Introduction to Python NumPy Arrays
Introduction to Python NumPy Arrays#
Learn the basics of Python Numpy Arrays
McKinney, Wes. Python for Data Analysis: Data Wrangling with Pandas, NumPy, and IPython. O’Reilly Media. Kindle Edition
What is NumPy?#
NumPy is short for “Numerical Python” and it is a fundamental python package for scientific computing.
It uses a high-performance data structure known as the n-dimensional array or ndarray, a multi-dimensional array object, for efficient computation of arrays and matrices.
What is an Array?#
Python arrays are data structures that store data similar to a list, except the type of objects stored in them is constrained.
Elements of an array are all of the same type and indexed by a tuple of positive integers.
The python module array allows you to specify the type of array at object creation time by using a type code, which is a single character. You can read more about each type code here: https://docs.python.org/3/library/array.html?highlight=array#module-array
array_one = array.array('i',[1,2,3,4]) type(array_one)
What is a NumPy N-Dimensional Array (ndarray)?#
It is an efficient multidimensional array providing fast array-oriented arithmetic operations.
An ndarray as any other array, it is a container for homogeneous data (Elements of the same type)
In NumPy, data in an ndarray is simply referred to as an array.
As with other container objects in Python, the contents of an ndarray can be accessed and modified by indexing or slicing operations.
For numerical data, NumPy arrays are more efficient for storing and manipulating data than the other built-in Python data structures.
import numpy as np np.__version__
list_one = [1,2,3,4,5]
numpy_array = np.array(list_one) type(numpy_array)
Advantages of NumPy Arrays#
The key difference between an array and a list is, arrays are designed to handle vectorized operations while a python list is not.
NumPy operations perform complex computations on entire arrays without the need for Python for loops.
In other words, if you apply a function to an array, it is performed on every item in the array, rather than on the whole array object.
In a python list, you will have to perform a loop over the elements of the list.
list_two = [1,2,3,4,5] # The following will throw an error: list_two + 2
Performing a loop to add 2 to every integer in the list
for index, item in enumerate(list_two): list_two[index] = item + 2 list_two
With a NumPy array, you can do the same simply by doing the following:
numpy_array + 2
Any arithmetic operations between equal-size arrays applies the operation element-wise:
numpy_array_one = np.array([1,2]) numpy_array_two = np.array([4,6])
numpy_array_one + numpy_array_two
numpy_array_one > numpy_array_two
NumPy internally stores data in a contiguous block of memory, independent of other built-in Python objects.
NumPy arrays takes significantly less amount of memory as compared to python lists.
import numpy as np import sys
python_list = [1,2,3,4,5,6] python_list_size = sys.getsizeof(1) * len(python_list) python_list_size
python_numpy_array = np.array([1,2,3,4,5,6]) python_numpy_array_size = python_numpy_array.itemsize * python_numpy_array.size python_numpy_array_size
Basic Indexing and Slicing#
One Dimensional Array#
When it comes down to slicing and indexing, one-dimensional arrays are the same as python lists
You can slice the array and pass it to a variable. Remember that variables just reference objects.
Any change that you make to the array slice, it will be technnically done on the original array object. Once again, variables just reference objects.
numpy_array_slice = numpy_array[1:4] numpy_array_slice
numpy_array_slice = 10 numpy_array_slice
In a two-dimensional array, elements of the array are one-dimensional arrays
numpy_two_dimensional_array = np.array([[1,2,3],[4,5,6],[7,8,9]])
Instead of looping to the one-dimensional arrays to access specific elements, you can just pass a second index value
Slicing two-dimensional arrays is a little different than one-dimensional ones.