Plotting at set intervals in a for loop

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NotSoWiseman
NotSoWiseman 2015-9-30
编辑： Kirby Fears 2017-1-5
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
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Shigeo Nakajima
Shigeo Nakajima 2017-1-5
If I was looking for what you stated (Kirby Fears), how would I do that?
Kirby Fears
Kirby Fears 2017-1-5
编辑：Kirby Fears 2017-1-5
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,

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Kirby Fears
Kirby Fears 2015-9-30
编辑：Kirby Fears 2015-9-30
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.
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NotSoWiseman
NotSoWiseman 2015-9-30
编辑：NotSoWiseman 2015-9-30
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
Kirby Fears 2015-9-30
编辑：Kirby Fears 2015-9-30
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.

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