[Numpy-discussion] Nonzero behaving strangely?

Christopher Barker Chris.Barker@noaa....
Thu Mar 17 17:31:18 CDT 2011


On 3/17/11 3:17 PM, santhu kumar wrote:

> nid = (res[:,4]==2).nonzero()
> nid tuple turns out to be empty. But the very first row satisfies the
> criteria.

it works for me:

In [45]: arr
Out[45]:
array([[ 33.35053669,  49.4615004 ,  44.27631299,   1.        ,   2. 
     ],
        [ 32.84263059,  50.24752036,  43.92291659,   1.        ,   0. 
      ],
        [ 33.68999668,  48.90554673,  43.51746687,   1.        ,   0. 
      ],
        [ 34.11564931,  49.77487763,  44.83843076,   1.        ,   0. 
      ],
        [ 32.4641859 ,  48.65469145,  45.09300791,   1.        ,   3. 
      ],
        [ 32.15428526,  49.26922262,  45.92959026,   1.        ,   0. 
      ],
        [ 31.23860825,  48.21824628,  44.30816331,   1.        ,   0. 
      ],
        [ 30.71171138,  47.45600573,  44.9282456 ,   1.        ,   0. 
      ],
        [ 30.53843426,  49.07713258,  44.20899822,   1.        ,   0. 
      ],
        [ 31.54722284,  47.61953925,  42.95235178,   1.        ,   0. 
      ],
        [ 32.44334635,  48.10500653,  42.51103537,   1.        ,   0. 
      ],
        [ 31.77269609,  46.53603145,  43.06468455,   1.        ,   0. 
      ],
        [ 30.1820843 ,  47.80819604,  41.77667819,   1.        ,   0. 
      ],
        [ 30.78652668,  46.82907769,  40.38586451,   1.        ,   0. 
      ],
        [ 30.05963091,  46.84268609,  39.54583693,   1.        ,   0. 
      ],
        [ 31.75239177,  47.22768463,  40.00717713,   1.        ,   0. 
      ],
        [ 30.94617127,  45.76986265,  40.68226643,   1.        ,   0. 
      ],
        [ 33.20069679,  47.42127403,  45.66738249,   1.        ,   0. 
      ],
        [ 34.39608116,  47.25481126,  45.4438599 ,   1.        ,   0. 
      ]])

In [46]: arr.shape
Out[46]: (19, 5)

In [47]: nid = (arr[:,4]==2).nonzero()

In [48]: nid
Out[48]: (array([0]),)

Maybe you are having a floating point comparison problem -- i.e.e that 
"2.0" is really "1.9999999999" or something.

Checking equality with floating point numbers is fraught with problems.

Also, y9ou amy not need teh nonzero:

In [56]: arr[:,4]==2
Out[56]:
array([ True, False, False, False, False, False, False, False, False,
        False, False, False, False, False, False, False, False, False, 
False], dtype=bool)

that's a boolean array that can be used for indexing, operating with 
"where", etc.

HTH,

-Chris


-- 
Christopher Barker, Ph.D.
Oceanographer

Emergency Response Division
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Chris.Barker@noaa.gov


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