# Using Heun's Method but Problem With Increment

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Muhammad Hadyan Utoro on 28 Aug 2021
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!
##### 2 CommentsShow 1 older commentHide 1 older comment
Muhammad Hadyan Utoro on 29 Aug 2021
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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### Accepted Answer

darova on 29 Aug 2021
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;
##### 1 CommentShow NoneHide None
Muhammad Hadyan Utoro on 30 Aug 2021
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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