My code keeps saying the variable u appears to be changing as well as index exceeds the number of array elements(1). Could someone help me with this

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Waleed Nofal 2019-12-5

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评论： JESUS DAVID ARIZA ROYETH 2019-12-5

L = 1 % length of domain in x direction 
tmax = 10  % end time 
nx = 11 % number of nodes in x direction 
nt = 101  % number of time steps dx = L/(nx-1); 
dt = tmax/(nt-1); 
alpha = 0.02
r = alpha*dt/dx^2; 
r2 = 1 - 2*r; 
 
t = 0 
u = 0 % initial condition 
for m=1:nt 
uold = u; % prepare for next step
 t = t + dt; 
for i=2:nx-1 
u(i) = r*uold(i-1) + r2*uold(i) + r*uold(i+1); %%This is where the issue is
end 
end

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Walter Roberson 2019-12-5

在 MATLAB Online 中打开

u = 0 % initial condition

A scalar

uold = u; % prepare for next step

copies the scalar

for i=2:nx-1

i starts from 2

u(i) = r*uold(i-1) + r2*uold(i) + r*uold(i+1); %%This is where the issue is

needs uold(i-1) and uold(i) and uold(i+1) which starts as uold(1), uold(2), uold(3), but uold is only a scalar.

Maybe you should initialize

u = zeros(1,nx);

but if you do, then be aware that you are going to be reading from those 0's as you go. That might be acceptable for something like heat propagation (though if you are doing heat, watch out for units: sometimes you should be using Kelvin instead of Celcius)

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Answer 1

JESUS DAVID ARIZA ROYETH 2019-12-5

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/495089-my-code-keeps-saying-the-variable-u-appears-to-be-changing-as-well-as-index-exceeds-the-number-of-ar#answer_405022

编辑：JESUS DAVID ARIZA ROYETH 2019-12-5

在 MATLAB Online 中打开

correction:

L = 1 % length of domain in x direction 
tmax = 10  % end time 
nx = 11 % number of nodes in x direction 
nt = 101  % number of time steps dx = L/(nx-1); 
dt = tmax/(nt-1); 
alpha = 0.02
r = alpha*dt/dx^2; 
r2 = 1 - 2*r; 
 
t = 0 
u = zeros(1,nx); % initial condition 
for m=1:nt 
uold = u; % prepare for next step
 t = t + dt; 
for i=2:nx-1 
u(i) = r*uold(i-1) + r2*uold(i) + r*uold(i+1); %%This is where the issue is
end 
end7