Chapter 1 :

Informatics Practices

Class XII ( As per CBSE Board)

Advance operations on dataframes (pivoting, sorting & aggregation/Descri- ptive statistics)

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New Syllabus 2019-20

Pivoting - dataframe

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DataFrame -It is a 2-dimensional data structure with columns of different types. It is just similar to a spreadsheet or SQL table, or a dict of Series objects. It is generally the most commonly used pandas object.

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Pivot –Pivot reshapes data and uses unique values from index/ columns to form axes of the resulting dataframe. Index is column name to use to make new frame’s index.Columns is column name to use to make new frame’s columns.Values is column name to use for populating new frame’s values.

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Pivot table - Pivot table is used to summarize and aggregate data inside dataframe.

Pivoting - dataframe

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Example of pivot:

PIVOT

DATAFRAME

Pivoting - dataframe

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There are two functions available in python for pivoting dataframe.

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1. Pivot()
2. pivot_table()

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1. pivot() - This function is used to create a new derived table(pivot) from existing dataframe. It takes 3 arguments : index, columns, and values. As a value for each of these parameters we need to specify a column name in the original table(dataframe). Then the pivot function will create a new table(pivot), whose row and column indices are the unique values of the respective parameters. The cell values of the new table are taken from column given as the values parameter.

Pivoting - dataframe

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#pivot() e.g. program

from collections import OrderedDict from pandas import DataFrame import pandas as pd

import numpy as np

​

table = OrderedDict((

("ITEM", ['TV', 'TV', 'AC', 'AC']),

('COMPANY',['LG', 'VIDEOCON', 'LG', 'SONY']),

('RUPEES', ['12000', '10000', '15000', '14000']),

('USD', ['700', '650', '800', '750'])

))

d = DataFrame(table) print("DATA OF DATAFRAME")

print(d)

p = d.pivot(index='ITEM', columns='COMPANY', values='RUPEES')

​

print("\n\nDATA OF PIVOT") print(p)

print (p[p.index=='TV'].LG.values)

#pivot() creates a new table/DataFrame whose columns are the unique values in COMPANY and whose rows are indexed with the unique values of ITEM.Last statement of above program retrun value of TV item LG company i.e. 12000

Pivoting - dataframe

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#Pivoting By Multiple Columns

Now in previous example, we want to pivot the values of both RUPEES an USD together, we will have to use pivot function in below manner.

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p = d.pivot(index='ITEM', columns='COMPANY')

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This will return the following pivot.

Pivoting - dataframe

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#Common Mistake in Pivoting

pivot method takes at least 2 column names as parameters - the index and the columns named parameters. Now the problem is that,What happens if we have multiple rows with the same values for these columns? What will be the value of the corresponding cell in the pivoted table using pivot method? The following diagram depicts the problem:

d.pivot(index='ITEM', columns='COMPANY', values='RUPEES') It throws an exception with the following message: ValueError: Index contains duplicate entries, cannot reshape

Pivoting - dataframe

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#Pivot Table

The pivot_table() method comes to solve this problem. It works like pivot, but it aggregates the values from rows with duplicate entries for the specified columns.

d.pivot_table(index='ITEM', columns='COMPANY', values='RUPEES‘,aggfunc=np.mean)

In essence pivot_table is a generalisation of pivot, which allows you to aggregate multiple values with the same destination in the pivoted table.

Sorting - dataframe

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Sorting means arranging the contents in ascending or descending order.There are two kinds of sorting available in pandas(Dataframe).

1. By value(column)
2. By index

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1. By value - Sorting over dataframe column/s elements is supported by sort_values() method. We will cover here three aspects of sorting values of dataframe.

• Sort a pandas dataframe in python by Ascending and Descending
• Sort a python pandas dataframe by single column
• Sort a pandas dataframe by multiple columns.

Sorting - dataframe

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Sort the python pandas Dataframe by single column – Ascending order

import pandas as pd import numpy as np

​

#Create a Dictionary of series

d = {'Name':pd.Series(['Sachin','Dhoni','Virat','Rohit','Shikhar']),

'Age':pd.Series([26,27,25,24,31]),

'Score':pd.Series([87,89,67,55,47])}

#Create a DataFrame df = pd.DataFrame(d)

print("Dataframe contents without sorting")

print (df)

df=df.sort_values(by='Score') print("Dataframe contents after sorting") print (df)

#In above example dictionary object is used to create the dataframe.Elements of dataframe object df is s orted by sort_value() method.As argument we are passing value score for by parameter only.by default it is sorting in ascending manner.

