import numpy as np
oneDim = np.array([1.0,2,3,4,5]) # a 1-dimensional array (vector)
print(oneDim)
print("#Dimensions =", oneDim.ndim)
print("Dimension =", oneDim.shape)
print("Size =", oneDim.size)
print("Array type =", oneDim.dtype, '\n')
twoDim = np.array([[1,2],[3,4],[5,6],[7,8]]) # a two-dimensional array (matrix)
print(twoDim)
print("#Dimensions =", twoDim.ndim)
print("Dimension =", twoDim.shape)
print("Size =", twoDim.size)
print("Array type =", twoDim.dtype, '\n')
arrFromTuple = np.array([(1,'a',3.0),(2,'b',3.5)]) # create ndarray from tuple
print(arrFromTuple)
print("#Dimensions =", arrFromTuple.ndim)
print("Dimension =", arrFromTuple.shape)
print("Size =", arrFromTuple.size)Introduction to Numpy and Pandas
The following tutorial contains examples of using the numpy and pandas library modules. The notebook can be downloaded from http://www.cse.msu.edu/~ptan/dmbook/tutorials/tutorial2/tutorial2.ipynb. Read the step-by-step instructions below carefully. To execute the code, click on the cell and press the SHIFT-ENTER keys simultaneously.
2.1 Introduction to Numpy
Numpy, which stands for numerical Python, is a Python library package to support numerical computations. The basic data structure in numpy is a multi-dimensional array object called ndarray. Numpy provides a suite of functions that can efficiently manipulate elements of the ndarray.
2.1.1 Creating ndarray
An ndarray can be created from a list or a tuple object as shown in the examples below. It is possible to create a 1-dimensional or multi-dimensional array from the list objects as well as tuples.
There are also built-in functions available in numpy to create the ndarrays.
print('Array of random numbers from a uniform distribution')
print(np.random.rand(5)) # random numbers from a uniform distribution between [0,1]
print('\nArray of random numbers from a normal distribution')
print(np.random.randn(5)) # random numbers from a normal distribution
print('\nArray of integers between -10 and 10, with step size of 2')
print(np.arange(-10,10,2)) # similar to range, but returns ndarray instead of list
print('\n2-dimensional array of integers from 0 to 11')
print(np.arange(12).reshape(3,4)) # reshape to a matrix
print('\nArray of values between 0 and 1, split into 10 equally spaced values')
print(np.linspace(0,1,10)) # split interval [0,1] into 10 equally separated values
print('\nArray of values from 10^-3 to 10^3')
print(np.logspace(-3,3,7)) # create ndarray with values from 10^-3 to 10^3print('A 2 x 3 matrix of zeros')
print(np.zeros((2,3))) # a matrix of zeros
print('\nA 3 x 2 matrix of ones')
print(np.ones((3,2))) # a matrix of ones
print('\nA 3 x 3 identity matrix')
print(np.eye(3)) # a 3 x 3 identity matrix2.1.2 Element-wise Operations
You can apply standard operators such as addition and multiplication on each element of the ndarray.
x = np.array([1,2,3,4,5])
print('x =', x)
print('x + 1 =', x + 1) # addition
print('x - 1 =', x - 1) # subtraction
print('x * 2 =', x * 2) # multiplication
print('x // 2 =', x // 2) # integer division
print('x ** 2 =', x ** 2) # square
print('x % 2 =', x % 2) # modulo
print('1 / x =', 1 / x) # divisionx = np.array([2,4,6,8,10])
y = np.array([1,2,3,4,5])
print('x =', x)
print('y =', y)
print('x + y =', x + y) # element-wise addition
print('x - y =', x - y) # element-wise subtraction
print('x * y =', x * y) # element-wise multiplication
print('x / y =', x / y) # element-wise division
print('x // y =', x // y) # element-wise integer division
print('x ** y =', x ** y) # element-wise exponentiation2.1.3 Indexing and Slicing
There are various ways to select a subset of elements within a numpy array. Assigning a numpy array (or a subset of its elements) to another variable will simply pass a reference to the array instead of copying its values. To make a copy of an ndarray, you need to explicitly call the .copy() function.
x = np.arange(-5,5)
print('Before: x =', x)
y = x[3:5] # y is a slice, i.e., pointer to a subarray in x
print(' y =', y)
y[:] = 1000 # modifying the value of y will change x
print('After : y =', y)
print(' x =', x, '\n')
z = x[3:5].copy() # makes a copy of the subarray
print('Before: x =', x)
print(' z =', z)
z[:] = 500 # modifying the value of z will not affect x
print('After : z =', z)
print(' x =', x)There are many ways to access elements of an ndarray. The following example illustrates the difference between indexing elements of a list and elements of ndarray.
