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

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{:});

Answer 1

[Walter Roberson]

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.

[Alvaro]

so is there a function that helps you find only real solutions?

[Torsten]

A number z is (numerically) real if abs(imag(z)) < epsilon for a small number epsilon > 0.

[Alvaro]

But I still don't get how can I display my results with or without imaginary numbers

[Torsten]

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]

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