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Unable to perform assignment because the indices on the left side are not compatible with the size of the right side.

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Alvaro
Alvaro 2023-10-16
评论： Walter Roberson 2023-10-17
Ran in:
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
% Van der Waals
Tc=647.1;
Pc=convpres(22060000,'Pa','atm');
a1=(27*R^2*Tc^2)/(64*Pc);
b1=(R*Tc)/(8*Pc);
VW=((R*T)/(V-b1))-(a1/(V^2))==P;
for i=1:6
B(1,i)=solve(VW(i),V)
end
Unable to perform assignment because the indices on the left side are not compatible with the size of the right side.
Error in sym/privsubsasgn (line 1200)
L_tilde2 = builtin('subsasgn',L_tilde,struct('type','()','subs',{varargin}),R_tilde);
Error in indexing (line 1031)
C = privsubsasgn(L,R,inds{:});

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回答（1 个）

Walter Roberson
Walter Roberson 2023-10-16
Ran in:
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
% Van der Waals
Tc=647.1;
Pc=convpres(22060000,'Pa','atm');
a1=(27*R^2*Tc^2)/(64*Pc);
b1=(R*Tc)/(8*Pc);
VW=((R*T)/(V-b1))-(a1/(V^2))==P
VW = 
for i=1:6
B{1,i}=solve(VW(i),V);
end
B
B = 1×6 cell array {3×1 sym} {3×1 sym} {3×1 sym} {3×1 sym} {3×1 sym} {3×1 sym}
Your equation is a cubic in V, and so it has three roots.
format long g
cell2mat(cellfun(@double, B, 'uniform', 0))
ans =
Columns 1 through 3 3.22856391464644 - 0.71413166224905i 3.23235043527525 - 0.710926542708146i 3.24382222761193 - 0.701023878142358i 3.22856391464644 + 0.71413166224905i 3.23235043527525 + 0.710926542708146i 3.24382222761193 + 0.701023878142358i 2658.31153778874 + 0i 1320.0066466243 + 0i 524.578769236741 + 0i Columns 4 through 6 3.26344891359564 - 0.683381010798684i 3.45021491277963 - 0.450133424499785i 3.11984383082587 + 0i 3.26344891359564 + 0.683381010798684i 3.45021491277963 + 0.450133424499785i 4.51149984762074 + 0i 259.852401068882 + 0i 47.6648286645631 + 0i 20.5054138525534 + 0i
Only one of the roots is real-valued -- but you asked to solve the equation, not to find real-valued solutions.
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Torsten
Torsten 2023-10-16
编辑：Torsten 2023-10-17
Ran in:
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
% Van der Waals
Tc=647.1;
Pc=convpres(22060000,'Pa','atm');
a1=(27*R^2*Tc^2)/(64*Pc);
b1=(R*Tc)/(8*Pc);
VW=((R*T)/(V-b1))-(a1/(V^2))==P
VW = 
for i=1:6
B{1,i}=solve(VW(i),V);
end
format long g
M = cell2mat(cellfun(@double, B, 'uniform', 0))
M =
Columns 1 through 3 3.22856391464644 - 0.71413166224905i 3.23235043527525 - 0.710926542708146i 3.24382222761193 - 0.701023878142358i 3.22856391464644 + 0.71413166224905i 3.23235043527525 + 0.710926542708146i 3.24382222761193 + 0.701023878142358i 2658.31153778874 + 0i 1320.0066466243 + 0i 524.578769236741 + 0i Columns 4 through 6 3.26344891359564 - 0.683381010798684i 3.45021491277963 - 0.450133424499785i 3.11984383082587 + 0i 3.26344891359564 + 0.683381010798684i 3.45021491277963 + 0.450133424499785i 4.51149984762074 + 0i 259.852401068882 + 0i 47.6648286645631 + 0i 20.5054138525534 + 0i
for i = 1:size(M,2)
indices = find(abs(imag(M(:,i))) < 1e-6);
sol{i} = M(indices,i); % better save in cell array for the case of 3 real solutions
end
sol
sol = 1×6 cell array {[2658.31153778874]} {[1320.0066466243]} {[524.578769236741]} {[259.852401068882]} {[47.6648286645631]} {3×1 double}
Walter Roberson
Walter Roberson 2023-10-17
Ran in:
Your last P has three real-valued solutions.
clc;clear;format compact;
syms V real
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
% Van der Waals
Tc=647.1;
Pc=convpres(22060000,'Pa','atm');
a1=(27*R^2*Tc^2)/(64*Pc);
b1=(R*Tc)/(8*Pc);
VW=((R*T)/(V-b1))-(a1/(V^2))==P;
for i=1:6
B{1,i} = solve(VW(i),V, 'maxdegree', 3);
end
cellfun(@double, B, 'uniform', 0)
ans = 1×6 cell array {[2.6583e+03]} {[1.3200e+03]} {[524.5788]} {[259.8524]} {[47.6648]} {3×1 double}
B{end}
ans = 
format long g
double(B{end})
ans =
20.5054138525534 - 7.21326454514511e-130i 3.11984383082587 + 3.60663227257255e-130i 4.51149984762074 + 3.60663227257255e-130i

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