Python Numpy Tutorial



This Blog is enough to get you started for hand ons with numpy, because it covers
What is Numpy
Getting started is Numpy
Arrays in numpy
Operations with numpy
Statistics with Numpy
Estimate Mean ,Median and Percentile Using Numpy
Estimate Standard Deviation and Variance Using Numpy
Random Data Distribution with Numpy and Matplotlib

What Is Numpy?

Numpy or Numerical python is a free and open source library for working with n-dimensional arrays or ndarrays.It is often used with other libraries such as scipy,matplotlib,Pandas,scikit for scientific computations for various data science or machine learning applications.Numpy is partly written in Python and the rest with C C++.

Getting Started With Numpy

Numpy does not come pre installed with python.So you will need to install it with either pip or pipenv installed on your system, then the following command is enough to get started with numpy

Pip install numpy

Or if you are using any python distribution like Anaconda, Spyder etc., then they have numpy pre installed and you can just start from importing numpy in your environment.

Throughout the blog i will use an alias name np for numpy,so i will import it in this way,
from numpy import np

Arrays In Numpy

The number lists passed to the array function is the number or rows and the number of elements in the list is the number of columnsThe numpy.array() function takes an optional argument as dtype,which you can use to define the data type of the elements of the array,by default the dtype will be set to the data type of the elements guessed by the numpy.For example

To create an arbitrary array, use numpy.array()
n = np.array([[1, 2, 3], [1, 3, 4]]) print(n)
This is an array with 2×3 dimension or with 2 rows and 3 columns. Output
[[1 2 3] [1 3 4]]
To create an array filled with 0s use numpy.zeros()
print(np.zeros((2,2)))
Here the (2,2) specifies the dimension or 2 rows and 2 columns. Output
[[0. 0.] [0. 0.]]
To create an array filled with 1s use numpy.ones()
print(np.ones((2,2)))
Output
[[1. 1.] [1. 1.]]
To create nxn array filled with a given number use numpy.fill()
print(np.fill((2,2),4))
Output
[[4. 4.] [4. 4.]]
To create an identity matrix use numpy.eye()
print(np.eye((3)))
Output
[[1. 0. 0.] [0. 1. 0.] [0. 0. 1.]]

Python Lists Vs Numpy Arrays

In python we used lists for storing multiple data instead of arrays, because arrays can only include homogenous data or elements of the same data type.Then why use arrays instead of lists?

Array elements are stored at contiguous memory locations,and use less memory and space which make them convenient to use.

Consider the following example where we are adding numbers,(1 to 100000 with 1 to 100000) with lists and arrays and comparing their time difference (with time module) to do this math.

import time import numpy as np vector = 100000   def pure_python_ver(): t = time.time() x = range(vector) y = range(vector) z = [x[i] + y[i] for i in range(len(y))] return time.time() – t   def numpy_ver(): t = time.time() x = np.arange(vector) y = np.arange(vector) z = x + y return time.time() – t   t1 = pure_python_ver() t2 = numpy_ver()   print(‘time taken by python version:’,t1) print(‘time taken by numpy version: ‘, t2) print(“Numpy is” + str(t1/t2) + ” faster!”)
Output
time taken by python version: 0.19698405265808105 time taken by python version: 0.0011696815490722656 Numpy is in this example 168.40827558092133 faster!
Check outthis Online Python Course by Fita, which includes Supervised,Unsupervised machine learning algorithms,Data Analysis Manipulation and visualisation,reinforcement testing, hypothesis testing and much more to make an industry required data scientist at an affordable price, which includes certification, support with career guidance assistance.

Array Indexing

Accessing an array element from 1-D array
arr = np.array([1, 2,3, 5, 11])   print(arr[2],arr[3]) # outputs: 3 5
Accessing elements from 2-D array
arr = np.array([[1,2,3],[6,7,8]]) print(‘3rd element from row 1: ‘, arr[0,2]) # outputs 3
where the first index passed is row number and the second is the column number .
Accessing elements from 3-D array
arr = np.array([[[0, 1, 1], [3, 3, 4]], [[4, 4, 5], [3, 3, 5]]])   print(arr[0][1][2]) #outputs 6 print(arr[0, 1, 2]) # outputs 6
Consider this to understand why the above code outputs 6
print(arr)   print(arr[0])   print(arr[0][1])   print(arr[0][1][2])
Output
[[[0 1 1] [3 3 4]] [[4 4 5] [3 3 5]]]   [[0 1 1] [3 3 4]]   [3 3 4]   4

Slicing Elements from the array

For 1-D arrays
We’ll have the same slicing method as the list that is using indexing or [start:end:step].
arr = np.array([1, 3, 5, 7, 11, 13, 17])   print(arr[1:7:2]) #output: [ 3 7 13]
The above program returns every element from index 1 to index 7 by skipping 2 elements in between.
For 2-D arrays
We will have to specify the slicing for both the rows and column, separated by a comma respectively.
arr = np.array([[1, 1, 2, 3, 5], [1, 2, 3, 5, 7]])   print(arr[1, 2:5]) # outputs [3 5 7]
Here the program slices for row 1 and for the columns of the respective row 1 it gives values from index 2 to index 5. Another example for slicing 2-D arrays
arr = np.array([[1, 1, 2, 3, 5], [1, 2, 3, 5, 7]])   print(arr[0:2, 2:4])
Here we are taking rows from 0 to 1(2 excluded because of colon) and printing values from index 2 to index 3 (4 excluded because of colon). Output
[[2 3] [3 5]]

