[Numpy-discussion] dates, np.where finding months

[Keith Goodman]

On Wed, Jan 20, 2010 at 6:01 AM, John [H2O] <washakie at gmail.com> wrote:
>
> I have an array with the leading column a series of datetime objects. It
> covers several years. What is the most efficient way to pull out all the
> 'January' dates?
>
> Right now I do this:
>
> A = array with column 0 datetime objects
>
> January = [i for i in A if i[0].month ==1 ]
>
> It works, but I would rather use np.where and get indices and not have to
> convert my list back into an array.

Instead of doing this:

>> A

array([[2010-01-01, 1],
       [2010-01-02, 2],
       [2010-02-01, 3],
       [2010-01-05, 4]], dtype=object)

>> [i for i in A if i[0].month==1]

[array([2010-01-01, 1], dtype=object),
 array([2010-01-02, 2], dtype=object),
 array([2010-01-05, 4], dtype=object)]

You could do this:

>> [i for i, date in enumerate(A[:,0]) if date.month==1]
   [0, 1, 3]