When I try to run a while loop for kinematcs I get the error:'index exceeds number of array elements'.

h(1)=150000;            %initial height
g(1)=(40*10^7)/(6371+h(1))^2;        %acceleration dependant on height 
i=1;        %loop counter
dt=0.005;        %time step
t(1)=0;         %initial time
v(1)=g(1)*t(1);         %velocity 
while h>=100000
    t(i+1)=t(i)+dt;
    h(i+1)=150000-(g(i)*(t(i+1))^2);
    g(i)=(40*10^7)/(6371+h(i+1))^2;
    v(i)=g(i)*t(i+1);
    i=(i+1);
end

When i try to run this loop i get the error 'Index exceeds number of array elements (1)'. Any help would be appriciated. Ideally I would like to be able see the last values of g and v when h>=100000.

[Ahmed Anas]

Can you send the m file please..?

[Sam Potter]

I think I have sent it correctly, apologies if not.

[Ahmed Anas]

I have found the error, but its important that what you want from this code.

In while loop you are using the whole h matrix, better to use that particular value i.e. h(1).. and i suggest you to use a combination of for and if loop instead of while loop. If you explain me what you want then able to do this for you.

Thanks

[Ahmed Anas]

Error lies in these two lines..

h(i+1)=150000-(g(i)*(t(i+1))^2)

g(i)=(40*10^7)/(6371+h(i+1))^2

In the h formula, it is using g which is calculated in the next step, so matlab is unable to solve it..

Hope you got the point

Answer 1

[Ameer Hamza]

It appears there is an issue with the indexing of g and v inside the while loop. The condition of while loop also seems wrong. Try this

h(1)=150000;            %initial height
g(1)=(40*10^7)/(6371+h(1))^2;        %acceleration dependant on height
dt=0.005;        %time step
t(1)=0;         %initial time
v(1)=g(1)*t(1);         %velocity

i=1; %loop counter
while h(end)>=100000
    t(i+1)=t(i)+dt;
    h(i+1)=150000-(g(i)*(t(i+1))^2);
    g(i+1)=(40*10^7)/(6371+h(i+1))^2;
    v(i+1)=g(i)*t(i+1);
    i=i+1;
end

[Sam Potter]

Thank you, this seems to have solved it! If you dont mind please could you explain the changes you made so i dont make the same mistake in the future. Thank you.

[Ameer Hamza]

These are the changes I made.

g(i+1)=(40*10^7)/(6371+h(i+1))^2;

The next iteration needed to use the value of g(i+1), but you were assigning it to g(i), therefore you got error "Index exceeds number of array elements", the element was not present at next iteration.

Same with

v(i+1)=g(i)*t(i+1);

Infact, i think following might be correct

v(i+1)=g(i+1)*t(i+1);

You can compare both and see which of them seems correct.

Last, I changed condition of while loop h(end)>=100000. You were comparing complete array, whereas only last element needed to be compared.