# MATLAB plotting issue

• MATLAB
Triathlete
I am trying to plot the magnitude of velocity and position against time but keep running into issues.
The error I am getting here is:

Subscript indices must either be real positive integers or logicals.

Error in project2 (line 46)
position(t) = w(1:3);

Here is the relevant portion of my script(everything else work fine except the plot):

N = 540;
a = 1;
b = 5400;
h = (b-a)/N;
t = a;
w = [r(1);r(2);r(3);v(1);v(2);v(3)];

for t = 1:10:5400
k1 = h*rky(t,w);
k2 = h*rky(t+h/2,w+k1/2);
k3 = h*rky(t+h/2,w+k2/2);
k4 = h*rky(t+h,w+k3);
w = w + (h/6)*(k1+2*k2+2*k3+k4);
t = t+h;
position(t) = w(1:3);
velocity(t) = w(4:6);
time(t) = t;
end

subplot(2,1,1),plot(time,norm(position))
title('position');
xlabel('Time');
ylabel('Displacement');
axis([1 5400 0 10000]);
subplot(2,1,2),plot(time,norm(velocity))
title('velocity');
xlabel('Time');
ylabel('Displacement');
axis([1 5400 0 5]);

Homework Helper
t is your integration coordinate, not an integer index to point to a position in an array.
Depending on how position is declared you get something like

position(i) = w(1:3)

or

for j = 1 to 3
position(i,j) = w(j)
next j

but hen in MATLAB language

Mentor
Matlab:
N = 540;
a = 1;
b = 5400;
h = (b-a)/N;
t = a;
w = [r(1);r(2);r(3);v(1);v(2);v(3)];

for t = 1:10:5400 % don't use t here
k1 = h*rky(t,w);
k2 = h*rky(t+h/2,w+k1/2);
k3 = h*rky(t+h/2,w+k2/2);
k4 = h*rky(t+h,w+k3);
w = w + (h/6)*(k1+2*k2+2*k3+k4);
t = t+h;
position(t) = w(1:3); % or here
velocity(t) = w(4:6); % or here
time(t) = t; % or on the lhs here
end
You are using t both as the index of the for loop and as the variable containing the actual time. Also, position and velocity will need to be matrices, not vectors.

BvU
Triathlete
Thanks for the responses!
How would I convert to matrices?

Gold Member
Thanks for the responses!
How would I convert to matrices?

The two lines like "position(t) = w(1:3);" are curious, since you are trying to assign 3 values w(1:3) to the single spot position(t) in a vector. Why is that? Don't you just want a single value for the position at time t?

If you wanted those three values to be a row in a matrix you could do something like position(i,:)=w(1:3) using a new indexing variable i.

The current error you're receiving is because the line t = t+h makes t some decimal value that doesn't make sense as an index. Once you resolve that, you'll also get another error because you can't assign 3 values to 1 spot in a vector.

Can you do your best to update the code with the suggestions presented so far and repost it afterward? If you post a self-contained set of runnable code it is easier to debug.