error in Euler method

. could you please, check my code I did not get as the following pic.

for i=1:length(T)-1
      x1(i+1)=x1(i)+dX1*h;
      x2(i+1)=x2(i)+dX2*h;
      v1(i+1)=v1(i)+dX3*h;
      v2(i+1)=v2(i)+dX4*h;
  end

[James Tursa]

Closing your question and deleting your code and comments is not appreciated on this forum since it does not help anyone else later on. Please keep your questions and comments intact for everyone to see. Thanks.

[Momo]

I have not received assistance that could be benefit to me or others. Thanks anyway.

Answer 1

[James Tursa]

You need to recalculate the derivatives at each time step. So all of the stuff that goes into calculating dX needs to be inside of your for-loop. Which means that all of the equations for this calculation need to be indexed. E.g., this

r=x1-x2 ;

should be this

r=x1(i)-x2(i) ;

etc.

[Momo]

I still get the same result. could please help me with that

[James Tursa]

Please post your current code.

[James Tursa]

So, when I wrote that you needed to have your derivative calculations inside of the for-loop, I was talking about your existing for-loop in your original posted code:

for i=1:length(T)-1
    % Put all of your derivative calculations here
    x1(i+1)=x1(i)+dX(1)*h;
    x2(i+1)=x2(i)+dX(3)*h;
    v1(i+1)=v1(i)+dX(2)*h;
    v2(i+1)=v2(i)+dX(4)*h;
end

What you have done is create an additional for-loop (not my intention), and changed your original for-loop indexing (also not my intention). Instead, go back to your original for-loop indexing, and put your derivative code where I have the comment. Basically, at each step you need to recalculate the derivatives based on the current state.

[James Tursa]

Why are you using i=1:N instead of the original i=1:length(T)-1? Why are you overwriting dx1(i) and dx2(i) each step without doing the if-test part?