my2dlist = [[1,2,3,4],[5,6,7,8],[9,10,11,12]] # a 2-dim list
print('my2dlist =', my2dlist)
print('my2dlist[2] =', my2dlist[2]) # access the third sublist
print('my2dlist[:][2] =', my2dlist[:][2]) # can't access third element of each sublist
# print('my2dlist[:,2] =', my2dlist[:,2]) # invalid way to access sublist, will cause syntax error
my2darr = np.array(my2dlist)
print('\nmy2darr =\n', my2darr)
print('my2darr[2][:] =', my2darr[2][:]) # access the third row
print('my2darr[2,:] =', my2darr[2,:]) # access the third row
print('my2darr[:][2] =', my2darr[:][2]) # access the third row (similar to 2d list)
print('my2darr[:,2] =', my2darr[:,2]) # access the third column
print('my2darr[:2,2:] =\n', my2darr[:2,2:]) # access the first two rows & last two columnsNumpy arrays also support boolean indexing.
my2darr = np.arange(1,13,1).reshape(3,4)
print('my2darr =\n', my2darr)
divBy3 = my2darr[my2darr % 3 == 0]
print('\nmy2darr[my2darr % 3 == 0] =', divBy3) # returns all the elements divisible by 3 in an ndarray
divBy3LastRow = my2darr[2:, my2darr[2,:] % 3 == 0]
print('my2darr[2:, my2darr[2,:] % 3 == 0] =', divBy3LastRow) # returns elements in the last row divisible by 3More indexing examples.
my2darr = np.arange(1,13,1).reshape(4,3)
print('my2darr =\n', my2darr)
indices = [2,1,0,3] # selected row indices
print('indices =', indices, '\n')
print('my2darr[indices,:] =\n', my2darr[indices,:]) # this will shuffle the rows of my2darr
rowIndex = [0,0,1,2,3] # row index into my2darr
print('\nrowIndex =', rowIndex)
columnIndex = [0,2,0,1,2] # column index into my2darr
print('columnIndex =', columnIndex, '\n')
print('my2darr[rowIndex,columnIndex] =', my2darr[rowIndex,columnIndex])2.1.4 Numpy Arithmetic and Statistical Functions
Numpy provides many built-in mathematical functions available for manipulating elements of an ndarray.
y = np.array([-1.4, 0.4, -3.2, 2.5, 3.4])
print('y =', y, '\n')
print('np.abs(y) =', np.abs(y)) # convert to absolute values
print('np.sqrt(abs(y)) =', np.sqrt(abs(y))) # apply square root to each element
print('np.sign(y) =', np.sign(y)) # get the sign of each element
print('np.exp(y) =', np.exp(y)) # apply exponentiation
print('np.sort(y) =', np.sort(y)) # sort arrayx = np.arange(-2,3)
y = np.random.randn(5)
print('x =', x)
print('y =', y, '\n')
print('np.add(x,y) =', np.add(x,y)) # element-wise addition x + y
print('np.subtract(x,y) =', np.subtract(x,y)) # element-wise subtraction x - y
print('np.multiply(x,y) =', np.multiply(x,y)) # element-wise multiplication x * y
print('np.divide(x,y) =', np.divide(x,y)) # element-wise division x / y
print('np.maximum(x,y) =', np.maximum(x,y)) # element-wise maximum max(x,y)y = np.array([-3.2, -1.4, 0.4, 2.5, 3.4])
print('y =', y, '\n')
print("Min =", np.min(y)) # min
print("Max =", np.max(y)) # max
print("Average =", np.mean(y)) # mean/average
print("Std deviation =", np.std(y)) # standard deviation
print("Sum =", np.sum(y)) # sum 2.1.5 Numpy linear algebra
Numpy provides many functions to support linear algebra operations.
X = np.random.randn(2,3) # create a 2 x 3 random matrix
print('X =\n', X, '\n')
print('Transpose of X, X.T =\n', X.T, '\n') # matrix transpose operation X^T
y = np.random.randn(3) # random vector
print('y =', y, '\n')
print('Matrix-vector multiplication')
print('X.dot(y) =\n', X.dot(y), '\n') # matrix-vector multiplication X * y
print('Matrix-matrix product')
print('X.dot(X.T) =', X.dot(X.T)) # matrix-matrix multiplication X * X^T
print('\nX.T.dot(X) =\n', X.T.dot(X)) # matrix-matrix multiplication X^T * XX = np.random.randn(5,3)
print('X =\n', X, '\n')
C = X.T.dot(X) # C = X^T * X is a square matrix
print('C = X.T.dot(X) =\n', C, '\n')
invC = np.linalg.inv(C) # inverse of a square matrix
print('Inverse of C = np.linalg.inv(C)\n', invC, '\n')
detC = np.linalg.det(C) # determinant of a square matrix
print('Determinant of C = np.linalg.det(C) =', detC)
S, U = np.linalg.eig(C) # eigenvalue S and eigenvector U of a square matrix
print('Eigenvalues of C =\n', S)
print('Eigenvectors of C =\n', U)2.2 Introduction to Pandas
Pandas provide two convenient data structures for storing and manipulating data–Series and DataFrame. A Series is similar to a one-dimensional array whereas a DataFrame is a tabular representation akin to a spreadsheet table.
