x0);

Your function becomes:

function F = jac(x,n,E)
h=3;
F=0;
for i=1:n
    D= 0.5;
%     V=0;
%     b=0;
    for j=1:n
        if (mod(j,2)==0)
           D = D + cos(h*x(j));
        else
           D = D - cos(h*x(j)); 
        end 
    end
    D=(2*E*sqrt(2)/(pi*h))*abs(D);
    F = F + 1/h*(D^2);
    h=h+2;
end
end

These changes should get you started. Again, I didn’t run this because it’s not obvious to me what you want to do.

3 个评论
显示 1更早的评论隐藏 1更早的评论

Physiker192 2014-8-24

thank you very much it works. I believe that my mistake was not defining f as a function of (x,n,E) the last code that i posted was an attempt to deal with it as a single file so it wouldn't ask me repetevely to enter E. Anyway thanks a lot.

Star Strider 2014-8-24

My pleasure!

请先登录，再进行评论。

Minimizing a multivariable function

2 个评论
显示无隐藏无

采纳的回答

3 个评论
显示 1更早的评论隐藏 1更早的评论

更多回答（0 个）

另请参阅

类别

标签

Community Treasure Hunt

Minimizing a multivariable function

2 个评论 显示 无隐藏 无

采纳的回答

3 个评论 显示 1更早的评论隐藏 1更早的评论

更多回答（0 个）

另请参阅

类别

标签

Community Treasure Hunt

2 个评论
显示无隐藏无

3 个评论
显示 1更早的评论隐藏 1更早的评论