how do I pre-allocate the program. It highlights it in red and I am not able to do that. Thanks for helping.

Question

Dereje 2017-11-30

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评论： Greg 2017-11-30

for i=1:numel(rho)-1
rho0=17.1;
zspan = [z(i) z(i+1)];
NS =(rho(i+1)-rho(i))/(z(i+1)-z(i));
[t,v] = ode45(@(t,v)rhs(t,v,NS), zspan, v0);
v0 = [v(end,1) ; v(end,2) ; v(end,3)];
zsol =  [zsol;t];
v1sol = [v1sol;v(:,1)];
v2sol = [v2sol;v(:,2)];
v3sol = [v3sol;v(:,3)];
end

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Greg 2017-11-30

Please format your question so we can read it as code. Use the "{} Code" button if you need help with formatting syntax.

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Answer 1

Guillaume 2017-11-30

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Assuming your ODE returns a 3x3 array:

v1sol = zeros(3, numel(rho) - 1);
v2sol = zeros(3, numel(rho) - 1);
v3sol = zeros(3, numel(rho) - 1);
zsol = zeros(numel(rho) - 1, 1);
rho0=17.1;
for i=1:numel(rho)-1
   zspan = [z(i) z(i+1)];
   NS =(rho(i+1)-rho(i))/(z(i+1)-z(i));
   [t,v] = ode45(@(t,v)rhs(t,v,NS), zspan, v0);
   v0 = [v(end,1) ; v(end,2) ; v(end,3)]; 
   zsol(i) = t;
   v1sol(:, i) = v(:, 1);
   v2sol(:, i) = v(:, 2);
   v3sol(:, i) = v(:, 3);
end

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Greg 2017-11-30

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I would go ahead and use

nrho = numel(rho) - 1;

Just a little cleaner in my opinion. Nothing to do with pre-allocation.

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Answer 2

Dereje 2017-11-30

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@Guillame there is an error(Matrix mis-match) on

sol(i) = t;

@Greg I don't understand your point.

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Greg 2017-11-30

Please use the "Comment on this Question" and "Comment on this Answer" links instead of creating a new answer. I know the web form is confusing the first time.

You can also edit your original question to attach the code. This makes it easier for others (people may have the same question you have) to find.

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