ODE function in a loop

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Physics 2017-3-2

0
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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/327778-ode-function-in-a-loop

评论： Walter Roberson 2017-3-4

I have a function of x and t, and am creating a loop to find the average x/t for every U. I have successfully used this method before, however my x values are zero every time.

My function is:

function dx=myfunction(t,x,U)
de=24.35;
dd=294.5.*10^(-9);
hbar=6.5.*10^(-16);%
lambda=20*d;
k=(2.*pi)/(lambda);
v=2.5.*10^(-3);
dx=zeros(3,1);
dx(1)=((de)/(2.*hbar.*v.*k)).*sin(x(2));
dx(2)=(dd.*U)./(hbar.*2*v).*(cos(((pi.*x(1))/10)-(t)));
end

In the loop:

dU=1;
U=0:dU:200;%
tspan=[1 30];
vx=zeros(size(U));
for j=1:length(U)
    [t,x]=ode45(@(t,x) myfunction(t,x,U(j)),tspan,[0 0 0]);
    vx(j)=(sum((x(:,1))./(t))./(length(t));
end

Any ideas as to why the changing of U gives x a value of 0 would be useful.

Thanks in advance.

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Physics 2017-3-2

编辑：Physics 2017-3-2

I have edited the constants and names. Changed it for ease.

Jan 2017-3-3

Note: While dd=294.5.*10^(-9) is an exüpensive power operation, dd=294.5e-9 is a cheap constant.

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Answer 1

Jan 2017-3-3

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The code will not run at all, because the number of parenthesis in

vx(j) = (sum((x(:,1))./(t))./(length(t));

is wrong. I guess you want:

vx(j) = sum(x(:,1) ./ t) ./ length(t);

It is useful to use additional parenthesis, if it improves the readability of the code, but here this was an overdoing.

If this is fixed, runnign your code fails in the line:

lambda=20*d;

due to an undefined variable "d".

Finally all I can suggest to use the debugger to examine, what's going on. Set a breakpoint in your code and step through the program line by line. This helps to understand, where which values are defined. Please post the running code afterwards, if you still have problems.