Reindexing in Pandas DataFrame

Reindexing in Pandas can be used to change the index of rows and columns of a DataFrame. Indexes can be used with reference to many index DataStructure associated with several pandas series or pandas DataFrame. Let’s see how can we Reindex the columns and rows in Pandas DataFrame. 

Reindexing the Rows

One can reindex a single row or multiple rows by using reindex() method. Default values in the new index that are not present in the dataframe are assigned NaN.

Example #1:

Python3

# import numpy and pandas module
import pandas as pd
import numpy as np

column=['a','b','c','d','e']
index=['A','B','C','D','E']

# create a dataframe of random values of array
df1 = pd.DataFrame(np.random.rand(5,5),
            columns=column, index=index)

print(df1)

print('\n\nDataframe after reindexing rows: \n',
df1.reindex(['B', 'D', 'A', 'C', 'E']))

Output:

Example #2: 

Python3

# import numpy and pandas module
import pandas as pd
import numpy as np

column = ['a', 'b', 'c', 'd', 'e']
index = ['A', 'B', 'C', 'D', 'E']

# create a dataframe of random values of array
df1 = pd.DataFrame(np.random.rand(5, 5),
        columns = column, index = index)

# create the new index for rows
new_index =['U', 'A', 'B', 'C', 'Z']

print(df1.reindex(new_index))

Output:

 Reindexing the columns using axis keyword

One can reindex a single column or multiple columns by using reindex() method and by specifying the axis we want to reindex. Default values in the new index that are not present in the dataframe are assigned NaN.

Example #1: 

Python3

# import numpy and pandas module
import pandas as pd
import numpy as np

column=['a','b','c','d','e']
index=['A','B','C','D','E']

#create a dataframe of random values of array
df1 = pd.DataFrame(np.random.rand(5,5),
           columns=column, index=index)

column=['e','a','b','c','d']

# create the new index for columns
print(df1.reindex(column, axis='columns'))

Output:

Example #2: 

Python3

# import numpy and pandas module
import pandas as pd
import numpy as np

column =['a', 'b', 'c', 'd', 'e']
index =['A', 'B', 'C', 'D', 'E']

# create a dataframe of random values of array
df1 = pd.DataFrame(np.random.rand(5, 5),
        columns = column, index = index)

column =['a', 'b', 'c', 'g', 'h']

# create the new index for columns
print(df1.reindex(column, axis ='columns'))

Output:

Replacing the missing values

Code #1: Missing values from the dataframe can be filled by passing a value to the keyword fill_value. This keyword replaces the NaN values. 

Python3

# import numpy and pandas module
import pandas as pd
import numpy as np

column =['a', 'b', 'c', 'd', 'e']
index =['A', 'B', 'C', 'D', 'E']

# create a dataframe of random values of array
df1 = pd.DataFrame(np.random.rand(5, 5),
        columns = column, index = index)

column =['a', 'b', 'c', 'g', 'h']

# create the new index for columns
print(df1.reindex(column, axis ='columns', fill_value = 1.5))

Output:

Code #2: Replacing the missing data with a string. 

Python3

# import numpy and pandas module
import pandas as pd
import numpy as np

column =['a', 'b', 'c', 'd', 'e']
index =['A', 'B', 'C', 'D', 'E']

# create a dataframe of random values of array
df1 = pd.DataFrame(np.random.rand(5, 5),
       columns = column, index = index)

column =['a', 'b', 'c', 'g', 'h']

# create the new index for columns
print(df1.reindex(column, axis ='columns', fill_value ='data missing'))

Output: