reconstruct matrix from vector coordinates

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Nicolas 2017-1-24

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评论： Walter Roberson 2017-1-25

Hello, I have 3 vectors (x,y,data) each equals 1x6358. How can I recreate the matrix DATA based on the indices x and y? Thank you

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Answer 1

Walter Roberson 2017-1-25

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Possibly you should be using scatteredInterpolant() or TriScatteredInterp to create an interpolant that you would then sample at points on a 2D grid.

Or possibly you should use

[xu, ~, xuidx] = unique(x);
[yu, ~, yuidx] = unique(y);
nxu = length(xu);
nyu = length(yu);
DATA = accumarray( [xuidx(:), yuidx(:)], data(:) );

This would be primarily for the situation where your x and y values form a grid but not necessarily of integer values and not necessarily equi-distant. This particular form of the code expects that all the x values that are intended to be treated as equal are exactly equal -- for example although 1 and 1-eps both display as 1, they are not exactly equal and this code would treat them as different. (If you do have a grid but the values are not exactly equal then there are ways to deal with that, such as by using uniquetol())

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Nicolas 2017-1-25

Yes, the mesh is more compact in some areas than some others.

Walter Roberson 2017-1-25

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The nxu and nyu lines above are not needed; I forgot to edit those out. You can get away with

[xu, ~, xuidx] = unique(x);
[yu, ~, yuidx] = unique(y);
DATA = accumarray( [xuidx(:), yuidx(:)], data(:) );

or, if needed, uniquetol() instead of unique()

This would produce a grid that would not necessarily be equally spaced. The vectors xu and yu give the actual coordinates.

If you wanted to create an equally spaced mesh, then probably after the above you would use griddedInterpolant together with a mesh of the points you wanted to sample at.

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Answer 2

Stephen23 2017-1-24

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编辑：Stephen23 2017-1-24

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You could use sub2ind:

 sz = [max(x),max(y)];
 ix = sub2ind(sz,x,y);
 mat = NaN(sz);
 mat(ix) = data;

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Walter Roberson 2017-1-25

Your x and y are not indices in the MATLAB sense.

Nicolas 2017-1-25

no they are spatial coordinates from the mesh of a numerical model. do I need to find a way to create the indices from the mesh in spatial coordinates?

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