How to Index Points and Store them all in an Array

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Tom 2018-10-26

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评论： Tom 2018-11-10

I am working with a surface on which I place an arbitrary number of nodes. The points are indexed in spherical polar coordinates via a formula for each coordinate (plus 2 points at the poles). Is there a simple way that I can set out the formulae with MATLAB, choose the index number I would like to go up to and then have the coordinates for each node placed one by one into the columns of a matrix (so the first row would be the theta coordinates for the position vectors of all the nodes, and so on with the phi coordinates for the nodes on the second row and the radial coordinates on the third row).

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Walter Roberson 2018-10-27

That would depend upon the available formula, whether index is a parameter to it. If only the coordinates are input the formula then it would depend upon whether there is a good way to calculate the coordinates from the index number, or an effective way to generate enough of the coordinates to determine the index order.

Tom 2018-10-27

编辑：Walter Roberson 2018-10-27

在 MATLAB Online 中打开

I can post the formula here as it is taken from a paper which I will reference here if necessary:

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/E94808588462001CF5CD982503DB1612/S0022112016006066a.pdf/fundamental_solutions_to_moment_equations_for_the_simulation_of_microscale_gas_flows.pdf

The first two points are located at the poles of a unit sphere and the other points are then distributed around the sphere at coordinates according to these formula:

\theta = i*\Pi/(N_\theta + 1)
\phi = 2*\Pi(j-1)/N_\phi
r = 1

where i runs from 1 to N_\theta and j runs from 1 to N_\phi, such that the total number of points is N = (N_\theta * N_\phi) +2. I can follow the formula by hand up to a small number of points on the sphere, but I feel like there must be a way to specify the maximum values of N_theta and N_phi such that the points are generated over the sphere and an array is created for their positions, as it would obviously be impossible to find the values of 100 points by hand, say.

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Answer 1

Walter Roberson 2018-10-27

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https://ww2.mathworks.cn/matlabcentral/answers/426363-how-to-index-points-and-store-them-all-in-an-array#answer_343770

在 MATLAB Online 中打开

[i, j] = ndgrid(1:N_theta, 1:N_phi);
theta = i .* pi ./ (N_theta + 1);
phi = 2 .* pi .* (j-1) ./ N_phi;
r = 1;
coords = [theta(:), phi(:)];
coords(:,3) = r;

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Walter Roberson 2018-10-29

在 MATLAB Online 中打开

[i, j] = ndgrid(1:N_theta, 1:N_phi);
theta = i .* pi ./ (N_theta + 1);
phi = 2 .* pi .* (j-1) ./ N_phi;
r = 1;
b = [theta(:).'; phi(:).'; r * ones(1,numel(theta))];

Tom 2018-11-10

This seems to have worked but I also need to add the vectors for the nodes at the north and south poles, so I need to add the code in which adds two vectors to the array for [0 0 r] and [0 0 -r]. I tried something like

b(1:3,i+1)=[0 0 r];

b(1:3,i+2)=[0 0 -r];

but it didn't seem to work.

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