[Numpy-discussion] Reading in a mesh file

Robert Kern robert.kern at gmail.com
Wed Aug 31 11:06:57 EDT 2016


On Wed, Aug 31, 2016 at 4:00 PM, Florian Lindner <mailinglists at xgm.de>
wrote:
>
> Hello,
>
> I have mesh (more exactly: just a bunch of nodes) description with values
associated to the nodes in a file, e.g. for a
> 3x3 mesh:
>
> 0   0   10
> 0   0.3 11
> 0   0.6 12
> 0.3 0   20
> 0.3 0.3 21
> 0.3 0.6 22
> 0.6 0   30
> 0.6 0.3 31
> 0.6 0.6 32
>
> What is best way to read it in and get data structures like the ones I
get from np.meshgrid?
>
> Of course, I know about np.loadtxt, but I'm having trouble getting the
resulting arrays (x, y, values) in the right form
> and to retain association to the values.

For this particular case (known shape and ordering), this is what I would
do. Maybe throw in a .T or three depending on exactly how you want them to
be laid out.

[~/scratch]
|1> !cat mesh.txt

0   0   10
0   0.3 11
0   0.6 12
0.3 0   20
0.3 0.3 21
0.3 0.6 22
0.6 0   30
0.6 0.3 31
0.6 0.6 32

[~/scratch]
|2> nodes = np.loadtxt('mesh.txt')

[~/scratch]
|3> nodes
array([[  0. ,   0. ,  10. ],
       [  0. ,   0.3,  11. ],
       [  0. ,   0.6,  12. ],
       [  0.3,   0. ,  20. ],
       [  0.3,   0.3,  21. ],
       [  0.3,   0.6,  22. ],
       [  0.6,   0. ,  30. ],
       [  0.6,   0.3,  31. ],
       [  0.6,   0.6,  32. ]])

[~/scratch]
|4> reshaped = nodes.reshape((3, 3, -1))

[~/scratch]
|5> reshaped
array([[[  0. ,   0. ,  10. ],
        [  0. ,   0.3,  11. ],
        [  0. ,   0.6,  12. ]],

       [[  0.3,   0. ,  20. ],
        [  0.3,   0.3,  21. ],
        [  0.3,   0.6,  22. ]],

       [[  0.6,   0. ,  30. ],
        [  0.6,   0.3,  31. ],
        [  0.6,   0.6,  32. ]]])

[~/scratch]
|7> x = reshaped[..., 0]

[~/scratch]
|8> y = reshaped[..., 1]

[~/scratch]
|9> values = reshaped[..., 2]

[~/scratch]
|10> x
array([[ 0. ,  0. ,  0. ],
       [ 0.3,  0.3,  0.3],
       [ 0.6,  0.6,  0.6]])

[~/scratch]
|11> y
array([[ 0. ,  0.3,  0.6],
       [ 0. ,  0.3,  0.6],
       [ 0. ,  0.3,  0.6]])

[~/scratch]
|12> values
array([[ 10.,  11.,  12.],
       [ 20.,  21.,  22.],
       [ 30.,  31.,  32.]])

--
Robert Kern
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