In an assignment A(I) = B, the number of elements in B and I must be the same.

Hello everyone! I'm having some issues trying to solve my code.

%%%---MAIN---%%%

clear all

close all

clc

global P ps

Ti=273.15;

Tf=1273.15;

T=linspace(Ti,Tf,10);

P=0.1*1e3;

k=[0 1 2 3 4 5];

ak=[6.1138e-1 4.4053e-2 1.4594e-3 2.6092e-6 2.8332e-7 2.7316e-9];

psat=ak(1)*(T-Ti).^k(1)+ak(2)*(T-Ti).^k(2)+ak(3)*(T-Ti).^k(3)...

+ak(4)*(T-Ti).^k(4)+ak(5)*(T-Ti).^k(5)+ak(6)*(T-Ti).^k(6); %[kPa]

options=optimset('TolFun',1e-5, 'MaxFunEvals',1e5, 'MaxIter',1e3,'Display','off');

for i=1:length(psat)

ps=psat(i);

mol_frac(i)=fsolve(@molar_fraction,[0.5 0.5],options);

end

%%%---FUNCTION---%%%

function f=molar_fraction (x)

global P ps

f=[x(1)-ps./P.*x(2); x(1)+x(2)-1];

end

this is the error I get from matlab:

In an assignment A(I) = B, the number of elements in B and I must be the same.

Error in Curve_sat_p (line 27) mol_frac(i)=fsolve(@molar_fraction,[0.5 0.5],options);

If I put a value of ps inside the function works but I don't know why it doesn't work with the for cycle.

Thanks to whoever is going to answer!

PM

Answer 1

[Thorsten]

Your function f returns a 2x1 column vector that cannot be assigned to a scalar mol_frac(i); instead, use

 mol_frac(:,i)= fsolve(@molar_fraction,[0.5 0.5],options);

Answer 2

[Pietro]

Thanks! That was very helpful!