On 12/6/06, Christian Marquardt <christian@[...].sc>  wrote:
> 
>  Dear list,
> 
>  apologies if the answer to my question is obvious...
> 
>  Is the following intentional?
> 
>    $>python
> 
>    Python 2.4 (#1, Mar 22 2005, 21:42:42)
>    [GCC 3.3.5 20050117 (prerelease) (SUSE Linux)] on linux2
>    Type "help", "copyright", "credits" or "license" for more information.
> 
>    >>> import numpy as np
>    >>> print np.__version__
>    1.0
> 
>    >>> x = np.array([1., 2., 3., 4., 5.])
> 
>    >>> idx = np.where(x > 6.)
>    >>> print len(idx)
>    1
> 
>  The reason is of course that where() returns a tuple of index arrays
>  instead of simply an index array:
> 
>    >>> print idx
>    (array([], dtype=int32),)
> 
>  Does that mean that one always has to explicitely request the first
>  element of the returned tuple in order to check how many matches were
>  found, even for 1d arrays? What's the reason for designing it that way?


Fancy indexing.

In [1]: a = arange(10).reshape(2,5)

In [2]: i = where(a> 3)

In [3]: i
Out[3]: (array([0, 1, 1, 1, 1, 1]), array([4, 0, 1, 2, 3, 4]))

In [4]: a[i] = 0

In [5]: a
Out[5]:
array([[0, 1, 2, 3, 0],
       [0, 0, 0, 0, 0]])


If you just want a count, try

In [6]: a = arange(10).reshape(2,5)

In [7]: sum(a> 3)
Out[7]: 6

Chuck