Why do I get "Array indices must be positive integers or logical values"

I am trying to solve the following differential equation ode3. Tc_p is a cfit that I change to a symfun so I can susbtitute inside my differential equation and solve the whole thing. I can`t understand why I am getting the error "Array indices must be positive integers or logical values.". Thx in advanace.

clear all
datos_TH;
syms Tfav(t) Tcav(t) Tm(t)
cond1=Tfav(0)==(Tf0-Tm0)/(Tf0-Tm0);
cond2=Tcav(0)==(Tc0-Tm0)/(Tf0-Tm0);
%cond3=Tm(0)==Tm0;
conds=[cond1; cond2];

ode1=diff(Tfav)==-2*Bigf*(Tfav-Tcav)/Rf0 + 100;
ode2=diff(Tcav)==2*K*(Bigc*Rci*(Tfav-Tcav)-Bi(1)*Tcav)/(1-Rci^2);
%ode3=diff(Tm)==(Sc*hi(1)*(Tc-Tm)-2*m*cp*(Tm-Tm0))/(Mc*cp);
odes=[ode1;ode2];
[TcavSol(t), TfavSol(t)]=dsolve(odes, conds);

Tf=[];
Tc=[];
n=1;
t_0=25;
time_ad=t_0*kf/(Rhof*cf*rc0^2);
for i=0:0.01:time_ad
    Tf(n)=double(TfavSol(i))*(Tf0-Tm0) + Tm0;
    Tc(n)=double(TcavSol(i))*(Tf0-Tm0) + Tm0;
    n=n+1;
    m=n+1;
end


%Moderator Temperature ODE
save('Tc','Tc');
t=linspace(0,t_0,m-2);
t=transpose(t);
Tc=transpose(Tc);
Tc_p=fit(t,Tc,'poly8');
Tc_p=subs(str2sym(formula(Tc_p)),coeffnames(Tc_p),num2cell(coeffvalues(Tc_p).'));
Tc_p=subs(Tc_p,'t');

cond3=Tm(0)==Tm0;
ode3=diff(Tm,'t')==(Sc*hi(1)*(Tc_p-Tm)-2*m*cp*(Tm-Tm0))/(Mc*cp);
odes=ode3;
TmSol(t)=dsolve(ode3,cond3);

And the complete error is:

Array indices must be positive integers or logical values.

Error in sym/privsubsasgn (line 1311)
            L_tilde2 = builtin('subsasgn',L_tilde,struct('type','()','subs',{varargin}),R_tilde);

Error in indexing (line 1142)
            C = privsubsasgn(L,R,inds{:});

Error in Sim_TH (line 41)
TmSol(t)=dsolve(ode3,cond3);

Answer 1

[Steven Lord]

You use Tm in your code but nowhere do you define it. My suspicion is that you've defined it as a numeric vector like you did Tf and Tc. But vector indices cannot be 0.

[Diego Mondaray Muñoz]

Sorry for not putting that part in my question, I define Tm here:

(This is my complete code, after this I just plot my answers)

%%SIMPLIFIED THERMOHYDRAULIC MODEL%%
clear all
datos_TH;
syms Tfav(t) Tcav(t) Tm(t)
cond1=Tfav(0)==(Tf0-Tm0)/(Tf0-Tm0);
cond2=Tcav(0)==(Tc0-Tm0)/(Tf0-Tm0);
%cond3=Tm(0)==Tm0;
conds=[cond1; cond2];

ode1=diff(Tfav)==-2*Bigf*(Tfav-Tcav)/Rf0 + 150;
ode2=diff(Tcav)==2*K*(Bigc*Rci*(Tfav-Tcav)-Bi(1)*Tcav)/(1-Rci^2);
%ode3=diff(Tm)==(Sc*hi(1)*(Tc-Tm)-2*m*cp*(Tm-Tm0))/(Mc*cp);
odes=[ode1;ode2];
[TcavSol(t), TfavSol(t)]=dsolve(odes, conds);

Tf=[];
Tc=[];
n=1;
t_0=25;
time_ad=t_0*kf/(Rhof*cf*rc0^2);
for i=0:0.01:time_ad
    Tf(n)=double(TfavSol(i))*(Tf0-Tm0) + Tm0;
    Tc(n)=double(TcavSol(i))*(Tf0-Tm0) + Tm0;
    n=n+1;
    m=n+1;
end


%Moderator Temperature ODE
save('Tc','Tc');
t=linspace(0,t_0,m-2);
t=transpose(t);
Tc=transpose(Tc);
Tc_p=fit(t,Tc,'poly8');
Tc_p=subs(str2sym(formula(Tc_p)),coeffnames(Tc_p),num2cell(coeffvalues(Tc_p).'));
Tc_p=subs(Tc_p,'t');

cond3=Tm(0)==Tm0;
ode3=diff(Tm,'t')==(Sc*hi(1)*(Tc_p-Tm)-2*m*cp*(Tm-Tm0))/(Mc*cp);
odes=ode3;
TmSol(t)=dsolve(ode3,cond3);

[Steven Lord]

t=linspace(0,t_0,m-2);
% snip intervening lines of code
TmSol(t)=dsolve(ode3,cond3);

The first element of t is 0. 0 is not a valid index into an array in MATLAB. Remove the indexing on that last line.

[Diego Mondaray Muñoz]

Thank you!!!

Answer 2

[Image Analyst]

See the FAQ for a thorough discussion:

https://matlab.fandom.com/wiki/FAQ#%22Subscript_indices_must_either_be_real_positive_integers_or_logicals.%22