Data wrangling involves processing the data in various formats like – merging, grouping, concatenating etc. for the purpose of analysing or getting them ready to be used with another set of data. Python has built-in features to apply these wrangling methods to various data sets to achieve the analytical goal. In this chapter we will look at few examples describing these methods.
Merging Data
The Pandas library in python provides a single function, merge, as the entry point for all standard database join operations between DataFrame objects −
pd.merge(left, right, how='inner', on=None, left_on=None, right_on=None, left_index=False, right_index=False, sort=True)
Let us now create two different DataFrames and perform the merging operations on it.
# import the pandas library
import pandas as pd
left = pd.DataFrame({
'id':[1,2,3,4,5],
'Name': ['Alex', 'Amy', 'Allen', 'Alice', 'Ayoung'],
'subject_id':['sub1','sub2','sub4','sub6','sub5']})
right = pd.DataFrame(
{'id':[1,2,3,4,5],
'Name': ['Billy', 'Brian', 'Bran', 'Bryce', 'Betty'],
'subject_id':['sub2','sub4','sub3','sub6','sub5']})
print left
print right
Its output is as follows −
Name id subject_id
0 Alex 1 sub1
1 Amy 2 sub2
2 Allen 3 sub4
3 Alice 4 sub6
4 Ayoung 5 sub5
Name id subject_id
0 Billy 1 sub2
1 Brian 2 sub4
2 Bran 3 sub3
3 Bryce 4 sub6
4 Betty 5 sub5
Grouping Data
Grouping data sets is a frequent need in data analysis where we need the result in terms of various groups present in the data set. Panadas has in-built methods which can roll the data into various groups.
In the below example we group the data by year and then get the result for a specific year.
# import the pandas library
import pandas as pd
ipl_data = {'Team': ['Riders', 'Riders', 'Devils', 'Devils', 'Kings',
'kings', 'Kings', 'Kings', 'Riders', 'Royals', 'Royals', 'Riders'],
'Rank': [1, 2, 2, 3, 3,4 ,1 ,1,2 , 4,1,2],
'Year': [2014,2015,2014,2015,2014,2015,2016,2017,2016,2014,2015,2017],
'Points':[876,789,863,673,741,812,756,788,694,701,804,690]}
df = pd.DataFrame(ipl_data)
grouped = df.groupby('Year')
print grouped.get_group(2014)
Its output is as follows −
Points Rank Team Year 0 876 1 Riders 2014 2 863 2 Devils 2014 4 741 3 Kings 2014 9 701 4 Royals 2014
Concatenating Data
Pandas provides various facilities for easily combining together Series, DataFrame, and Panel objects. In the below example the concat function performs concatenation operations along an axis. Let us create different objects and do concatenation.
import pandas as pd
one = pd.DataFrame({
'Name': ['Alex', 'Amy', 'Allen', 'Alice', 'Ayoung'],
'subject_id':['sub1','sub2','sub4','sub6','sub5'],
'Marks_scored':[98,90,87,69,78]},
index=[1,2,3,4,5])
two = pd.DataFrame({
'Name': ['Billy', 'Brian', 'Bran', 'Bryce', 'Betty'],
'subject_id':['sub2','sub4','sub3','sub6','sub5'],
'Marks_scored':[89,80,79,97,88]},
index=[1,2,3,4,5])
print pd.concat([one,two])
Its output is as follows −
Marks_scored Name subject_id 1 98 Alex sub1 2 90 Amy sub2 3 87 Allen sub4 4 69 Alice sub6 5 78 Ayoung sub5 1 89 Billy sub2 2 80 Brian sub4 3 79 Bran sub3 4 97 Bryce sub6 5 88 Betty sub5
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