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Arrays with different numbers of rows can be stacked on top of each other using the numpy.vstack() function. This function takes a tuple of arrays as input and stacks them vertically. The arrays being stacked must have the same number of columns. If the arrays have different numbers of columns, they can be horizontally stacked using the numpy.hstack() function.

Here is an example of stacking arrays with different numbers of rows using numpy.vstack():

import numpy as np

arr1 = np.array([[1, 2, 3],
                [4, 5, 6]])

arr2 = np.array([[7, 8, 9]])

arr3 = np.array([[10, 11, 12],
                [13, 14, 15],
                [16, 17, 18]])

result = np.vstack((arr1, arr2, arr3))

print(result)

The output of this code will be:

[[ 1  2  3]
 [ 4  5  6]
 [ 7  8  9]
 [10 11 12]
 [13 14 15]
 [16 17 18]]

In this example, arr1 has two rows, arr2 has one row, and arr3 has three rows. By using np.vstack(), these arrays are stacked vertically to create a single array with six rows.