Empty sym: 0-by-1

Question

Alvaro 2023-10-12

0
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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/2032569-empty-sym-0-by-1

评论： Walter Roberson 2023-10-12

When I run this code, my answer for A comes out as 'Empty sym: 0-by-1'

clc;clear;format compact;
syms V 
P=[0.98 1.97 4.93 9.86 49.36 98.69]; %atm
T=[573 573 573 573 573 573]; %K
R=461.5; %Pa*m3/mol*K
R=convpres(R,'Pa','atm');
R=R*1000; %L*atm/mol*K
GI=(P*V)-(R*T);
A=solve(GI,V)
A =
Empty sym: 0-by-1

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Answer 1

Torsten 2023-10-12

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/2032569-empty-sym-0-by-1#answer_1331874

在 MATLAB Online 中打开

Use

A = double(arrayfun(@(i)solve(GI(i),V),1:numel(P)))

instead of

A = solve(GI,V)

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Dyuman Joshi 2023-10-12

在 MATLAB Online 中打开

which can be simplified to

out = double(arrayfun(@(x) solve(x,V), GI))

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Answer 2

Steven Lord 2023-10-12

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/2032569-empty-sym-0-by-1#answer_1331884

在 MATLAB Online 中打开

Let's look at the equations you're trying to solve.

syms V

P=[0.98 1.97 4.93 9.86 49.36 98.69]; %atm

T=[573 573 573 573 573 573]; %K

R=461.5; %Pa*m3/mol*K

R=convpres(R,'Pa','atm');

R=R*1000; %L*atm/mol*K

GI=(P*V)-(R*T)

GI =

The first element of GI has a solution, as does the second element of GI. Is the solution for those two elements exactly the same? To ask that question another way, is there a value of V that satisfies all the elements in GI simultaneously?

A1=solve(GI(1), V)

A1 =

A2 = solve(GI(2), V)

A2 =

isAlways(A1 == A2)

ans = logical

0

No, there isn't. So your original solve call that's looking for that one value of V that satisfies all the elements in GI simultaneously is correct to return an empty sym.

Now if you wanted a vector of values of V that satisfy each corresponding element in GI, that's possible to compute.

A = zeros(size(GI), 'sym');

for whichTerm = 1:numel(GI)

A(whichTerm) = solve(GI(whichTerm), V);

end

A

A =

Do they satisfy their equations?

result = zeros(size(GI), 'sym');

for whichEquation = 1:numel(GI)

result(whichEquation) = subs(GI(whichEquation), V, A(whichEquation));

end

result

result =

Yes. Are they close to the same value?

vpa(A, 5)

ans =

Nope. Not even same order of magnitude.

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Walter Roberson 2023-10-12

This is an important point: solve is always looking for simultaneous solution of all the inputs. It is designed for simultaneous equation solving, and does not have any option for solving each equation individually.

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