Want to limit the lines of output. (for example, output after every 1000 iterations)

time = 0;
tstop=input('enter tstop now');
m=input('enter value of m now');
k= input('enter value of k now');
b= input('enter b value now');
dt=input(' enter value of time step now');
nstep=input('enter value of nstep');
pos=input('enter starting position now');
vel=input('enter starting velocity now');
i=1;
j=1;
while time < tstop
    t(i) = time;
    xdot(i) = vel;
    x(i) = pos;
    acc= -(k/m)*pos -(b/m)*vel;
    x2dot(i)=acc;
    vel = vel + acc*dt;
    pos = pos + vel*dt;
    time = time+dt;
    i=i+1;
end

out=[t',x2dot',xdot',x']

[Image Analyst]

Why is "out" outside your loop? Do you want to break out of the loop when i = 1000? If you want to continue the loop you should put it inside the loop like Sven showed you.

Answer 1

[Sven]

Hi Brian, your loop has a number of places where arrays are growing inside a loop. I believe this is the main reason why your code runs for 25 seconds. Things will run much faster if you preallocate. In fact, try the code below which should be almost instantaneous:

tstop=25; dt=0.001;
m=2; k=5; b=1;
pos=20; vel=3;
time = 0:dt:tstop;
out = zeros(length(time),4); % This is the big time-saver
out(:,1) = time';
for i = 1:length(time)
    acc= -(k/m)*pos -(b/m)*vel;
    out(i,2:4) = [acc vel pos];
    vel = vel + acc*dt;
    pos = pos + vel*dt;
end

Now, there are further speed-ups that could be made, but I've tried to keep the code similar to your original.

If you need the last row displayed to the screen once every 1000 iterations, then add this small if-statement inside the for-loop:

      if mod(i,1000)==0
          disp(out(i,:))
      end

I hope that answers the question

[Brian]

If its placed inside the loop it creates an infinite loop

[Jan]

No, the loops gets slower due to a missing pre-allocation, but it is not an infinite loop.

[Sven]

Yes, it's not infinite, it's just not ideal.

Brian, can you clarify exactly what your question is?

If you just want to display the last 1000 results to the command window, then it doesn't need to be stored in "out". If you want to just tell your user every 1000 loops that the program is still running, you don't need my "out=..." command inside the loop.

It will all depend on what you're actually trying to do. Squeezing the question into the title of this post seems to have led to some confusion.

[Brian]

Basically if the program prints out 25000 lines of data, I want only 25 lines. The simulation lasts for 25 seconds so I need the data at every 1 second interval.

[Sven]

I've updated the answer to what you've said here Brian. Is it what you were looking for?

[Brian]

Its close. I just need one line per 1000 iterations. Not sure why the if statement with mod doesn't work in the original program. Thanks for the help and sorry about the confusion.

[Sven]

Oh, that's easy then... I've edited the answer to print just one line ... ie, "out(i,:)" instead of "out(i-24:i,:)"

The reason the original program was slow was that *every* time the loop was run, the size of "out" (and "t" and "x" and "xdot") grew. This means that MATLAB kept on making new space in memory for a 99-by-4 array, 100-by-4 array, 101-by-4 array, etc, all the way up to 25000. That's a lot of "finding and allocating new space for a slightly different array". See the help docs for the word "preallocation".

If the question's all done, hit the "accept" button. Cheers,

Sven.

[Sven]

And just to clarify, the if-stament with mod in the original program was displaying *all* of "out" each time it was reached, rather than just some subset of "out".