How to solve the "Subscript indices must either be real positive integers or logicals." error.

Question

Naiara Tabanez 2019-9-11

0
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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/479808-how-to-solve-the-subscript-indices-must-either-be-real-positive-integers-or-logicals-error

回答： Shunichi Kusano 2019-9-11

Hi, guys.

First of all, I am trying to solve a heat transfer problem with the finite volume method, but I don't know where am I failing in, and how to solve it.

Thank you all!

%unidimensional heat transfer
Ts=50;
i=6;
s=100;
T=zeros(i);
alfa=6.82*10^-6;
x=[1 2 3 4 5 6];
L=1; 
dt=0.01;
for t=1:s
Tc=(2*Ts-T(1));
Tf=(2*Ts-T(6));
T(1)=alfa*dt((1/alfa*t*dt)*T(1)+(1/alfa*t*dt)*T(1)-(2/alfa*(L/6))*T(1)+(1/alfa*(L/6))*T(2)+(1/alfa*(L/6))*Tc);
T(2)=alfa*dt((1/alfa*t*dt)*T(2)+(1/alfa*t*dt)*T(2)-(2/alfa*(L/6))*T(2)+(1/alfa*(L/6))*T(3)+(1/alfa*(L/6))*T(1));
T(3)=alfa*dt((1/alfa*t*dt)*T(3)+(1/alfa*t*dt)*T(3)-(2/alfa*(L/6))*T(3)+(1/alfa*(L/6))*T(4)+(1/alfa*(L/6))*T(2));
T(4)=alfa*dt((1/alfa*t*dt)*T(4)+(1/alfa*t*dt)*T(4)-(2/alfa*(L/6))*T(4)+(1/alfa*(L/6))*T(5)+(1/alfa*(L/6))*T(3));
T(5)=alfa*dt((1/alfa*t*dt)*T(5)+(1/alfa*t*dt)*T(5)-(2/alfa*(L/6))*T(5)+(1/alfa*(L/6))*T(6)+(1/alfa*(L/6))*T(4));
T(6)=alfa*dt((1/alfa*t*dt)*T(6)+(1/alfa*t*dt)*T(6)-(2/alfa*(L/6))*T(6)+(1/alfa*(L/6))*Tf+(1/alfa*(L/6))*T(5));
end
plot(x,T);

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Answer 1

Shunichi Kusano 2019-9-11

0
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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/479808-how-to-solve-the-subscript-indices-must-either-be-real-positive-integers-or-logicals-error#answer_391307

Hi Naiara,

The error occurs, since you use "dt" as an array, though it is a scalar. The problem is the first "dt" of each sentence in the following codes.

T(1)=alfa*dt((1/alfa*t*dt)*T(1)+(1/alfa*t*dt)*T(1)-(2/alfa*(L/6))*T(1)+(1/alfa*(L/6))*T(2)+(1/alfa*(L/6))*Tc);
T(2)=alfa*dt((1/alfa*t*dt)*T(2)+(1/alfa*t*dt)*T(2)-(2/alfa*(L/6))*T(2)+(1/alfa*(L/6))*T(3)+(1/alfa*(L/6))*T(1));
T(3)=alfa*dt((1/alfa*t*dt)*T(3)+(1/alfa*t*dt)*T(3)-(2/alfa*(L/6))*T(3)+(1/alfa*(L/6))*T(4)+(1/alfa*(L/6))*T(2));
T(4)=alfa*dt((1/alfa*t*dt)*T(4)+(1/alfa*t*dt)*T(4)-(2/alfa*(L/6))*T(4)+(1/alfa*(L/6))*T(5)+(1/alfa*(L/6))*T(3));
T(5)=alfa*dt((1/alfa*t*dt)*T(5)+(1/alfa*t*dt)*T(5)-(2/alfa*(L/6))*T(5)+(1/alfa*(L/6))*T(6)+(1/alfa*(L/6))*T(4));
T(6)=alfa*dt((1/alfa*t*dt)*T(6)+(1/alfa*t*dt)*T(6)-(2/alfa*(L/6))*T(6)+(1/alfa*(L/6))*Tf+(1/alfa*(L/6))*T(5));