8000 Bad results of SVD on some (non-contiguous?) arrays (Trac #178) · Issue #776 · numpy/numpy · GitHub
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Bad results of SVD on some (non-contiguous?) arrays (Trac #178) #776

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numpy-gitbot opened this issue Oct 19, 2012 · 2 comments
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Bad results of SVD on some (non-contiguous?) arrays (Trac #178) #776

numpy-gitbot opened this issue Oct 19, 2012 · 2 comments
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00 - Bug component: Other

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Original ticket http://projects.scipy.org/numpy/ticket/178 on 2006-07-13 by @bthyreau, assigned to unknown.

Hi, The svd functions may return fully wrong values on the svn trunk (Here: 0.9.9.2800).
It worked with 0.9.8, still.
Short script to trigger the problem:

from numpy import *
from numpy.linalg import svd
from numpy.random import rand
K = rand(10003 * 2).reshape(10003, 2).astype(float32)
K3 = K[:,0:1]
r1 = svd(K3.copy(), 0)[0]
r2 = svd(K3, 0)[0]
(r1 == r2).all()

The last line should return True, but returns False presently. Note that strangely enough, it starts failing for me at sizes of about 10010. Thanks
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@alberts wrote on 2006-07-13

This looks similar to the problem mentioned in #763. The following error from Valgrind 3.1.0 seems to indicate the main culprit (tested with r2813):

==7522== Conditional jump or move depends on uninitialised value(s)
==7522==    at 0x4632701: PyArray_Scalar (arrayobject.c:1289)
==7522==    by 0x4632EF5: PyArray_Return (arrayobject.c:1444)
==7522==    by 0x4696EF6: PyUFunc_GenericReduction (ufuncobject.c:2559)
==7522==    by 0x5F15CFC: PyCFunction_Call (in /usr/lib/libpython2.4.so.1.0)
==7522==    by 0x5EEB586: PyObject_Call (in /usr/lib/libpython2.4.so.1.0)
==7522==    by 0x463D318: PyArray_GenericReduceFunction (arrayobject.c:3211)
==7522==    by 0x46669E3: PyArray_All (multiarraymodule.c:781)
==7522==    by 0x4666A77: array_all (arraymethods.c:1380)
==7522==    by 0x5F15CFC: PyCFunction_Call (in /usr/lib/libpython2.4.so.1.0)
==7522==    by 0x5F50E2A: PyEval_EvalFrame (in /usr/lib/libpython2.4.so.1.0)
...

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@teoliphant wrote on 2006-07-18

This was a problem with _broacast_cast and strided_cast in particular where the index was not being updated to reflect all the converted values.

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