Append to Empty Array in Numpy

  1. Append to Numpy Empty Array With the numpy.append() Function
  2. Append to Numpy Empty Array With the List Method in Python

This tutorial will introduce the methods to append new rows to an empty NumPy array in Python.

Append to Numpy Empty Array With the numpy.append() Function

If we have an empty array and want to append new rows to it inside a loop, we can use the numpy.empty() function. Since no data type is assigned to a variable before initialization in Python, we have to specify the data type and structure of the array elements while creating the empty array. This can be done inside the numpy.empty() function. We can then append new rows to the empty array with the numpy.append() function. See the following code example.

import numpy as np

array = np.empty((0,3), int)

array = np.append(array, np.array([[1,3,5]]), axis=0)
array = np.append(array, np.array([[2,4,6]]), axis=0)

print(array)

Output:

[[1 3 5]
 [2 4 6]]

We first created an empty array and defined its structure and data type with the np.empty() function. We then appended two rows along the 0 axis of the array with the np.append() function.

Append to Numpy Empty Array With the List Method in Python

We can also achieve the same goal by using the list data structure in Python. We can create empty lists and append rows to them in Python. The list.append() function appends new elements to a list in Python. We can then convert this list to a NumPy array with the numpy.array() function. See the following code example.

import numpy as np

list = []

list.append([1,3,5])
list.append([2,4,6])

array2 = np.array(list)

print(array2)

Output:

[[1 3 5]
 [2 4 6]]

We first created an empty list list and appended new rows to the list with the list.append() function. In the end, we converted the list to the NumPy array array2 with the np.array(list) function in Python.

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