Numpy bug? How do I align a numpy record array ("recarray")?

Question

Unfortunately the following:

import numpy as np
a = np.zeros(4, dtype=np.dtype([('t', '<f8'), ('d', [('a', '<i4'), ('b', '<f8')], (100,))], align=True))
b = np.require(a, requirements=['ALIGNED'])
print([x.flags['ALIGNED'] for x in [a, b]])

prints [False, False]!

How do I align a?

Correct me if I am wrong! when you are printing "b.flags['ALIGNED']" you get False ? ('cause when I run this code I get True value). — user3378649, Oct 21 '14 at 22:58
@user3378649: I get False on numpy 1.9. (I wrote it as a program so that there can be no doubt.) — Neil G, Oct 21 '14 at 23:02
Is this on 64 bits? @user3378649, is yours on 32 bits with 1.9? — Charles Harris, Oct 22 '14 at 18:03
@CharlesHarris: I'm running Yosemite on an i7, so I think I'm 64 bits. — Neil G, Oct 22 '14 at 18:07

score 5 · Accepted Answer · answered Oct 22 '14 at 18:52

5

You can specify the alignment requirement in the dtype.

In [9]: a = np.zeros(4, dtype= np.dtype([('x', '<f8'), ('y', '<i4')], align=False))

In [10]: a.data
Out[10]: <read-write buffer for 0x2f94440, size 48, offset 0 at 0x2f8caf0>

In [11]: a = np.zeros(4, dtype=np.dtype([('x', '<f8'), ('y', '<i4')], align=True))
In [12]: a.data
Out[12]: <read-write buffer for 0x2f94030, size 64, offset 0 at 0x2f8c5b0>

Note the difference in size. For structured types, the alignment flag was misleading in previous versions of Numpy, the requirement is now 16 bytes for strings and structured types in order to make things work correctly on SPARC. Julian Taylor gives a more extended explanation at http://article.gmane.org/gmane.comp.python.numeric.general/59123

answered Oct 22 '14 at 18:52

Charles Harris

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Thanks for your answer. While it works with this simple test case. It doesn't work in general unfortunately. Please see my updated question. – Neil G Oct 23 '14 at 00:42
1

OK, looks like a bug. The `recarray` class accepts an aligned parameter, but looks to lose it in `format_parser`. Please open a Numpy issue and we can move the conversation there. – Charles Harris Oct 23 '14 at 03:06
1

Thanks, sounds good. Please let me know if you have seen the motivation for my question: http://stackoverflow.com/questions/26484665/opengl-says-from-param-received-a-non-contiguous-array This bug is breaking PyOpenGL I think. – Neil G Oct 23 '14 at 03:17

Numpy bug? How do I align a numpy record array ("recarray")?

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