Pandas: How to Use factorize() to Encode Strings as Numbers


The pandas factorize() function can be used to encode strings as numeric values.

You can use the following methods to apply the factorize() function to columns in a pandas DataFrame:

Method 1: Factorize One Column

df['col1'] = pd.factorize(df['col'])[0]

Method 2: Factorize Specific Columns

df[['col1', 'col3']] = df[['col1', 'col3']].apply(lambda x: pd.factorize(x)[0])

Method 3: Factorize All Columns

df = df.apply(lambda x: pd.factorize(x)[0])

The following example shows how to use each method with the following pandas DataFrame:

import pandas as pd

#create DataFrame
df = pd.DataFrame({'conf': ['West', 'West', 'East', 'East'],
                   'team': ['A', 'B', 'C', 'D'],
                   'position': ['Guard', 'Forward', 'Guard', 'Center'] })

#view DataFrame
df

   conf team position
0  West    A    Guard
1  West    B  Forward
2  East    C    Guard
3  East    D   Center

Example 1: Factorize One Column

The following code shows how to factorize one column in the DataFrame:

#factorize the conf column only
df['conf'] = pd.factorize(df['conf'])[0]

#view updated DataFrame
df

	conf	team	position
0	0	A	Guard
1	0	B	Forward
2	1	C	Guard
3	1	D	Center

Notice that only the ‘conf’ column has been factorized.

Every value that used to be ‘West’ is now 0 and every value that used to be ‘East’ is now 1.

Example 2: Factorize Specific Columns

The following code shows how to factorize specific columns in the DataFrame:

#factorize conf and team columns only
df[['conf', 'team']] = df[['conf', 'team']].apply(lambda x: pd.factorize(x)[0])

#view updated DataFrame
df

        conf	team	position
0	0	0	Guard
1	0	1	Forward
2	1	2	Guard
3	1	3	Center

Notice that the ‘conf’ and ‘team’ columns have both been factorized.

Example 3: Factorize All Columns

The following code shows how to factorize all columns in the DataFrame:

#factorize all columns
df = df.apply(lambda x: pd.factorize(x)[0])

#view updated DataFrame
df

     conf	team	position
0	0	0	0
1	0	1	1
2	1	2	0
3	1	3	2

Notice that all of the columns have been factorized.

Additional Resources

The following tutorials explain how to perform other common operations in pandas:

How to Convert Pandas DataFrame Columns to Strings
How to Convert Categorical Variable to Numeric in Pandas
How to Convert Pandas DataFrame Columns to Integer

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