Can someone please explain to me how numpy indexing works for 2d arrays. I am finding it difficult to wrap my head around.

Specifically, if i create a 2d 8x8 array, what would each value represent in this instance:

array[x:y:i, t:n:m]

CodePudding user response：

In your array:

• The x and t, are the beginning of the slice;
• The y and t, are the end of the slice;
• The i and m, are the step of the slice.

For example, let's define an 8x8 array:

z=[[x*y x y for x in range(8)] for y in range(8)]
z=np.asarray(z)

Out[1]:
array([[ 0,  1,  2,  3,  4,  5,  6,  7],
       [ 1,  3,  5,  7,  9, 11, 13, 15],
       [ 2,  5,  8, 11, 14, 17, 20, 23],
       [ 3,  7, 11, 15, 19, 23, 27, 31],
       [ 4,  9, 14, 19, 24, 29, 34, 39],
       [ 5, 11, 17, 23, 29, 35, 41, 47],
       [ 6, 13, 20, 27, 34, 41, 48, 55],
       [ 7, 15, 23, 31, 39, 47, 55, 63]])

z.shape
Out[2]: (8, 8)

From row 0 until row 3 (excluding it) every 2 rows, will index like:

z[0:3:2]

Out[3]: 
array([[ 0,  1,  2,  3,  4,  5,  6,  7],
       [ 2,  5,  8, 11, 14, 17, 20, 23]])

For columns:

z[:,1:6:3]

Out[4]: 
array([[ 1,  4],
       [ 3,  9],
       [ 5, 14],
       [ 7, 19],
       [ 9, 24],
       [11, 29],
       [13, 34],
       [15, 39]])

Combining rows and columns:

z[0:3:2, 0:3:2]

Out[5]: 
array([[0, 2],
       [2, 8]])