Other Numpy operations

The shape returns the size of each dimension.
arr = np.array([[1, 1, 2, 3, 5], [1, 2, 3, 5, 7], [1, 2, 3, 4, 5]]) print(arr.shape) # output: (3, 5)
The reshape will change the shape of the array, most often you will see a -1 passed as the second argument, this is to let the numpy decide the shape of the respective dimension of the position.
arr = np.array([1, 1, 2, 3, 5, 8, 13, 21])   newarr_1 = arr.reshape(-1, 2) # unknown value passed for row newarr_2 = arr.reshape(2, -1) # unknown value passed for column print(newarr_1) print() print(newarr_2)
Output
[[ 1 1] [ 2 3] [ 5 8] [13 21]]   [[ 1 1] [ 2 3] [ 5 8] [13 21]]
The ravel will convert an array with any dimensions into a 1-D array or array with one column,but with a view copy.
arr = np.array([[1, 2], [3, 4], [5, 6]]) new_arr = arr.ravel() print(new_arr)
The flatten will also convert an array with any dimensions into a 1-D array or array with one column,but with a copy.

Copy And View In Numpy

The difference between view and copy of an array is that the former one(view) will create a new array with the elements as of the parents elements,but changing any of the elements of either the parent’s or the view copy’s element will be reflected in both the arrays.Whereas the later one(copy) is like deep copy of lists, where changing elements of either the parents or the copied array will not change the other one.

Math operations on arrays

 Doing addition, multiplication division or finding the square root is just as you would do in python with 2 or more operators.I will implement all these operations in the program below.

Consider the following example for converting a list of temperatures in celsius to fahrenheit and converting an array of temperatures in Celsius to Fahrenheit.

Creating an array
cel_list = [20.2, 20.4, 22.9, 21.5, 23.7, 25.3, 21.8, 24.2, 20.9, 22.1] cel_arr = np.array(cel_list)   # or create with this # cel_arr = np.array([20.2,20.4,22.9, 21.5,23.7, 25.3,21.8,24.2,20.9, 22.1])   print(cel_arr)   # output: [20.2 20.4 22.9 21.5 23.7 25.3 21.8 24.2 20.9 22.1]
Converting celsius to fahrenheit
For lists this can be done by looping over each value with list comprehension and performing the operation.
f_list = [ x*(9/5) + 32 for x in cel_list] print(f_list)   # output # [68.36, 68.72, 73.22, 70.7, 74.66, 77.53999999999999, 71.24000000000001, 75.56, 69.62, 71.78]
For arrays this can be done with scalar multiplication as follows
feh_arr = cel_arr * (9 / 5) + 32 print(feh_arr)   # Output: [68.36 68.72 73.22 70.7 74.66 77.54 71.24 75.56 69.62 71.78]

Statistics with Numpy

Estimate mean with numpy
The mean of a data set is the average value or the sum of values divided by the number of values.We can calculate the mean of any data set with numpy.mean(). Now let us find the average value of the speed of the given 10 people.
speed = [91, 20, 81, 86, 120, 86, 91, 122, 91, 78]   x = np.mean(speed)   print(x) # output: 86.6
Estimate median with numpy
Median is the middle most value of the given values, after arranged in ascending or descending order,we can calculate the median of any data set with numpy.median(). Now let’s calculate the middle most value(where all the values surround) for those 10 people’s speed.
speed = [91, 20, 81, 86, 120, 86, 91, 122, 91, 78]   x = np.median(speed)   print(x) # output: 88.5
Estimate Percentile with numpy
A Percentile shows the percentage of the values that are less than the given value,we can calculate percentile with numpy.percentile(). Now let us calculate the speed where the percentage of speed is 50 or less than that.
speed = [91, 20, 81, 86, 120, 86, 91, 122, 91, 78]   x = np.percentile(speed, 34)   print(x) # output: 86.0
The mode of the data set can be calculated with the scipy library of python.
Estimate standard deviation with numpy
The standard deviation indicates how far the values are from the mean or the sum of the difference of the value from the mean, we can calculate the standard deviation with numpy.std() Now lets calculate the standard deviation of the speeds registered.
speed = [91, 20, 81, 86, 120, 86, 91, 122, 91, 78]   x = np.std(speed)   print(x) # output: 26.3977271748914
Estimate variance with numpy
Variance is the square root of the standard deviation.We can estimate the variance using numpy.var(). Now calculate how far the values are from the mean and every other value.
speed = [91, 20, 81, 86, 120, 86, 91, 122, 91, 78]   x = np.var(speed)   print(x) # output: 696.8399999999999

Random Data Distribution With Numpy and Matplotlib

Let us create 2 arrays with random values and specified mean and variance.
import numpy as np   set_1 = np.random.normal(10.0,1.0,100) set_2 = np.random.normal(17.0,2.5,100)
Here the first array will have 100 random values, with a mean of 10.0 and variance of 1.0. The second array has 100 random values, with a mean of 17.0 and variance of 1.5. Now we will use matplotlib library to plot this data as a scatter diagram.
import matplotlib.pyplot as plt %matplotlib inline #to prevent opening of new window for showing diagram   plt.scatter(set_1,set_2) plt.show()
We can observe that the values are concentrated more around the 10 (mean of x) on the x-axis and near 17 (mean of y) on the y-axis. Here is the output scatter plot diagram.

