How to Extract Elements from a 2D Array Using Indices from Another Array?

Using NumPy Array as Indices for the 2nd Dimension of Another Array

To extract specific elements from a 2D array based on indices provided by a second array, you can leverage NumPy's integer array indexing.

Consider this example:

A = np.array([[0,1], [2,3], [4,5]])
B = np.array([[1], [0], [1]], dtype='int')

To obtain the following desired output:

C = np.array([[1], [2], [5]])

You can employ the following method:

A[np.arange(A.shape[0]),B.ravel()]

How it Works:

• np.arange(A.shape[0]) creates an array containing indices for each row of A.
• B.ravel() flattens B, removing any additional dimensions.
• The combined index array [np.arange(A.shape[0]),B.ravel()] specifies the row and column indices in A to extract.

Alternatively, if B is a 1D array or a list of column indices, you can skip flattening with .ravel():

A[np.arange(A.shape[0]),B]

This method provides a straightforward approach for extracting elements from a 2D array using indices derived from another array.

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