Store a number of calculations within a matrix

Hi All

I am writing a small script with some functions to calculate 3 things. Velocity, acceleration and distance of a particle.

I use a while loop to increment the variable "time".

Each run through the loop, velocity, acceleration and distance are calculated using functions (i.e. I pass variables to a function and get the answer back).

This works fine. However, each loop overwrites the last. Ideally I would like to store each of the variables with time. i.e. a 2 column, n row matrix of [time, V], [time, a], [time, s]. Even better would be to have them all in one matrix ie [time, V, a, s], though I am unsure then how I would plot separate graphs for each.

Once I have that done I would then be aiming to save that matrix to a data file to open in Origin Labs. But that is another question (when I get to it!). This is my simple script:

D = 1e-6
R = D/2
Vol = (4/3)*pi*(D/2)^3
ro = 2000
mass = Vol*ro
viscosity = 1.81e-5
gravity = 9.81
time = 0

while(time<1)

      V = Velocity(mass, viscosity, gravity, time, R);
      a = Acceleration(mass, viscosity, gravity, time, R);
      s = Distance(mass, viscosity, gravity, time, R);
      time = time+1e-6;
      percent = (time/1)*100    

end

So before the time is stepped, I need to have stored my calculations with the time in a matrix of some sort. It is probably a simple thing to do but I am not a big user of matlab and I just can't see how it might be done.

Answer 1

[Thorsten]

Store all results in variable R

i = 1;

while(time<1)
    R(i, 1 ) = time;
    R(i, 2) = Velocity(mass, viscosity, gravity, time, R);
    R(i, 3) = Acceleration(mass, viscosity, gravity, time, R);
    R(i, 4) = Distance(mass, viscosity, gravity, time, R);
    time = time+1e-6;
    percent = (time/1)*100 
    i = i + 1;   
end

To plot

plot(R(:, 1), R(:, 2))

Write to file

dlmwrite('mydata.txt', R)

[Alex Mason]

Ok I can see how that works.

I just tried it and I think, due to the time step I am using (1e-6, so there will be about 1 million rows) that this will not work. I got to 15% and could visibly see the process getting slower and slower with each loop.

Not sure there is any other way to do it. I may have to reduce my time step.

[Thorsten]

Pre-allocate R; if this does not speed up the computation, reduce stepsize.

stepsize = 1e-6;
R = nan(1/stepsize, 4); % preallocate to speed up

while...

[Alex Mason]

Great thanks that works a treat!

Quick question re plotting. It is not critical but I cannot get Matlab to plot 3 separate graphs. I tried doing:

figure(1) plot.... figure(2) plot....

as I have seen elsewhere on the net, but matlab just produces a single figure with whichever plot was processed last. What is more is that it seems to like plotting a line between the first and last points. Not sure what is going on there, but I am intending to import the data into OriginLabs anyway.