Finding elements in a numpy array with multiple criteria

Question

How can i find the indices of elements in a Numpy array which satisfy multiple criteria?

Example: The function numpy.nonzero lets me find the indices according to some criterion:

In [1]: from numpy import *
In [2]: a = array([1,0,1,-1])
In [5]: nonzero(a != 0)
Out[5]: (array([0, 2, 3]),)

However, giving multiple criteria like this does not work:

In [6]: nonzero((a != 0) and (a < 0))
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
/Users/cls/<ipython-input-6-85fafffc5d1c> in <module>()
----> 1 nonzero((a != 0) and (a < 0))

ValueError: The truth value of an array with more than one element is ambiguous. Use a.any() or a.all()

In MATLAB, it's possible to write:

find((d != 0) & (d < 0))

How can I do this with Numpy?

DSM · Accepted Answer · 2012-05-14 16:28:43Z

4

IIUC, you can use & instead of and:

>>> from numpy import *
>>> a = array([1,0,1,-1])
>>> nonzero(a!=0)
(array([0, 2, 3]),)
>>> nonzero((a != 0) and (a < 0))
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: The truth value of an array with more than one element is ambiguous. Use a.any() or a.all()
>>> nonzero((a != 0) & (a < 0))
(array([3]),)
>>> where((a != 0) & (a < 0))
(array([3]),)

answered May 14, 2012 at 16:28

DSM

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1 Comment

clstaudt Over a year ago

Thanks. What does "IIUC" mean?

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