How can I substitute a variable given by a struct?

Question

Abner Ojeda 2020-5-2

0
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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/522368-how-can-i-substitute-a-variable-given-by-a-struct

评论： Abner Ojeda 2020-5-3

Hey, everyone. I'm trying to solve an optimization problem without the Optimization toolbox. I already have the roots from the equation (by "solve"), it gives to me an struct array, with fields x1, x2... xn, my problem is I already have an x1, x2... xn. I've been trying by so many ways but simply I couldn't solve it, I'm looking to asign the value of the fields to the syms variables and then apply "subs". Code below.

Thanks in advance for help. :)

clear; close all; clc;
var= input ('Dimention of domain');
syms x [1 var];
func= input ('Enter the function, i.e [x1+x2+...+xn]: \n');
y=[];
for s=x(1:end)
    dp= diff (func, [s]);
    y=[y, dp];
end
pc=solve (y);
pucr= fieldnames (pc);
for i= 1:numel(pucr)
    vals(i, 1)= getfield(pc,pucr{i});
end
Hf=hessian (func);
Hff= subs (Hf);

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darova 2020-5-2

在 MATLAB Online 中打开

Try this

for i= 1:numel(pucr)
    v = getfield(pc,pucr{i});
    v1 = subs(v,oldv, newv);
    vals(i, 1) = doulbe(v1);    
end

Abner Ojeda 2020-5-3

Thanks for taking your time in answer my question, it doesn't work, but I appreciate your answer.

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Answer 1

Walter Roberson 2020-5-2

0
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/522368-how-can-i-substitute-a-variable-given-by-a-struct#answer_429777

在 MATLAB Online 中打开

subs(ExpressionToSubstituteInto, pc)

When pc is a struct with field names, then the corresponding variable names will have the appropriate values substituted.

Note: this will not work if any of the field names contain an empty value, even if the corresponding variable is not used in the expression. In particular, if you happened to do

pc = solve(y, 'returnconditions', true)

then pc.parameters would be created even if no parameters were generated, and with pc.parameters being empty, subs() with pc would fail. You can get it working by removing the empty field from the struct.