Empty sym: 0-by-1
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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)
采纳的回答
Torsten
2023-10-12
Use
A = double(arrayfun(@(i)solve(GI(i),V),1:numel(P)))
instead of
A = solve(GI,V)
1 个评论
Dyuman Joshi
2023-10-12
which can be simplified to
out = double(arrayfun(@(x) solve(x,V), GI))
更多回答（1 个）
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)
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)
A2 = solve(GI(2), V)
isAlways(A1 == A2)
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
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
Yes. Are they close to the same value?
vpa(A, 5)
Nope. Not even same order of magnitude.
1 个评论
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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