根据python中的值获取数组行和列号

Question

For example I have a 5*5 np.array like this:

a=[[1,2,3,4,5],
   [6,7,8,9,10],
   [11,12,13,14,15],
   [16,17,18,19,20],
   [21,22,23,24,25]]

if I want to get the range of row and column where number<=15 , how can I do this?

On the contrary, if I know the range of row and column, like i in xrange(1,4) and j in xrange(1,4) , how can I get the number like:

[[7,8,9],
 [12,13,14],
 [17,18,19]]

Answer 1

To get the range based on a condition, you can either apply the condition directly, or use np.where :

>>> a
array([[ 1,  2,  3,  4,  5],
       [ 6,  7,  8,  9, 10],
       [11, 12, 13, 14, 15],
       [16, 17, 18, 19, 20],
       [21, 22, 23, 24, 25]])
>>> a < 15
array([[ True,  True,  True,  True,  True],
       [ True,  True,  True,  True,  True],
       [ True,  True,  True,  True, False],
       [False, False, False, False, False],
       [False, False, False, False, False]], dtype=bool)
>>> np.where(a < 15)
(array([0, 0, 0, 0, 0, 1, 1, 1, 1, 1, 2, 2, 2, 2]),
 array([0, 1, 2, 3, 4, 0, 1, 2, 3, 4, 0, 1, 2, 3]))

In the latter case, the return value is a tuple of the matching indices.

To achieve the opposite operation, you can simply slice your array :

>>> ar[1:4, 1:4]
array([[ 7,  8,  9],
       [12, 13, 14],
       [17, 18, 19]])