[Numpy-discussion] Reading in a mesh file

[Florian Lindner]

Hello,

thanks for your reply which was really helpful!

My problem is that I discovered that the data I got is rather unordered.

The documentation for reshape says: Read the elements of a using this index order, and place the elements into the
reshaped array using this index order. ‘C’ means to read / write the elements using C-like index order, with the last
axis index changing fastest, back to the first axis index changing slowest. ‘F’ means to read / write the elements using
Fortran-like index order, with the first index changing fastest, and the last index changing slowest.

With my data both dimensions change, so there is no specific ordering of the points, just a bunch of arbitrarily mixed
"x y z value" data.

My idea is:

out = np.loadtxt(...)
x = np.unique(out[:,0])
y = np.unique[out]:,1])
xx, yy = np.meshgrid(x, y)

values = lookup(xx, yy, out)

lookup is ufunc (I hope that term is correct here) that looks up the value of every x and y in out, like
x_filtered = out[ out[:,0] == x, :]
y_filtered  = out[ out[:,1] == y, :]
return y_filtered[2]

(untested, just a sketch)

Would this work? Any better way?

Thanks,
Florian


Am 31.08.2016 um 17:06 schrieb Robert Kern:
> On Wed, Aug 31, 2016 at 4:00 PM, Florian Lindner <mailinglists at xgm.de <mailto: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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