# Loop never stops?

2 visualizaciones (últimos 30 días)
Amanda Liu el 16 de Jun. de 2021
Comentada: Amanda Liu el 16 de Jun. de 2021
Why does my loop never stops after I assign k1 & k2 as symbolic variables?
n=10;
T=sym(zeros(n,n));
T(1,:) = 410; % Top
T(end,:) = 420; % Bottom
T(:,1) = 400; % Left
T(:,end) = 400; % Right
T_old = T;
for k = 1:20 %time steps
for j = 2: (n-1)
for i = 2: (n-1)
%k1 = 0.02; k2=0.33;
syms k1 k2
T(i,j) = T_old(i,j)*(1-2*k1-2*k2)+k1*(T_old(i-1,j)+T_old(i+1,j))+k2*(T_old(i,j-1)+T_old(i,j+1));
end
end
T_old = T;
end
##### 2 comentariosMostrar NingunoOcultar Ninguno
KSSV el 16 de Jun. de 2021
This line
syms k1 k2
need not to be loop, keep it at the begining. Are you sure you want k1, k2 to be symbolic?
Amanda Liu el 16 de Jun. de 2021
Yes as I have 2 different values of k1 and of k2.

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Walter Roberson el 16 de Jun. de 2021
The code does finish. It just takes a long time. You are building some complicated expressions. You can improve performance by putting in an expand() call:
On my system, execution time is about 22 seconds, but on the demonstration system here it is over 55 seconds so I cannot show you the result.
tic
n = 10;
T=sym(zeros(n,n));
T(1,:) = 410; % Top
T(end,:) = 420; % Bottom
T(:,1) = 400; % Left
T(:,end) = 400; % Right
T_old = T;
syms k1 k2
for k = 1:20 %time steps
for j = 2: (n-1)
for i = 2: (n-1)
T(i,j) = expand(T_old(i,j)*(1-2*k1-2*k2)+k1*(T_old(i-1,j)+T_old(i+1,j))+k2*(T_old(i,j-1)+T_old(i,j+1)));
end
end
T_old = T;
end
toc
T(1:5,1:5)
Tn = subs(T, {k1, k2}, {0.02, 0.33});
Tn(1:5,1:5)
vpa(Tn,5)
##### 2 comentariosMostrar NingunoOcultar Ninguno
Walter Roberson el 16 de Jun. de 2021
Numeric results look like
[400.0, 410.0, 410.0, 410.0, 410.0, 410.0, 410.0, 410.0, 410.0, 400.0]
[400.0, 346.97, 300.07, 265.17, 246.58, 246.58, 265.17, 300.07, 346.97, 400.0]
[400.0, 327.52, 263.81, 216.56, 191.43, 191.43, 216.56, 263.81, 327.52, 400.0]
[400.0, 323.92, 257.06, 207.47, 181.11, 181.11, 207.47, 257.06, 323.92, 400.0]
[400.0, 323.48, 256.23, 206.35, 179.84, 179.84, 206.35, 256.23, 323.48, 400.0]
[400.0, 323.48, 256.23, 206.36, 179.84, 179.84, 206.36, 256.23, 323.48, 400.0]
[400.0, 323.94, 257.09, 207.51, 181.15, 181.15, 207.51, 257.09, 323.94, 400.0]
[400.0, 327.67, 264.07, 216.89, 191.8, 191.8, 216.89, 264.07, 327.67, 400.0]
[400.0, 348.09, 301.89, 267.39, 248.99, 248.99, 267.39, 301.89, 348.09, 400.0]
[400.0, 420.0, 420.0, 420.0, 420.0, 420.0, 420.0, 420.0, 420.0, 400.0]
Amanda Liu el 16 de Jun. de 2021
Thank you for replying. I will proceed with this method and get back to you soon.

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