How to plot the surface of an arbitrary given TABLE (not matrix) with X,Y,Z values?

I want to plot the surface Z of arbitrary given values for each X,Y-pair.

I usually IMPORT the tables with the X,Y,Z data, so I they are not a matrix.

An example is displayed below:

1 1 0.171121066356432

1 2 0.0326008205305280

1 3 0.561199792709660

2 1 0.881866500451810

2 2 0.669175304534394

2 3 0.190433267179954

3 1 0.368916546063895

3 2 0.460725937260412

3 3 0.981637950970750

I tried the following lines to plot the surface

[X,Y] = meshgrid(1:1:3, 1:1:3);

Z=rand(9,1);

surf(X,Y,Z)

But I get the following error:

??? Error using ==> surf at 78

Z must be a matrix, not a scalar or vector.

Error in ==> Untitled2 at 5

surf(X,Y,Z)

Question: I replaced the x,y coordinate with the function meshgrid but what do I make wih the Z values?

I'm aware that Z is not a matrix, but this is exactly the problem!!!

I hope someone knows how to correct these commands

Thanks,

Emerson

Answer 1

[Walter Roberson]

[ux, ax, bx] = unique(YourTable(:,1));
[uy, ay, by] = unique(YourTable(:,2));
[X, Y] = ndgrid(ux, uy);
Z = accumarray( [bx(:),by(:)], YourTable(:,3), [], NaN );
surf(X,Y,Z);

This code will even average the z entries if there are multiple (x,y) pairs with different z.

If I have done it right, it does not need to be fed non-negative integers in the first two table columns.

I set the code up so that if there is some combination of (x,y) for which there is no table value, then NaN will be inserted as the Z value. surf() understands NaN as meaning not to plot that square. But you could put any other fixed value numeric there instead. In particular, in the special case where you want 0 for missing values, you can abbreviate the accumarray call to

Z = accumarray( [bx(:),by(:)], YourTable(:,3) );

[Emerson De Souza]

Thank you Walter,

your suggestion works.

The line

Z = accumarray( [bx(:),by(:)], YourTable(:,3), [], NaN ); has a problem. It returns the error:

??? Error using ==> accumarray

Fourth input FUN must be a function handle.

Error in ==> Copy_of_Z4_PLOT_SCALAR_FIELD at 29

Z = accumarray( [bx(:),by(:)], SFIELD(:,3), [], NaN );

%it is not working

But I used only the last line and everything was fine.

Thank you again

Emerson

[Andrei Bobrov]

Z = accumarray( [bx(:),by(:)], YourTable(:,3), [],[], NaN );

[Walter Roberson]

Some day I'll memorize the order of parameters for accumarray...

Answer 2

[Jan]

You can reshape Z such that it becomes a matrix:

[X,Y] = meshgrid(1:1:3, 1:1:3);
Z = reshape(rand(9, 1), 3, 3);
surf(X, Y, Z)

Of course you would use "rand(3,3)" in a real application.

[Emerson De Souza]

Thank you Jan,

your suggestion works as long the length of X and Y are identical which must not be the case for real applications.

For the case that X and Y have different length I used the method suggested by Walter.

But thank you again for your attention

Emerson