Question

I have a 2d numpy.array, where the first column contains datetime.datetime objects, and the second column integers:

A = array([[2002-03-14 19:57:38, 197],
       [2002-03-17 16:31:33, 237],
       [2002-03-17 16:47:18, 238],
       [2002-03-17 18:29:31, 239],
       [2002-03-17 20:10:11, 240],
       [2002-03-18 16:18:08, 252],
       [2002-03-23 23:44:38, 327],
       [2002-03-24 09:52:26, 334],
       [2002-03-25 16:04:21, 352],
       [2002-03-25 18:53:48, 353]], dtype=object)

What I would like to do is select all rows for a specific date, something like

A[first_column.date()==datetime.date(2002,3,17)]
array([[2002-03-17 16:31:33, 237],
           [2002-03-17 16:47:18, 238],
           [2002-03-17 18:29:31, 239],
           [2002-03-17 20:10:11, 240]], dtype=object)

How can I achieve this?

Thanks for your insight 🙂

Answer 1

You could do this:

from_date=datetime.datetime(2002,3,17,0,0,0)
to_date=from_date+datetime.timedelta(days=1)
idx=(A[:,0]>from_date) & (A[:,0]<=to_date)
print(A[idx])
# array([[2002-03-17 16:31:33, 237],
#        [2002-03-17 16:47:18, 238],
#        [2002-03-17 18:29:31, 239],
#        [2002-03-17 20:10:11, 240]], dtype=object)

A[:,0] is the first column of A.

Unfortunately, comparing A[:,0] with a datetime.date object raises a TypeError. However, comparison with a datetime.datetime object works:

In [63]: A[:,0]>datetime.datetime(2002,3,17,0,0,0)
Out[63]: array([False,  True,  True,  True,  True,  True,  True,  True,  True,  True], dtype=bool)

Also, unfortunately,

datetime.datetime(2002,3,17,0,0,0)<A[:,0]<=datetime.datetime(2002,3,18,0,0,0)

raises a TypeError too, since this calls datetime.datetime‘s __lt__ method instead of the numpy array’s __lt__ method. Perhaps this is a bug.

Anyway, it’s not hard to work-around; you can say

In [69]: (A[:,0]>datetime.datetime(2002,3,17,0,0,0)) & (A[:,0]<=datetime.datetime(2002,3,18,0,0,0))
Out[69]: array([False,  True,  True,  True,  True, False, False, False, False, False], dtype=bool)

Since this gives you a boolean array, you can use it as a “fancy index” to A, which yields the desired result.