How to I convert this to a 2d array?

Question

Ewen Chan 2016-4-2

0
链接

此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/276683-how-to-i-convert-this-to-a-2d-array

评论： Star Strider 2016-4-4

I have a space delimited text file in the form of x y z and I want to create a 2d array so that I can plot the contour (using contourf) - what's the best way I can do that?

This is what the data looks like: (x y z)

48497963  13.19587040  60.91899872
03828049  13.26920033  61.01649857
63999939  13.42000008  60.59999847
76104927  13.37959957  61.06637955
97845078  12.31340027  61.88624954
23686028  12.50454998  62.81980896
55764008  12.43560028  61.80752945

And for the contourf function, it says that I need to format that into a 2d array (and I need to have the x and y be the indices.

I tried this:

f=fopen('68 data set.txt');
c=textscan(f,'%f %f %f','CollectOutput',true);
fclose(f);
out=accumarray(c{1}+1,c{2});

(from this: http://www.mathworks.com/matlabcentral/answers/116044-how-to-read-data-from-txt-file-and-to-create-a-2d-matrix )

And it gave an error: "Index exceeds matrix dimensions"

Also, is it true that I would have to sort the indices for the contourf function?

Your help is greatly appreciated.

0 个评论
显示 -2更早的评论隐藏 -2更早的评论

请先登录，再进行评论。

请先登录，再回答此问题。

Answer 1

Star Strider 2016-4-2

2
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/276683-how-to-i-convert-this-to-a-2d-array#answer_216060

编辑：per isakson 2016-4-2

在 MATLAB Online 中打开

You can do everything with the data you have and the scatteredInterpolant function. It is a core MATLAB function, and no Toolboxes are required. I was able to do this much with the data you presented:

   XYZ = [ 23.48497963  13.19587040  60.91899872
           24.03828049  13.26920033  61.01649857
           24.63999939  13.42000008  60.59999847
           24.76104927  13.37959957  61.06637955
           18.97845078  12.31340027  61.88624954
           19.23686028  12.50454998  62.81980896
           19.55764008  12.43560028  61.80752945];
   F = scatteredInterpolant(XYZ(:,1:2), XYZ(:,3), 'linear');      % ScatteredInterpolant, Linear Interpolation
   [Gx,Gy] = meshgrid(XYZ(:,1), XYZ(:,2));                        % Define ‘meshgrid’ As Query Points
  %
  figure(1)
  contourf(F(Gx,Gy))                                                % Create Filled Contour Plot
  grid

With all our data it would likely look much better. It does create a contourf plot from your data.

I am not certain where you want to go with it from here, but this code works. I didn’t post the plot because of the small amount of data you presented. With all your data it should give you the information you want. You can generate the plot I created from this code.

10 个评论
显示 8更早的评论隐藏 8更早的评论

Star Strider 2016-4-4

在 MATLAB Online 中打开

If it does not model your surface accurately, there are a number of interpolation methods that might be better, such as 'nearest', 'linear', natural, and 'v4'. I would experiment with each to see which produces the best result. I chose 'cubic' because that is usually the best for complicated curved surfaces, but it may not be ideal in your application.

With respect to the functions connecting points on the upper (with respect to ‘Y’) concave part of your surface, the problem may be in the way your data are organised. The plot functions want to ‘connect the lines’ to produce a convex outline. Even delaunay and boundary (which should produce an accurate boundary even along concave regions of your surface) colour outside your intended border. Every function I’ve experimented with wants to connect the points along the concave curvature. Even the surf and mesh plots connect that region. Your data have defeated everything I can think of to not connect that region.

You can get some idea of what the problem is by plotting:

figure(3)
plot3(X, Y, Z, '-x', 'MarkerSize',5)
grid on
% view([0 90])

Enlarging it in the plot GUI will give you some idea. The plotted lines zig-zag.

Since I have no idea how your data were collected or how they are organised, I’m not certain what to suggest. Sorting them could be an option, but I have no idea how best to do that, since I don’t know anything about your data. (You would use sortrows to sort ‘D{:}’ by one of its columns.)

Ewen Chan 2016-4-4

Generically, the data is collected by running a series of experiments, varying two parameters (one that is shown as the x-axis, and another that isn't shown) which is how I created the surface in CAD (think along the lines of a multi-section loft, except that in reality, it isn't quite that - but something similiar to that idea - if that helps you understand where the data is coming from).

And then the surface is meshed/tessellated inside the CAD environment and the resulting text file is the export of the mesh.

Hope that helps.

So what I am hoping to get out of this is to be able to plot the data "as-is" without any interpolation or extrapolation.

Thanks.

Star Strider 2016-4-4

My pleasure.

It helps a bit. When I sort your data by any one of the three columns, it doesn’t change the way MATLAB displays it. Using boundary helps somewhat, but several of the points are still connected where they shouldn’t be.

There are many undocumented features in MATLAB code, that only MathWorks knows of. At this point, I suggest that you Contact Support, submit your problem as a bug report or enhancement request, and see if MathWorks can fix it. This isn’t really an ‘extrapolation’ but the problem of MATLAB creating continuities where there are none.

Include the URL of this thread: http://www.mathworks.com/matlabcentral/answers/276683-how-to-i-convert-this-to-a-2d-array in your message to MathWorks so you don’t have to repeat yourself. Just mention that this thread fully describes your problems and my attempts to solve them.

Ask MathWorks to email me with any solution they come up with, because I’d like to know if one exists. I am out of ideas.

请先登录，再进行评论。