2.2.1 Series
A Series object consists of a one-dimensional array of values, whose elements can be referenced using an index array. A Series object can be created from a list, a numpy array, or a Python dictionary. You can apply most of the numpy functions on the Series object.
from pandas import Series
s = Series([3.1, 2.4, -1.7, 0.2, -2.9, 4.5]) # creating a series from a list
print('Series, s =\n', s, '\n')
print('s.values =', s.values) # display values of the Series
print('s.index =', s.index) # display indices of the Series
print('s.dtype =', s.dtype) # display the element type of the Seriesimport numpy as np
s2 = Series(np.random.randn(6)) # creating a series from a numpy ndarray
print('Series s2 =\n', s2, '\n')
print('s2.values =', s2.values) # display values of the Series
print('s2.index =', s2.index) # display indices of the Series
print('s2.dtype =', s2.dtype) # display the element type of the Seriess3 = Series([1.2,2.5,-2.2,3.1,-0.8,-3.2],
index = ['Jan 1','Jan 2','Jan 3','Jan 4','Jan 5','Jan 6',])
print('Series s3 =\n', s3, '\n')
print('s3.values =', s3.values) # display values of the Series
print('s3.index =', s3.index) # display indices of the Series
print('s3.dtype =', s3.dtype) # display the element type of the Seriescapitals = {'MI': 'Lansing', 'CA': 'Sacramento', 'TX': 'Austin', 'MN': 'St Paul'}
s4 = Series(capitals) # creating a series from dictionary object
print('Series s4 =\n', s4, '\n')
print('s4.values =', s4.values) # display values of the Series
print('s4.index=', s4.index) # display indices of the Series
print('s4.dtype =', s4.dtype) # display the element type of the Seriess3 = Series([1.2,2.5,-2.2,3.1,-0.8,-3.2],
index = ['Jan 1','Jan 2','Jan 3','Jan 4','Jan 5','Jan 6',])
print('s3 =\n', s3, '\n')
# Accessing elements of a Series
print('s3[2]=', s3[2]) # display third element of the Series
print('s3[\'Jan 3\']=', s3['Jan 3']) # indexing element of a Series
print('\ns3[1:3]=') # display a slice of the Series
print(s3[1:3])
print('\ns3.iloc([1:3])=') # display a slice of the Series
print(s3.iloc[1:3])There are various functions available to find the number of elements in a Series. Result of the function depends on whether null elements are included.
s3['Jan 7'] = np.nan
print('Series s3 =\n', s3, '\n')
print('Shape of s3 =', s3.shape) # get the dimension of the Series
print('Size of s3 =', s3.size) # get the number of elements of the Series
print('Count of s3 =', s3.count()) # get the number of non-null elements of the SeriesA boolean filter can be used to select elements of a Series
print(s3[s3 > 0]) # applying filter to select non-negative elements of the SeriesScalar operations can be performed on elements of a numeric Series
print('s3 + 4 =\n', s3 + 4, '\n')
print('s3 / 4 =\n', s3 / 4) Numpy functions can be applied to pandas Series.
print('np.log(s3 + 4) =\n', np.log(s3 + 4), '\n') # applying log function to a numeric Series
print('np.exp(s3 - 4) =\n', np.exp(s3 - 4), '\n') # applying exponent function to a numeric SeriesThe value_counts() function can be used for tabulating the counts of each discrete value in the Series.
colors = Series(['red', 'blue', 'blue', 'yellow', 'red', 'green', 'blue', np.nan])
print('colors =\n', colors, '\n')
print('colors.value_counts() =\n', colors.value_counts())2.2.2 DataFrame
A DataFrame object is a tabular, spreadsheet-like data structure containing a collection of columns, each of which can be of different types (numeric, string, boolean, etc). Unlike Series, a DataFrame has distinct row and column indices. There are many ways to create a DataFrame object (e.g., from a dictionary, list of tuples, or even numpy’s ndarrays).
from pandas import DataFrame
cars = {'make': ['Ford', 'Honda', 'Toyota', 'Tesla'],
'model': ['Taurus', 'Accord', 'Camry', 'Model S'],
'MSRP': [27595, 23570, 23495, 68000]}
carData = DataFrame(cars) # creating DataFrame from dictionary
carData # display the tableprint('carData.index =', carData.index) # print the row indices
print('carData.columns =', carData.columns) # print the column indicesInserting columns to an existing dataframe
carData2 = DataFrame(cars, index = [1,2,3,4]) # change the row index
carData2['year'] = 2018 # add column with same value
carData2['dealership'] = ['Courtesy Ford','Capital Honda','Spartan Toyota','N/A']
carData2 # display tableCreating DataFrame from a list of tuples.
