How to solve an ODE with ode45 in a parfor loop?

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e_frog 2020-10-29

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/630233-how-to-solve-an-ode-with-ode45-in-a-parfor-loop

评论： e_frog 2020-10-29

I have the following code:

function [t,S] = solve_ode(fg_vars,tspan)
        inital_conds = [0 0];
        t = zeros(65,1);
        S = zeros(65,2);
        parfor i = 1:100
        [t(:,i), S(:,i)] = ode45(@(t,s) fn(t,s,fg_vars),tspan,inital_conds)
        end
        t = t(:)
        S = S(:)
end

Which gives me the following error:

Unable to perform assignment because the size of the left side is 65-by-1 and the size of the right side is 65-by-2.

I dont understand why this error occures, because when I try to calculate the ODE without the pafor loop, the produced solutions are of this size:

t = 65x1 double

S = 65x2 double

Thanks!

回答（1 个）

KSSV 2020-10-29

0
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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/630233-how-to-solve-an-ode-with-ode45-in-a-parfor-loop#answer_527768

编辑：KSSV 2020-10-29

在 MATLAB Online 中打开

[t(:,i), S(:,i)] = ode45(@(t,s) fn(t,s,fg_vars),tspan,inital_conds)

In the above t will be a column vector and S will be a column matrix with two columns. You cannot save like that. You can do

T = cell(100,1) ;
S = cell(100,1) ; 
for i = 1:100 
    [t, s] = ode45(@(t,s) fn(t,s,fg_vars),tspan,inital_conds) ;
    T{i} = t ; 
    S{i} = s ; 
end

The above cannot be used with parfor...you cannot index the parfor loop.

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KSSV 2020-10-29

Did you notice there is s and S. The one which is saved are initialized as cell S. And s is saved into S. You should have messed up with this.

e_frog 2020-10-29

I actually did have to switch a t with a T and it is running now. BUT: now matlab just does the same calculation 100 times, instead of splitting the one calculation for tspan up between the cpu cores. So now the calculation takes actually 100 times longer than without any loop at all. Thats not what i wanted. I NEED to use the parfor loop. How can I achieve that?

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