Apply a function to single or selected columns or rows in Pandas Dataframe

In this article, we will learn different ways to apply a function to single or selected columns or rows in Dataframe. We will use Dataframe/series.apply() method to apply a function.

Syntax: Dataframe/series.apply(func, convert_dtype=True, args=())

Parameters: This method will take following parameters :
func: It takes a function and applies it to all values of pandas series.
convert_dtype: Convert dtype as per the function’s operation.
args=(): Additional arguments to pass to function instead of series.

Return Type: Pandas Series after applied function/operation.

Method 1: Using Dataframe.apply() and lambda function.
Example 1: For Column

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

# List of Tuples
matrix = [(1, 2, 3),
          (4, 5, 6),
          (7, 8, 9)
         ]

# Create a DataFrame object
df = pd.DataFrame(matrix, columns = list('xyz'),
                  index = list('abc'))

# Apply function numpy.square() to lambda
# to find the squares of the values of
# column whose column name is 'z'
new_df = df.apply(lambda x: np.square(x) if x.name == 'z' else x)

# Output
new_df

Output :

Example 2: For Row.

# import pandas and numpy library
import pandas as pd
import numpy as np
# List of Tuples
matrix = [(1, 2, 3),
          (4, 5, 6),
          (7, 8, 9)
         ]

# Create a DataFrame object
df = pd.DataFrame(matrix, columns = list('xyz'),
                   index = list('abc'))

# Apply function numpy.square() to lambda
# to find the squares of the values of row
# whose row index is 'b'
new_df = df.apply(lambda x: np.square(x) if x.name == 'b' else x,
                axis = 1)

# Output
new_df

Output :

Method 2: Using Dataframe/series.apply() & [ ] Operator.

Example 1: For Column.

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

# List of Tuples
matrix = [(1, 2, 3),
          (4, 5, 6),
          (7, 8, 9)
         ]

# Create a DataFrame object
df = pd.DataFrame(matrix, columns = list('xyz'),
                   index = list('abc'))

# Apply a function to one column 'z'
# and assign it back to the same column
df['z'] = df['z'].apply(np.square)

# Output
df

Output :

Example 2: For Row.

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

# List of Tuples
matrix = [(1, 2, 3),
          (4, 5, 6),
          (7, 8, 9)
         ]

# Create a DataFrame object
df = pd.DataFrame(matrix, columns = list('xyz'),
                  index = list('abc'))

# Apply a function to one row 'b'
# and assign it back to the same row
df.loc['b'] = df.loc['b'].apply(np.square)

# Output
df

Output :

Method 3: Using numpy.square() method and [ ] operator.
Example 1: For Column

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

# List of Tuples
matrix = [(1, 2, 3),
          (4, 5, 6),
          (7, 8, 9)
         ]

# Create a DataFrame object
df = pd.DataFrame(matrix, columns = list('xyz'),
                  index = list('abc'))

# Apply a function to one column 'z' and
# assign it back to the same column
df['z'] = np.square(df['z'])

# Output
print(df)

Output :

Example 2: For Row.

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

# List of Tuples
matrix = [(1, 2, 3),
          (4, 5, 6),
          (7, 8, 9)
         ]

# Create a DataFrame object
df = pd.DataFrame(matrix, columns = list('xyz'), index = list('abc'))

# Apply a function to one row 'b' and
# assign it back to the same row
df.loc['b'] = np.square(df.loc['b'])

# Output
df

Output :

We can also apply a function to more than one column or row in the dataframe.

Example 1: For Column

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

# List of Tuples
matrix = [(1, 2, 3),
          (4, 5, 6),
          (7, 8, 9)
         ]

# Create a DataFrame object
df = pd.DataFrame(matrix, columns = list('xyz'),
                  index = list('abc'))

# Apply function numpy.square()
# for square the values of
# two columns 'x' and 'y'
new_df = df.apply(lambda x: np.square(x) if x.name in ['x', 'y'] else x)

# Output
new_df

Output :

Example 2: For Row.

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

# List of Tuples
matrix = [(1, 2, 3),
          (4, 5, 6),
          (7, 8, 9)
         ]

# Create a DataFrame object
df = pd.DataFrame(matrix, columns = list('xyz'),
                  index = list('abc'))

# Apply function numpy.square() to
# square the values of two rows
# 'b' and 'c'
new_df = df.apply(lambda x: np.square(x) if x.name in ['b', 'c'] else x,
                 axis = 1)

# Output
new_df

Output :