[PYTHON] Create a 2D array by adding a row to the end of an empty array with numpy

Array You can add an array to a python array using the .append method.

>>> arr = []
>>> arr.append([1, 2, 3])
>>> arr.append([4, 5])
>>> arr
[[1, 2, 3], [4, 5]]

It is a two-dimensional array with free row lengths.

numpy.ndarray If you try to add it in the same way with numpy, it will not be a 2D array.

>>> arr = np.array([])
>>> arr = np.append(arr, np.array([1, 2, 3]))
>>> arr = np.append(arr, np.array([4, 5]))
>>> arr
array([ 1.,  2.,  3.,  4.,  5.])

To add a row to the end of numpy as a two-dimensional array, you need to write:

>>> arr = np.empty((0,3), int)
>>> arr = np.append(arr, np.array([[1, 2, 3]]), axis=0)
>>> arr = np.append(arr, np.array([[4, 5, 0]]), axis=0)
>>> arr
array([[1, 2, 3],
       [4, 5, 0]])


--Initialize with np.empty --The number of elements in the array to be added matches the length of the initialized row (3 in the above example). --ndarray, which is the second argument of np.append, is a multiple array. --Do not forget ʻaxis = 0`

Reference to numpy.ndarray

>>> arr
array([[1, 2, 3],
       [4, 5, 0]])
>>> arr[0,1]
>>> arr[1,:2]
array([4, 5])
>>> arr[:, 1]
array([2, 5])

Slicing is easy and convenient.

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