Using Heun's Method but Problem With Increment

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Muhammad Hadyan Utoro 2021-8-28

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https://ww2.mathworks.cn/matlabcentral/answers/1442319-using-heun-s-method-but-problem-with-increment

评论： Muhammad Hadyan Utoro 2021-8-30

Hi everyone,

I'm really new to programming and try to use Heun's Method to this skydiving problem, but I kept getting this error, "Unable to perform assignment because the left and right sides have a different number of elements. Error in EulervsHeunStart (line 17)

vh(i) = vh;"

I know I have to use the increment to get the second slope (corrector). Here's my script:

while t<=tend
    %save both velocities at time ti
    vh(i) = vh;
    ve(i) = ve;
    tim(i)= t;
    %good old Euler
    a = g-c/m*ve.^2;
    ve = ve+a*dt;
    % x = x+v*dt;
    %Heun: compute slope at ti
    a = g-c/m*vh.^2; 
    %compute predicted velocity at ti+1
    vh = vh+a*dt;  
    %compute second slope at v(ti+1)
    a = g-c/m*vh(i).^2;
    vh(i+1) = vh(i)+a*dt;
    %compute average of both slopes
    
    %use average  slope for extrapolating v from ti to ti+1
    %increments
    t=t+dt;
    i=i+1;
end

Thank you!

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darova 2021-8-29

PLease atach the whole code

Muhammad Hadyan Utoro 2021-8-29

EulervsHeunStart.m

Hi darova,

please find the attachment for the whole code, but they're pretty much the same as I put on the question and 've' and 'vh' variables are for velocity using euler and velocity using heun respectively. I'd like to compare the error.

Many thanks

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Answer 1

darova 2021-8-29

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I think you should use index (i) everywhere. vh variable has two elements after first iteration

You trying to assign two elements into one position in this line

vh(i) = vh;

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Muhammad Hadyan Utoro 2021-8-30

I've used all index on every ve and vh, but it even said, "Unable to perform assignment because the left and right sides have a different number of elements.

Error in EulervsHeunStart (line 17)

vh(i) = vh;"

I don't understand how this works, but thanks again darova. I really appreciate your help

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