MATLAB: ‘The variable appears to change size on every loop iteration’ Error is shown.

compressebility factordimensionsiterationvariables

I'm currently trying to create a function which depends on the compressibility factor of species in a mixture. This function uses subprogram of van der waals mixing rules.
The appears to an conistent error there which i can't fix.
Error Message is : The variable appears to change size on every loop iteration
The code used
Subprogram for Mixing rules
function [am, bm, aa] = MIXRULES_VDW(a, b, kappa, eta, x)
%The funtion MIXRULES_VDW calculates the am and bm of a mixture using  
%VDW MIXING RULES. 
%All input/output data are expressed in SI or are dimensionless.
%giving a firt value for c
c=length(x);
%initialization the values of am and bm
am=0;
bm=0;
%calculation of am, bm and aa
for i=1:c
    for j=1:c
        aa(i,j)=sqrt(a(i)*a(j))*(1-kappa(i,j));
        bb(i,j)=(b(i)+b(j))/2*(1-eta(i,j));
        am=am+x(i)*x(j)*aa(i,j);
        bm=bm+x(i)*x(j)*bb(i,j);
    end
end
Actual program for Calculating compressibilty:
function [Z,V,ZL,ZV,VL,VV] = ZMIX_PRVDW(Pc, Tc, w, kappa, eta, P, T, x)
%The funtion ZMIX_PRVDW calculates the compressibility factor z and the specific
%molar volume V for a mixture at given pressure P, temperature T and concentration
%using PR-CEoS and VDW MIXING RULES. This function requires REALROOTS_CARDANO and
% MIXRULES_VDW funtions to run.
%All input/output data are expressed in SI.
%P[Pa], T[K], w[dimensionless], V[m^3/mol], Z[dimensionless]
%universal gas constant
R=8.314;
%PR attractive constant and covolume at critical point
b=0.07780*(R*Tc)./Pc;
k=0.37464+1.54226*w-0.26992*w.^2;
alpha=(1+k.*(1-sqrt(T./Tc))).^2;
a=0.45724*alpha.*(R^2.*(Tc.^2))./Pc;
%calling MIXRULES_VDW function
[am, bm, aa] = MIXRULES_VDW(a, b, kappa, eta, x);
%calculation of Am and Bm
Am=am*P/(R*T)^2;
Bm=bm*P/(R*T);
%calculation of A and B
a2=-1+Bm;
a1=Am-3*Bm^2-2*Bm;
a0=-Am*Bm+Bm^2+Bm^3;
%calculation of Z and V using REALROOTS_CARDANO function
Z=REALROOTS_CARDANO(a2,a1,a0);
V=Z*R*T/P;
%selection of Z and V values for liquid and vapor
if min(V)<b
    ZL=max(Z);
    ZV=max(Z);
    VL=max(V);
    VV=max(V);
else
    ZL=min(Z);
    ZV=max(Z);
    VL=min(V);
    VV=max(V);
    
end
end
Both indicate same problem with the use of 'aa'. Same error is shown.
Any help would be appreciated.

Best Answer

Documentation on preallocation which explains why you're seeing this.
As Bob Nbob said, to make it go away, preallocate your arrays before the loop:
aa = zeros(c, c); %or zeros(c) it's the same

bb = zeros(c, c); %or zeros(c) it's the same
You may also consider giving more imaginative names to your variables, full words that explain what the variables contain, so any reader of your code can understand it.
Additionally, if you're calculating am and bm in a loop, there's no point of storing aa and bb as an array. So you could just have:
am=0;
bm=0;
%calculation of am, bm and aa
for i=1:c
    for j=1:c
        aa = sqrt(a(i)*a(j))*(1-kappa(i,j));
        bb = (b(i)+b(j))/2*(1-eta(i,j));
        am=am+x(i)*x(j)*aa;
        bm=bm+x(i)*x(j)*bb;
    end
end
However, the loops are completely unnecessary:
aa = sqrt(a .* a.') .* (1-kappa);  %calculate all the aa in one go
bb = (b + b.')/2 .* (1-eta);       %calculate all the bb in one go
am = sum(x .* x.' .* aa, 'all');   %requires R2018b or later for the 'all' option
bm = sum(x .* x.' .* bb, 'all');
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