OUTPUT

Dataframe contents without sorting Name Age Score

1. Sachin 26 87
2. Dhoni 27 89
3. Virat 25 67
4. Rohit 24 55
5. Shikhar 31 47

Dataframe contents after sorting Name Age Score

3 Rohit 24

2 Virat 25

1 Dhoni 27

0 Sachin 26

4 Shikhar 31 47

55

67

87

89

Sorting - dataframe

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Sort the python pandas Dataframe by single column – Descending order

import pandas as pd import numpy as np

​

#Create a Dictionary of series

d = {'Name':pd.Series(['Sachin','Dhoni','Virat','Rohit','Shikhar']),

'Age':pd.Series([26,27,25,24,31]),

'Score':pd.Series([87,89,67,55,47])}

#Create a DataFrame df = pd.DataFrame(d)

print("Dataframe contents without sorting") print (df) df=df.sort_values(by='Score',ascending=0) print("Dataframe contents after sorting") print (df)

#In above example dictionary object is used to create the dataframe.Elements of dataframe object df is s orted by sort_value() method.we are passing 0 for Ascending parameter ,which sort the data in desce- nding order of score.

OUTPUT

Dataframe contents without sorting Name Age Score

1. Sachin 26 89
2. Dhoni 27 87
3. Virat 25 67
4. Rohit 24 55
5. Shikhar 31 47

Dataframe contents after sorting Name Age Score

4 Shikhar 31

1 Dhoni 27 89

0 Sachin 26 87

2. Virat 25 67
3. Rohit 24 55

47

Sorting - dataframe

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Sort the pandas Dataframe by Multiple Columns

import pandas as pd import numpy as np

​

#Create a Dictionary of series

d = {'Name':pd.Series(['Sachin','Dhoni','Virat','Rohit','Shikhar']), 'Age':pd.Series([26,25,25,24,31]),

'Score':pd.Series([87,67,89,55,47])}

#Create a DataFrame df = pd.DataFrame(d)

print("Dataframe contents without sorting") print (df)

df=df.sort_values(by=['Age', 'Score'],ascending=[True,False])

print("Dataframe contents after sorting") print (df)

#In above example dictionary object is used to create the dataframe.Elements of dataframe object df is s

orted by sort_value() method.we are passing two columns as by parameter value and in ascending parameter also with two parameters first true and second false,which means sort in ascending order of age and descending order of score

OUTPUT

Dataframe contents without sorting Name Age Score

1. Sachin 26 87
2. Dhoni 25 67
3. Virat 25 89
4. Rohit 24 55
5. Shikhar 31 47

Dataframe contents after sorting Name Age Score

3 Rohit 24 55

2 Virat 25 89

1 Dhoni 25 67

Sorting - dataframe

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2. By index - Sorting over dataframe index sort_index() is supported by sort_values() method. We will cover here three aspects of sorting values of dataframe. We will cover here two aspects of sorting index of dataframe.

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• how to sort a pandas dataframe in python by index in Ascending order
• how to sort a pandas dataframe in python by index in Descending order

Sorting - dataframe

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sort the dataframe in python pandas by index in ascending order:

import pandas as pd import numpy as np

​

#Create a Dictionary of series

d = {'Name':pd.Series(['Sachin','Dhoni','Virat','Rohit','Shikhar']),

'Age':pd.Series([26,25,25,24,31]),

'Score':pd.Series([87,67,89,55,47])}

#Create a DataFrame df = pd.DataFrame(d)

df=df.reindex([1,4,3,2,0])

print("Dataframe contents without sorting")

print (df) df1=df.sort_index()

print("Dataframe contents after sorting") print (df1)

#In above example dictionary object is used to create the dataframe.Elements of dataframe object df is first reindexed by reindex() method,index 1 is positioned at 0,4 at 1 and so on.then sorting by sort_index() method. By default it is sorting in ascending order of index.

OUTPUT

Dataframe contents without sorting Name Age Score

3 Rohit 24

2 Virat 25

0 Sachin 26

1 Dhoni 25 67

4 Shikhar 31 47

55

89

87

Dataframe contents after sorting Name Age Score

0

1

2

3

4 Shikhar 31

Sachin 26 87

Dhoni 25 67

Virat 25 89

Rohit 24 55

47

index

Sorting - dataframe

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Sorting pandas dataframe by index in descending order:

import pandas as pd import numpy as np

​

#Create a Dictionary of series

d = {'Name':pd.Series(['Sachin','Dhoni','Virat','Rohit','Shikhar']),

'Age':pd.Series([26,25,25,24,31]),

'Score':pd.Series([87,67,89,55,47])}

#Create a DataFrame df = pd.DataFrame(d)

df=df.reindex([1,4,3,2,0])

print("Dataframe contents without sorting")

print (df) df1=df.sort_index(ascending=0) print("Dataframe contents after sorting") print (df1)

#In above example dictionary object is used to create the dataframe.Elements of dataframe object df is first reindexed by reindex() method,index 1 is positioned at 0,4 at 1 and so on.then sorting by sort_index() method.

