Python - Numpy/Pandas/Matplot Machine Learning Libraries

Python Numpy/Pandas Libraries
Machine Learning
Portland Data Science Group
Created by Andrew Ferlitsch
Community Outreach Officer
July, 2017

Libraries - Numpy
• A popular math library in Python for Machine Learning
is ‘numpy’.
import numpy as np
Keyword to import a library Keyword to refer to library by an alias (shortcut) name
Numpy.org : NumPy is the fundamental package for scientific computing with Python.
• a powerful N-dimensional array object
• sophisticated (broadcasting) functions
• tools for integrating C/C++ and Fortran code
• useful linear algebra, Fourier transform, and random number capabilities

Libraries - Numpy
The most import data structure for scientific computing in Python
is the NumPy array. NumPy arrays are used to store lists of numerical
data and to represent vectors, matrices, and even tensors.
NumPy arrays are designed to handle large data sets efficiently and
with a minimum of fuss. The NumPy library has a large set of routines
for creating, manipulating, and transforming NumPy arrays.
Core Python has an array data structure, but it’s not nearly as versatile,
efficient, or useful as the NumPy array.
http://www.physics.nyu.edu/pine/pymanual/html/chap3/chap3_arrays.html

Numpy – Multidimensional Arrays
• Numpy’s main object is a multi-dimensional array.
• Creating a Numpy Array as a Vector:
data = np.array( [ 1, 2, 3 ] )
Numpy function to create a numpy array
Value is: array( [ 1, 2, 3 ] )
• Creating a Numpy Array as a Matrix:
data = np.array( [ [ 1, 2, 3 ], [ 4, 5, 6 ], [ 7, 8, 9 ] ] )
Outer Dimension Inner Dimension (rows)
Value is: array( [ 1, 2, 3 ],
[ 4, 5, 6 ],
[ 7, 8, 9 ] )

Numpy – Multidimensional Arrays
• Creating an array of Zeros:
data = np.zeros( ( 2, 3 ), dtype=np.int )
Numpy function to create an array of zeros
Value is: array( [ 0, 0, 0 ],
[ 0, 0, 0 ] )
• Creating an array of Ones:
data = np.ones( (2, 3), dtype=np.int )
rows
columns
data type (default is float)
Numpy function to create an array of onesValue is: array( [ 1, 1, 1 ],
[ 1, 1, 1 ] )
And many more functions: size, ndim, reshape, arange, …

Libraries - Pandas
• A popular library for importing and managing datasets in Python
for Machine Learning is ‘pandas’.
import pandas as pd
PyData.org : high-performance, easy-to-use data structures and data analysis tools for the
Python programming language.
Used for:
• Data Analysis
• Data Manipulation
• Data Visualization

Pandas – Indexed Arrays
• Pandas are used to build indexed arrays (1D) and matrices (2D),
where columns and rows are labeled (named) and can be accessed
via the labels (names).
1 2 3 4
4 5 6 7
8 9 10 11
1 2 3 4
4 5 6 7
8 9 10 11
one
two
three
x1 x2 x3 x4
raw data
Row (samples)
index
Columns (features)
index
Panda Indexed Matrix

Pandas – Series and Data Frames
• Pandas Indexed Arrays are referred to as Series (1D) and
Data Frames (2D).
• Series is a 1D labeled (indexed) array and can hold any data type,
and mix of data types.
s = pd.Series( data, index=[ ‘x1’, ‘x2’, ‘x3’, ‘x4’ ] )
Series Raw data Column Index Labels
• Data Frame is a 2D labeled (indexed) matrix and can hold any
data type, and mix of data types.
df = pd.DataFrame( data, index=[‘one’, ‘two’], columns=[ ‘x1’, ‘x2’, ‘x3’, ‘x4’ ] )
Data Frame Row Index Labels Column Index Labels

Pandas – Selecting
• Selecting One Column
x1 = df[ ‘x1’ ]
Selects column labeled x1 for all rows
1
4
8
• Selecting Multiple Columns
x1 = df[ [ ‘x1’, ‘x3’ ] ]
Selects columns labeled x1 and x3 for all rows
1 3
4 6
8 10
x1 = df.ix[ :, ‘x1’:’x3’ ]
Selects columns labeled x1 through x3 for all rows
1 2 3
4 5 6
8 9 10
Note: df[‘x1’:’x3’ ] this python syntax does not work!
rows (all) columns
Slicing function
And many more functions: merge, concat, stack, …

Libraries - Matplotlib
• A popular library for plotting and visualizing data in Python
import matplotlib.pyplot as plt
matplotlib.org: Matplotlib is a Python 2D plotting library which produces publication quality
figures in a variety of hardcopy formats and interactive environments across platforms.
Used for:
• Plots
• Histograms
• Bar Charts
• Scatter Plots
• etc

Matplotlib - Plot
• The function plot plots a 2D graph.
plt.plot( x, y )
Function to plot
X values to plot
Y values to plot
• Example:
plt.plot( [ 1, 2, 3 ], [ 4, 6, 8 ] ) # Draws plot in the background
plt.show() # Displays the plot
X Y
1
2
4
6
8
2 3

Matplotlib – Plot Labels
• Add Labels for X and Y Axis and Plot Title (caption)
plt.plot( [ 1, 2, 3 ], [ 4, 6, 8 ] )
plt.xlabel( “X Numbers” ) # Label on the X-axis
plt.ylabel( “Y Numbers” ) # Label on the Y-axis
plt.title( “My Plot of X and Y”) # Title for the Plot
plt.show()
1
2
4
6
8
2 3
X Numbers
YNumbers
My Plot of X and Y

Matplotlib – Multiple Plots and Legend
• You can add multiple plots in a Graph
plt.plot( [ 1, 2, 3 ], [ 4, 6, 8 ], label=‘ 1st Line’ ) # Plot for 1st Line
plt.plot( [ 1, 2, 3 ], [ 2, 4, 6 ], label=‘2nd Line’ ) # Plot for 2nd Line
plt.xlabel( “X Numbers” )
plt.ylabel( “Y Numbers” )
plt.title( “My Plot of X and Y”)
plt.legend() # Show Legend for the plots
plt.show()
1
2
4
6
8
2 3
X Numbers
YNumbers
My Plot of X and Y
---- 1st Line
---- 2nd Line

Matplotlib – Bar Chart
• The function bar plots a bar graph.
plt.plot( [ 1, 2, 3 ], [ 4, 6, 8 ] ) # Plot for 1st Line
plt.bar() # Draw a bar chart
plt.show()
1
2
4
6
8
2 3
And many more functions: hist, scatter, …

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