MATLAB: Where is this error coming from

Question

I have relatively limited MATLAB experience so please, bare with me for this question. When I try and run the following code:

if true
function [y1, y2, y3, x1, x2, x3] = flash1(T, P, z1, z2, z3)
T = 250;
P=1520;
z1=0.5;
z2=0.3;
z3=0.2;
%Inlet Compositions - 1) Pentane, 2) Hexane, 3) Cyclohexane
%Determining Pisat
%Antoine Coefficients
A1=6.87362;
B1=1075.78;
C1=233.205;
A2=6.87024;
B2=1168.72;
C2=224.21;
A3=6.8413;
B3=1201.531;
C3=222.647;
%Calculating saturation pressure of each component, Temp is in Celsius
P1s=A1-B1/(T+C1);
P2s=A2-B2/(T+C2);
P3s=A3-B3/(T+C3);
% Calculating the equilibrium coefficient for each component
K1 = P1s/P;
K2 = P2s/P;
K3 = P3s/P;
% Rachford Rice Equation
S = solve('z1*(K1-1)/(q*(K1-1)+1)+z2*(K2-1)/(q*(K2-1)+1)+z3*(K3-1)/(q*(K3-1)+1)','q');
x1=z1/(S*(K1-1)+1); <-------This Line
x2=z2/(S*(K2-1)+1);
x3=z3/(S*(K3-1)+1);
y1=K1*x1;
y2=K2*x2;
y3=K3*x3;
end

I get the following errors at the indicated line.

??? Error using ==> maple at 129 Error, (in linalg:-linsolve) 2nd argument fails to evaluate to a vector or a matrix

Error in ==> sym.mldivide at 30 X = maple('linsolve',char(A),char(B),'''_rank''');

Error in ==> sym.mrdivide at 29 X = (A.'\B.').';

Error in ==> flash1 at 35 x1=z1/(S*(K1-1)+1);

Thanks for the help

Answer 1

That:

if true

is not part of the code, right? If it is, then you should be not even able to run the code....

Did you look at the output S? S has two solutions; it is a 2-by-1 one symbolic variable. To turn it into a 2-by-one double value, do:

S = subs(S);

Now use the element-by-element division:

x1=z1./(S*(K1-1)+1);
x2=z2./(S*(K2-1)+1);
x3=z3./(S*(K3-1)+1);
y1=K1*x1;
y2=K2*x2;
y3=K3*x3;