reconstruct matrix from vector coordinates

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

Answer 1

[Walter Roberson]

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())

[Nicolas]

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

[Walter Roberson]

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.

Answer 2

[Stephen23]

You could use sub2ind:

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

[Nicolas]

I got a: Error using sub2ind (line 43) Out of range subscript. x ranges from 0 to 10000; y ranges from -10000 to 0.0039. x, y and data are all vectors 1x6352

[Walter Roberson]

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

[Nicolas]

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?