tuplelist = [(2011,45.1,32.4),(2012,42.4,34.5),(2013,47.2,39.2),
(2014,44.2,31.4),(2015,39.9,29.8),(2016,41.5,36.7)]
columnNames = ['year','temp','precip']
weatherData = DataFrame(tuplelist, columns=columnNames)
weatherDataCreating DataFrame from numpy ndarray
import numpy as np
npdata = np.random.randn(5,3) # create a 5 by 3 random matrix
columnNames = ['x1','x2','x3']
data = DataFrame(npdata, columns=columnNames)
dataThere are many ways to access elements of a DataFrame object.
# accessing an entire column will return a Series object
print(data['x2'])
print(type(data['x2']))# accessing an entire row will return a Series object
print('Row 3 of data table:')
print(data.iloc[2]) # returns the 3rd row of DataFrame
print(type(data.iloc[2]))
print('\nRow 3 of car data table:')
print(carData2.iloc[2]) # row contains objects of different types# accessing a specific element of the DataFrame
print('carData2 =\n', carData2)
print('\ncarData2.iloc[1,2] =', carData2.iloc[1,2]) # retrieving second row, third column
print('carData2.loc[1,\'model\'] =', carData2.loc[1,'model']) # retrieving second row, column named 'model'
# accessing a slice of the DataFrame
print('\ncarData2.iloc[1:3,1:3]=')
print(carData2.iloc[1:3,1:3])print('carData2 =\n', carData2, '\n')
print('carData2.shape =', carData2.shape)
print('carData2.size =', carData2.size)# selection and filtering
print('carData2 =\n', carData2, '\n')
print('carData2[carData2.MSRP > 25000] =')
print(carData2[carData2.MSRP > 25000])2.2.3 Arithmetic Operations
print(data)
print('\nData transpose operation: data.T')
print(data.T) # transpose operation
print('\nAddition: data + 4')
print(data + 4) # addition operation
print('\nMultiplication: data * 10')
print(data * 10) # multiplication operationprint('data =\n', data)
columnNames = ['x1','x2','x3']
data2 = DataFrame(np.random.randn(5,3), columns=columnNames)
print('\ndata2 =')
print(data2)
print('\ndata + data2 = ')
print(data.add(data2))
print('\ndata * data2 = ')
print(data.mul(data2))print(data.abs()) # get the absolute value for each element
print('\nMaximum value per column:')
print(data.max()) # get maximum value for each column
print('\nMinimum value per row:')
print(data.min(axis=1)) # get minimum value for each row
print('\nSum of values per column:')
print(data.sum()) # get sum of values for each column
print('\nAverage value per row:')
print(data.mean(axis=1)) # get average value for each row
print('\nCalculate max - min per column')
f = lambda x: x.max() - x.min()
print(data.apply(f))
print('\nCalculate max - min per row')
f = lambda x: x.max() - x.min()
print(data.apply(f, axis=1))The value_counts() function can also be applied to a pandas DataFrame
objects = {'shape': ['circle', 'square', 'square', 'square', 'circle', 'rectangle'],
'color': ['red', 'red', 'red', 'blue', 'blue', 'blue']}
shapeData = DataFrame(objects)
print('shapeData =\n', shapeData, '\n')
print('shapeData.value_counts() =\n', shapeData.value_counts().sort_values())2.2.4 Plotting Series and DataFrame
There are many built-in functions available to plot the data stored in a Series or a DataFrame.
(a) Line plot
%matplotlib inline
s3 = Series([1.2,2.5,-2.2,3.1,-0.8,-3.2,1.4],
index = ['Jan 1','Jan 2','Jan 3','Jan 4','Jan 5','Jan 6','Jan 7'])
s3.plot(kind='line', title='Line plot')(b) Bar plot
s3.plot(kind='bar', title='Bar plot')(c) Histogram
s3.plot(kind='hist', title = 'Histogram')(d) Box plot
tuplelist = [(2011,45.1,32.4),(2012,42.4,34.5),(2013,47.2,39.2),
(2014,44.2,31.4),(2015,39.9,29.8),(2016,41.5,36.7)]
columnNames = ['year','temp','precip']
weatherData = DataFrame(tuplelist, columns=columnNames)
weatherData[['temp','precip']].plot(kind='box', title='Box plot')(e) Scatter plot
print('weatherData =\n', weatherData)
weatherData.plot(kind='scatter', x='temp', y='precip')