what does np.eye(n)[nparray] mean? [duplicate]

Question

I am going thru some code

y_enc = np.eye(21)[label]

where label is ndarray of shape (224, 224) y_enc is ndarray of shape (224, 224, 21)

Even with the shapes printed, I am having trouble understanding this statement. np.eye is supposed to generate a diagonal matrix of dimension 21 x 21. what does it mean to have [label] following it?

Answer 1

From Documentation. numpy.eye

Return a 2-D array with ones on the diagonal and zeros elsewhere.

Example:

>>np.eye(3)
array([[ 1.,  0.,  0.],
   [ 0.,  1.,  0.],
   [ 0.,  0.,  1.]])
>>> np.eye(3)[1]
array([ 0.,  1.,  0.])

[label] is array element indexing. So with only one element in it, it returns given number of rows element as array.

>>> np.eye(3)[1]
array([ 0.,  1.,  0.])
>>> np.eye(3)[2]
array([ 0.,  0.,  1.])

as it is 2d array you can also access the specific element by giving 2 index number on [row_index, column_index]

>>> np.eye(3)[2,1]
0.0
>>> np.eye(3)[2,2]
1.0
>>> np.eye(3)[1,1]
1.0