Modifying DataFrames with Pandas

Pandas apply() function

The Pandas apply() function can be used to apply a function on every value in a column or row of a DataFrame, and transform that column or row to the resulting values.

By default, it will apply a function to all values of a column. To perform it on a row instead, you can specify the argument axis=1 in the apply() function call.

# This function doubles the input value
def double(x):
  return 2*x

# Apply this function to double every value in a specified column
df.column1 = df.column1.apply(double)

# Lambda functions can also be supplied to `apply()`
df.column2 = df.column2.apply(lambda x : 3*x)

# Applying to a row requires it to be called on the entire DataFrame
df['newColumn'] = df.apply(lambda row: 
  row['column1'] * 1.5 + row['column2'],
  axis=1
)

Pandas DataFrames adding columns

Pandas DataFrames allow for the addition of columns after the DataFrame has already been created, by using the format df['newColumn'] and setting it equal to the new column’s value.

# Specifying each value in the new column:
df['newColumn'] = [1, 2, 3, 4]

# Setting each row in the new column to the same value:
df['newColumn'] = 1

# Creating a new column by doing a 
# calculation on an existing column:
df['newColumn'] = df['oldColumn'] * 5