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I have a big file with 3 columns: density, dimension, value.
example:

10 0.3 200  
10 0.4 300  
20 0.3 250  
20 0.4 320  
...  

I am trying to draw a 3d plot - mesh with mesh() function in octave, like this:

data = load ("file.txt");  
mesh(data(:,1), data (:,2), data (:,3));

Problem I have is , I always get error:

rows (z) must be the same as length (y), columns (z) must be the same as length (x).

It worked with function plot3(), but I would like a mesh kind of plot.

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  • $\begingroup$ Welcome to SciComp.SE! While Octave and MATLAB-related questions are on topic here, they are expected to have a scientific focus. This question can be answered by simply reading the MATLAB documentation for the mesh function. Note that if X is of size $n\times 1$ and Y is of size $m\times 1$, then Z is expected to be of size $n\times m$ in mesh(X,Y,Z). $\endgroup$ – Ben Jun 3 '13 at 3:41
  • $\begingroup$ Thanks. I am sorry for that, but purpose is purely scientific-al. I knew about the dimensions, just didn't know how to apply my data to that function. reshape() function is the answer! $\endgroup$ – kle pra Jun 3 '13 at 16:57
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The mesh function plots functions z=f(x,y). So to call the mesh() function, you must have 2D data. You can give vectors for x and y, but z must be an array with length(x) rows and length(y) columns, or x and y and z must be all be 2D arrays of the same size. Your data has been pulled out into a single, long vector which you need to two-dimensionalize.

I believe the reshape function will do what you need. E.g. something like:

x=reshape(data(:,1),m,n);
y=reshape(data(:,2),m,n);
z=reshape(data(:,3),m,n);
mesh(x,y,z);

where m and n are the appropriate dimensions in x and y of your data.

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  • $\begingroup$ Thanks for explaining and reshape function! I manually entered x,y vectors since I know the interval: start, end and step and reshaped z-data and it works now. $\endgroup$ – kle pra Jun 3 '13 at 16:53
  • $\begingroup$ There's also a vec2mat function in MATLAB, but I didn't find its counterpart in Octave. reshape goes back a long way, but vec2mat may be new. $\endgroup$ – Bill Barth Jun 4 '13 at 16:42
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Another way could be:

Call your 3 data vectors aa,bb,cc and assume that aa and bb represent a point on the plane and cc is the height above that plane.

Create a grid:

x_plot = linspace(min(aa),max(aa),100);
y_plot = linspace(min(bb),max(bb),100);
[XI,YI] = meshgrid(x_plot,y_plot);

Create the interpolant:

ZI = griddata(aa,bb,cc,XI,YI);

And plot

figure()
mesh(XI,YI,ZI)
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