Powell's Algorithm not obeying Upper Bound (UB)

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Debapriya Sengupta 2022-5-18

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https://ww2.mathworks.cn/matlabcentral/answers/1722230-powell-s-algorithm-not-obeying-upper-bound-ub

评论： Debapriya Sengupta 2022-5-21

Hello,

I am running Powell's algorithm with Golden Section method, to find the minimum of a function. My function call statement is as follows:

Q=pi/180;
S=Q*-8.5;
L=Q*-10;
U=Q*-8;
[xo,Ot,nS]=powell('My_Func',S,0,1,L,U,[],[],300);

My_Func is a user defined function. I am getting xo = -0.0727 as output, which is greater than U. Please explain why Powell's algorithm is not obeying boundaries.

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Jan 2022-5-18

@Debapriya Sengupta: How can we solve the problem without having your code and data? We cannot debug the function provided by Giovani Tonel 15 years ago - but you can. Use the debugger to step through the code line by line.

Debapriya Sengupta 2022-5-19

I have already done a step run. I could not find a check for L or U anywhere in the code.

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回答（1 个）

Jan 2022-5-19

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1722230-powell-s-algorithm-not-obeying-upper-bound-ub#answer_967180

编辑：Jan 2022-5-19

在 MATLAB Online 中打开

Yes, this is the documented behavior:

% Lb, Ub: lower and upper bound vectors to plot (default = x0*(1+/-2))

Lb and Ub are used for the graphics only.

Summary: You are using a function provided 15 years ago with a limited quality. It does not do, what you need and this is mentioned in the lean documentation. Then you explain in the forum, that this function does not do, what you need, but do not mention initially, which function you actually mean.

"Please explain why Powell's algorithm is not obeying boundaries."

Because this implementation of Powell's algorithm is not designed to consider the boundries for finding the result.

Sorry, you made it as hard as possible to find this trivial answer. You even found this answer by your own already: "I could not find a check for L or U anywhere in the code."

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John D'Errico 2022-5-19

Why would you expect code found on the file exchange, where anyone can post code, to be as robust asnd well written as code written by professionals in the field? Remember that anyone can post code to the FEX, including complete novices to programming. Trust that which you get for free to be sometimes worth what you paid for it. Admittedly, there are codes to be found on the FEX which were written by highly skilled coders who have written code as good as that which you will get from the MathWorks itself. But that is not the case for all such submissions.

Usually the comments can be good guides, but even there you can find false advertising. I could mention the case where someone used a fake account in the name of their own grandmother to post misleadingly admiring comments on their own useless code. So for any code you find, read the comments, but also look at the quality of the code you find. Good, well written code often has a look and feel that is very different from what would be posted by a novice. You will find clearly written help that explains the code and how to use it. You will find a well designed interface.

But in all cases, remember what you paid for free code.

Torsten 2022-5-20

编辑：Torsten 2022-5-20

The time when Powell's method became popular was FORTRAN time in software development. I guess you will find FORTRAN code for your problem in the internet.

Here is a link:

https://copasi.org/Support/User_Manual/Methods/Optimization_Methods/Praxis/

Debapriya Sengupta 2022-5-21