Introduction to numpy Session 1

Objective of the class
• What is numpy?
• numpy performance test
• Introduction to numpy arrays
• Introduction to numpy function
• Dealing with Flat files using numpy
• Mathematical functions
• Statisticals function
• Operations with arrays
Introduction to Numpy

NumPy
NumPy is an extension to the Python programming language, adding support for
large, multi-dimensional arrays and matrices, along with a large library of high-level
mathematical functions to operate on these arrays
To install NumPy run:
python setup.py install
To perform an in-place build that can be run from the source folder run:
python setup.py build_ext --inplace
The NumPy build system uses distutils and numpy.distutils. setuptools is only
used when building via pip or with python setupegg.py.

Official Website: http://www.numpy.org
• NumPy is licensed under the BSD license, enabling reuse with few restrictions.
• NumPy replaces Numeric and Numarray
• Numpy was initially developed by Travis Oliphant
• There are 225+ Contributors to the project (github.com)
• NumPy 1.0 released October, 2006
• Numpy 1.14.0 is the lastest version of numpy
• There are more than 200K downloads/month from PyPI
NumPy

NumPy performance Test

Getting Started with Numpy
>>> # Importing Numpy module
>>> import numpy
>>> import numpy as np
IPython has a ‘pylab’ mode where it imports all of NumPy, Matplotlib,
and SciPy into the namespace for you as a convenience. It also enables
threading for showing plots

• Arrays are the central feature of NumPy.
• Arrays in Python are similar to lists in Python, the only difference being the array
elements should be of the same type
import numpy as np
a = np.array([1,2,3], float) # Accept two arguments list and type
print a # Return array([ 1., 2., 3.])
print type(a) # Return <type 'numpy.ndarray'>
print a[2] # 3.0
print a.dtype # Print the element type
print a.itemsize # print bytes per element
print a.shape # print the shape of an array
print a.size # print the size of an array

a.nbytes # return the total bytes used by an array
a.ndim # provide the dimension of an array
a[0] = 10.5 # Modify array first index important decimal will
come if the array is float type else it will be hold only 10
a.fill(20) # Fill all the values by 20
a[1:3] # Slice the array
a[-2:] # Last two elements of an array

Airthmatic Operation with numpy

import numpy as np
a = np.array([[1,2,3], [4,5,6]], int) # Accept two arguments list and type
>>> a.shape # Return (2, 3)
>>> a.ndim # return 2
>>> a[0][0] # Return 1
>>> a[0,0] # Return 1
>>> a[1,2] # Return 6
>>> a[1:] # Return array([[4, 5, 6]])
>>> a[1,:] # Return array([4, 5, 6])
>>> a[:2] # Return array([[1, 2, 3],[4, 5, 6]])
>>> a[:,2] # Return array([3, 6])
>>> a[:,1] # Return array([2, 5])
>>> a[:,0] # Return array([1, 4])
Multi-Dimensional Arrays

import numpy as np
a = np.array([[1,2,3], [4,5,6]], int) # Accept two arguments list and type
>>> b = a.reshape(3,2) # Return the new shape of array
>>> b.shape # Return (3,2)
>>> len(a) # Return length of a
>>> 2 in a # Check if 2 is available in a
>>> b.tolist() # Return [[1, 2], [3, 4], [5, 6]]
>>> list(b) # Return [array([1, 2]), array([3, 4]), array([5, 6])]
>>> c = b
>>> b[0][0] = 10
>>> c # Return array([[10, 2], [ 3, 4], [ 5, 6]])
Reshaping array

Slices Are References
>>> a = array((0,1,2,3,4))
# create a slice containing only the
# last element of a
>>> b = a[2:4]
>>> b
array([2, 3])
>>> b[0] = 10
# changing b changed a!
>>> a
array([ 0, 1, 10, 3, 4])

arange function and slicing
>>> a = np.arange(1,80, 2)
array([ 1, 3, 5, 7, 9, 11, 13, 15, 17, 19, 21, 23, 25, 27, 29, 31, 33,
35, 37, 39, 41, 43, 45, 47, 49, 51, 53, 55, 57, 59, 61, 63, 65, 67,
69, 71, 73, 75, 77, 79])
>>> a[[1,5,10]] # Return values at index 1,5 and 10
>>> myIndexes = [4,5,6]
>>> a[myIndexes] # Return values at indexes 4,5,6..
>>> mask = a % 5 == 0 # Return boolean value at those indexes
>>> a[mask]
Out[46]: array([ 5, 15, 25, 35, 45, 55, 65, 75])

Where function
>>> where (a % 5 == 0)
# Returns - (array([ 2, 7, 12, 17, 22, 27, 32, 37], dtype=int64),)
>>> loc = where(a % 5 == 0)
>>> print a[loc]
[ 5 15 25 35 45 55 65 75]

Flatten Arrays
# Create a 2D array
>>> a = array([[0,1],
[2,3]])
# Flatten out elements to 1D
>>> b = a.flatten()
>>> b
array([0,1,2,3])
# Changing b does not change a
>>> b[0] = 10
>>> b
array([10,1,2,3])
>>> a
array([[0, 1],
[2, 3]])

>>> a = array([[0,1,2],
... [3,4,5]])
>>> a.shape
(2,3)
# Transpose swaps the order # of axes. For 2-D this # swaps rows and columns.
>>> a.transpose()
array([[0, 3], [1, 4],[2, 5]])
# The .T attribute is # equivalent to transpose().
>>> a.T
array([[0, 3],
[1, 4],
[2, 5]])
Transpose Arrays

csv = np.loadtxt(r'A:UPDATE PythonModule 11Programsconstituents-
financials.csv', skiprows=1, dtype=str, delimiter=",", usecols = (3,4,5))
Arrays from/to ASCII files

Other format supported by other similar package

Introduction to numpy Session 1

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