This was a quick start tutorial for Numpy and its implementations. To get in-depth knowledge of Python along with its various applications and real-time projects, you can enroll in Python Training in Chennai or Python Training in Bangalore by FITA or enroll for a Data science course in Chennai or Data science course in Bangalore which includes Supervised, Unsupervised machine learning algorithms, Data Analysis Manipulation and visualization, reinforcement testing, hypothesis testing and much more to make an industry required data scientist at an affordable price, which includes certification, support with career guidance assistance.






Quick Enquiry

Please wait while submission in progress...


Contact Us

Chennai

  93450 45466

Bangalore

 93450 45466

Coimbatore

 95978 88270

Online

93450 45466

Madurai

97900 94102

Pondicherry

93635 21112

For Hiring

 93840 47472
 hr@fita.in

Corporate Training

 90036 23340


Read More Read less

FITA Academy Branches

Chennai

Bangalore

Coimbatore

Other Locations

FITA Academy - Velachery
Plot No 7, 2nd floor,
Vadivelan Nagar,
Velachery Main Road,
Velachery, Chennai - 600042
Tamil Nadu

    :   93450 45466

FITA Academy - Anna Nagar
No 14, Block No, 338, 2nd Ave,
Anna Nagar,
Chennai 600 040, Tamil Nadu
Next to Santhosh Super Market

    :   93450 45466

FITA Academy - T Nagar
05, 5th Floor, Challa Mall,
T Nagar,
Chennai 600 017, Tamil Nadu
Opposite to Pondy Bazaar Globus

    :   93450 45466

FITA Academy - Tambaram
Nehru Nagar, Kadaperi,
GST Road, West Tambaram,
Chennai 600 045, Tamil Nadu
Opposite to Saravana Jewellers Near MEPZ

    :   93450 45466

FITA Academy - Thoraipakkam
5/350, Old Mahabalipuram Road,
Okkiyam Thoraipakkam,
Chennai 600 097, Tamil Nadu
Next to Cognizant Thoraipakkam Office and Opposite to Nilgris Supermarket

    :   93450 45466

FITA Academy - Porur
17, Trunk Rd,
Porur
Chennai 600116, Tamil Nadu
Above Maharashtra Bank

    :   93450 45466

FITA Academy Marathahalli
No 7, J J Complex,
ITPB Road, Aswath Nagar,
Marathahalli Post,
Bengaluru 560037

    :   93450 45466

FITA Academy - Saravanampatty
First Floor, Promenade Tower,
171/2A, Sathy Road, Saravanampatty,
Coimbatore - 641035
Tamil Nadu

    :   95978 88270

FITA Academy - Singanallur
348/1, Kamaraj Road,
Varadharajapuram, Singanallur,
Coimbatore - 641015
Tamil Nadu

    :   95978 88270

FITA Academy - Madurai
No.2A, Sivanandha salai,
Arapalayam Cross Road,
Ponnagaram Colony,
Madurai - 625016, Tamil Nadu

    :   97900 94102

FITA Academy - Pondicherry
410, Villianur Main Rd,
Sithananda Nagar, Nellitope,
Puducherry - 605005
Near IG Square

    :   93635 21112

Read More Read less
  • Are You Located in Any of these Areas

    Adyar, Adambakkam, Anna Salai, Ambattur, Ashok Nagar, Aminjikarai, Anna Nagar, Besant Nagar, Chromepet, Choolaimedu, Guindy, Egmore, K.K. Nagar, Kodambakkam, Koyambedu, Ekkattuthangal, Kilpauk, Meenambakkam, Medavakkam, Nandanam, Nungambakkam, Madipakkam, Teynampet, Nanganallur, Navalur, Mylapore, Pallavaram, Purasaiwakkam, OMR, Porur, Pallikaranai, Poonamallee, Perambur, Saidapet, Siruseri, St.Thomas Mount, Perungudi, T.Nagar, Sholinganallur, Triplicane, Thoraipakkam, Tambaram, Vadapalani, Valasaravakkam, Villivakkam, Thiruvanmiyur, West Mambalam, Velachery and Virugambakkam.

    FITA Velachery or T Nagar or Thoraipakkam OMR or Anna Nagar or Tambaram or Porur branch is just few kilometre away from your location. If you need the best training in Chennai, driving a couple of extra kilometres is worth it!