Modelling gas particles; "Attempted to access x(11); index out of bounds because numel(x)=10."

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Alastair Martin 2015-3-17

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评论： Alastair Martin 2015-5-4

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Hi, I am at the begging of trying to module gas particles currently the get a problem

Attempted to access x(11); index out of bounds because numel(x)=10.

Error in gasparticelbasic (line 20) x(i) = x(i)+velocityx*dt;

This is my current code;

figure(1) n = 10; % Number of data points x = 50*rand(n,1)-25; y = 50*rand(n,1)-25;

 velocityx = 10*rand(1);
 velocityy = 10*rand(1);
 dt = 0.2;
 for i = 1:1:50
    subplot (2,1,1)
    plot(x,y,'o');
    axis([-25,25,-25,25]);
    hold on
    pause(dt);
    hold off
    x(i) = x(i)+velocityx*dt;
    y(i) = y(i)+velocityy*dt;
 end

Any help would be great. Thanks

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Answer 1

Sebastian Castro 2015-3-17

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I have two thoughts here.

1. You have an out-of-bounds indexing error because you're saying that x(i) = x(i)+velocityx*dt; , thus making the variable reference an index that hasn't yet been created. If you want to do a basic forward integration, shouldn't you try x(i+1)= x(i)+velocityx*dt; ?

This would require you to define an initial velocity x(1) , and for efficiency, to preallocate your x and y vectors with a known size ... for example x = zeros(100,1);

2. Defining the velocities with rand before the loop means that the value of velocityx and velocityy will be the same for every particle and for every iteration.

If you want each particle to have a constant velocity for the whole duration, try generating 25 random numbers: velocityx = rand(25,1);
Else, if you want each particle to have a different random velocity at every time step, call the rand function inside the loop so its value changes at each iteration.

- Sebastian