OUTPUT

Dataframe contents without sorting Name Age Score

3 Rohit 24

2 Virat 25

0 Sachin 26

1 Dhoni 25 67

4 Shikhar 31 47

55

89

87

Dataframe contents after sorting Name Age Score

4 Shikhar 31 47

Passing ascending=0 as argument for descending order.

index

Aggregation/Descriptive statistics - dataframe

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Data aggregation –

Aggregation is the process of turning the values of a dataset (or a subset of it) into one single value or data aggregation is a multivalued function ,which require multiple values and return a single value as a result.There are number of aggregations possible like count,sum,min,max,median,quartile etc. These(count,sum etc.) are descriptive statistics and other related operations on DataFrame Let us make this clear! If we have a DataFrame like…

…then a simple aggregation method is to calculate the summary of the Score, which is 87+67+89+55+47= 345. Or a different aggregation method would be to count the number of Name, which is 5.

Aggregation/Descriptive statistics - dataframe

#Create a Dictionary of series

d = {'Name':pd.Series(['Sachin','Dhoni','Virat','Rohit','Shikhar']),

'Age':pd.Series([26,25,25,24,31]),

'Score':pd.Series([87,67,89,55,47])} #Create a DataFrame

df = pd.DataFrame(d) print("Dataframe contents") print (df)

print(df.count())

print("count age",df[['Age']].count())

print("sum of score",df[['Score']].sum())

print("minimum age",df[['Age']].min())

print("maximum score",df[['Score']].max())

print("mean age",df[['Age']].mean())

print("mode of age",df[['Age']].mode())

print("median of score",df[['Score']].median())

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#e.g. program for data aggregation/descriptive statistics

import pandas as pd import numpy as np

OUTPUT

Dataframe contents Name Age Score

0

1

2

3

4 Shikhar 31

Sachin 26 87

Dhoni 25 67

Virat 25 89

Rohit 24 55

47

Name Age Score

5

5

5

dtype: int64 count age Age 5 dtype: int64

sum of score Score 345 dtype: int64

minimum age Age 24 dtype: int64

maximum score Score 89 dtype: int64

26.2

mean age Age dtype: float64 mode of age Age 0 25

67.0

median of score Score dtype: float64

Aggregation/Descriptive statistics - dataframe

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Quantile -

Quantile statistics is a part of a data set. It is used to describe data in a clear and understandable way.

The 0,30 quantile is basically saying that 30 % of the observations in our data set is below a given line. On the other hand ,it is also stating that there are 70 % remaining above the line we set.

Common Quantiles

Certain types of quantiles are used commonly enough to have specific names. Below is a list of these:

• The 2 quantile is called the median
• The 3 quantiles are called terciles
• The 4 quantiles are called quartiles
• The 5 quantiles are called quintiles
• The 6 quantiles are called sextiles
• The 7 quantiles are called septiles
• The 8 quantiles are called octiles
• The 10 quantiles are called deciles
• The 12 quantiles are called duodeciles
• The 20 quantiles are called vigintiles
• The 100 quantiles are called percentiles
• The 1000 quantiles are called permilles

Aggregation/Descriptive statistics - dataframe

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#e.g. program on Quantile –

import pandas as pd import numpy as np

df = pd.DataFrame(np.array([[1, 1], [2, 10], [3, 100], [4, 1000]]),columns=['a', 'b']) print(df)

print(df.quantile(0.5))

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OUTPUT

a b

0 1 1

1 2 10

2 3 100

3 4 1000

a 2.5

b 55.0

Name: 0.5, dtype: float64

Aggregation/Descriptive statistics - dataframe

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var() – Variance Function in python pandas is used to calculate variance of a given set of numbers, Variance of a data frame, Variance of column and Variance of rows, let’s see an example of each.

#e.g.program

import pandas as pd import numpy as np

#Create a Dictionary of series

d = {'Name':pd.Series(['Sachin','Dhoni','Virat','Rohit','Shikhar']), 'Age':pd.Series([26,25,25,24,31]),

'Score':pd.Series([87,67,89,55,47])} #Create a DataFrame

df = pd.DataFrame(d) print("Dataframe contents") print (df)

print(df.var())

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#df.loc[:,“Age"].var() for variance of specific column

#df.var(axis=0) column variance #df.var(axis=1) row variance