Plotting at set intervals in a for loop

I'm plotting partical postions in a for-loop with t as followed:

t = 0:timeStep:timeStep*nrOfTimeSteps

Now I think the bulk of my code is not required for this question. I want to plot the particle positions in a scatter plot at four set intervals namely [0, 1/4, 1/2, 3/4, 1] of the total time. Everything I've tried so far plots either 0 points, the particle positions at the end, or all the positions at each time step (hold on).

What is the way to implement this within my loop?

Thanks in advance

[Kirby Fears]

Are you trying to scatterplot Y values for X between [0,1/4] in one plot, then repeat for X between [1/4,1/2] in a second plot, etc?

[NotSoWiseman]

No, at every time step I get two vectors which describe the x and y position of each particle. So I want to plot these positions using scatter(x,y) at four times when t = [0, 1/4, 1/2, 3/4, 1]*totalTime. So say I have 100 particles the scatterplot should contain 400 points.

[Shigeo Nakajima]

If I was looking for what you stated (Kirby Fears), how would I do that?

[Kirby Fears]

Shigeo,

This will plot all (X,Y) position pairs for time between 0-1/4 in one plot, time between 1/4-1/2 in a second plot, etc.

T=0.1:0.1:10;
X=rand(numel(T),100);
Y=rand(numel(T),100);
stepSize=numel(T)/4;
tPoints=round(0:stepSize:numel(T));

for t = 1:(numel(tPoints) - 1),
    xVals = X(tPoints(t)+1:tPoints(t+1),:);
    yVals = Y(tPoints(t)+1:tPoints(t+1),:);
    figure();
    scatter(xVals(:),yVals(:));
end,

Answer 1

[Kirby Fears]

Wrote a test script that scatters X and Y positions for 100 particles at 5 times as close to (0,1/4,1/2,3/4,1) portion of total time as possible. The legend shows the exact time that used for each scatter.

T=0.1:0.1:100;
X=rand(numel(T),100);
Y=rand(numel(T),100);
stepSize=(numel(T)-1)/4;
tPoints=round(1:stepSize:numel(T));

for t = 1:numel(tPoints),
    scatter(X(tPoints(t),:),Y(tPoints(t),:));
    if t==1,
        hold on;
    end,
end,
hold off;
legendTimes=num2cell(T(tPoints)');
legendTimes=cellfun(@(c)num2str(c),legendTimes,'UniformOutput',false);
legend(legendTimes);

Hope this helps.

[NotSoWiseman]

Thank you for the input, I'll definitely try using the dynamic legend but I think it will be hard to implement a similar code into my script. I'm looking for something more along the lines of a simple if-statement (which I tried unsuccessfully). My loop simply plots all positions simultaneously and then replaces them as the assignment asks.

for t = 0:timeStep:timeStep*nrOfTimeSteps

    a = rand(Np,1)*2*pi;
    R = 2*sqrt(Dmol*timeStep);

    xp = xp + vx * timeStep + R*cos(a);
    yp = yp + R*sin(a);
...

If I plot within the loop and ask it to 'drawnow' I can see the movement of the particles. I simply want to grab 5 snapshots at the stated times. Something like:

hold on

if t == [0, 1/4, 1/2, 3/4, 1]
  scatter(x,y)
end

Which I tried but didn't work.

[Kirby Fears]

In this example, you can use "hold on" after scatter(x,y) in the case that t is zero (hold after first scatter only). I used a similar technique in my answer above.

Also, try using the any() command to make your if statement work properly.

 for t=0:.05:1,
     if any(t == [0,.25,.5,.75,1]),
         disp(num2str(t));
     end,
 end,

This will only fail if t is not precisely equal to the values you're comparing against, so be sure that the values do occur in the loop. If you want to allow for a margin of error in equality, you can change it to something like:

 tol=1e-5;
 ...
 if any( abs([0,.25,.5,.75,1]-t) < tol ),
 ...

Hope this helps.