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

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Diego Mondaray Muñoz
评论： Diego Mondaray Muñoz 2023-4-28
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);

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采纳的回答

Steven Lord
Steven Lord 2023-4-27
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.
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显示 1更早的评论
Steven Lord
Steven Lord 2023-4-27
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.

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Image Analyst
Image Analyst 2023-4-27

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