How to use solver in function

Question

Kian 2012-2-11

0
链接

此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/28727-how-to-use-solver-in-function

编辑： Cedric 2013-10-8

采纳的回答： Walter Roberson

在 MATLAB Online 中打开

I have got a problem here.

I want to use solver within a function I m writing. To give you more details, suppose I want to write a function like:

function [a] = example
a = solve('14 = x^2')
end

Now the problem is how I can key in a variable instead of 14 in that solver. I mean suppose I have a variable been defined before like: d = 14, now I want to write my function as below, but it doesn't work :(

function [a] = example
d = 14
a = solve('d = x^2')
end

Any help?

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Answer 1

Walter Roberson 2012-2-11

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https://ww2.mathworks.cn/matlabcentral/answers/28727-how-to-use-solver-in-function#answer_37070

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syms x
a  = solve(x^2 - d)

Alternately,

d = 14
a = solve(subs(sym('d = x^2'), 'd', d));

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Kian 2012-2-11

Thank you Walter for the answer.

Actually it works as you wrote it. But, I think for the case I have it does not, cause I think I wrote my issue not exactly in such a way it is. I mean you gave me a good answer, but that's just not exactly my case, so I write the equation itself, and I just put it in the shape you told me to do. So,

a = mat2(1,2);

b = mat2(1,3);

r = solve(subs(sym('a=(((1/273)-((0.0001844)*(log(((r*0.001)*b)/(0.611*((r*0.001)+0.622))))))^-1)-273'),'a',a,'b',b));

where mat2 has already been defined. According to what you said, now it should work out. But when I run the program it gives me an error saying:

??? Error using ==> sym.subs

Too many input arguments.

So, I think the problem is with the number of arguments I have in my equation(a & b). I think I didn't explain my issue well in the first place, so sorry for that.

I have got another problem relating to this issue as well.

Later I have a line as:

content = f - r;

where r is the result given by the solver, (f has already been computed), and then I have:

fid = fopen('Result.txt','wt');

fprintf(fid,'Content of the parcel is %5.2f\n',content);

fclose(fid);

Again when I run the program it gives me an error relating to this code line saying:

??? Error using ==> fprintf

Function is not defined for 'sym' inputs.

I would appreciate any help, and sorry for my lengthy question. I think I have now explained everything in detail.

Thanks again.

Walter Roberson 2012-2-12

If you have a number in symbolic form which does not contain any unresolved symbols, then use double() to convert it to double precision number. Otherwise use char() and print the result as a string.

Kian 2012-2-12