The OrdinalCategoricalEncoder() replaces the categories by digits, from 1 to the number of different labels. If we select “arbitrary”, then the encoder will assign numbers as the labels appear in the variable (first come first served). If we select “ordered”, the encoder will assign numbers following the mean of the target value for that label. So labels for which the mean of the target is higher will get the number 1, and those where the mean of the target is smallest will get the number n.

The OrdinalCategoricalEncoder() works only with categorical variables. A list of variables can be indiacated, or the imputer will automatically select all categorical variables in the train set.

import numpy as np
import pandas as pd
import matplotlib.pyplot as plt
from sklearn.model_selection import train_test_split

from feature_engine import categorical_encoders as ce

# Load dataset
def load_titanic():
        data = pd.read_csv('https://www.openml.org/data/get_csv/16826755/phpMYEkMl')
        data = data.replace('?', np.nan)
        data['cabin'] = data['cabin'].astype(str).str[0]
        data['pclass'] = data['pclass'].astype('O')
        data['embarked'].fillna('C', inplace=True)
        return data

data = load_titanic()

# Separate into train and test sets
X_train, X_test, y_train, y_test = train_test_split(
                data.drop(['survived', 'name', 'ticket'], axis=1),
                data['survived'], test_size=0.3, random_state=0)

# set up the encoder
encoder = ce.OrdinalCategoricalEncoder(encoding_method='ordered',
                                                 variables=['pclass', 'cabin', 'embarked'])

# fit the encoder
encoder.fit(X_train, y_train)

# transform the data
train_t= encoder.transform(X_train)
test_t= encoder.transform(X_test)

{'pclass': {3: 0, 2: 1, 1: 2},
 'cabin': {'T': 0,
  'n': 1,
  'G': 2,
  'A': 3,
  'C': 4,
  'F': 5,
  'D': 6,
  'E': 7,
  'B': 8},
 'embarked': {'S': 0, 'Q': 1, 'C': 2}}

API Reference

class feature_engine.categorical_encoders.OrdinalCategoricalEncoder(encoding_method='ordered', variables=None)[source]

The OrdinalCategoricalEncoder() replaces categories by ordinal numbers (0, 1, 2, 3, etc). The numbers can be ordered based on the mean of the target per category, or assigned arbitrarily.

For the ordered ordinal encoding for example in the variable colour, if the mean of the target for blue, red and grey is 0.5, 0.8 and 0.1 respectively, blue is replaced by 1, red by 2 and grey by 0.

For the arbitrary ordinal encoding the numbers will be assigned arbitrarily to the categories, on a first seen first served basis.

The Encoder will encode only categorical variables (type ‘object’). A list of variables can be passed as an argument. If no variables are passed as argument, the encoder will only encode categorical variables (object type) and ignore the rest.

The encoder first maps the categories to the numbers for each variable (fit). The encoder then transforms the categories to the mapped numbers (transform).

  • encoding_method (str, default='ordered') – Desired method of encoding. ‘ordered’: the categories are numbered in ascending order according to the target mean per category. ‘arbitrary’ : categories are numbered arbitrarily.
  • variables (list, default=None) – The list of categorical variables that will be encoded. If None, the encoder will find and select all object type variables.

The dictionary containing the {ordinal number: category} pairs used to replace categories for every variable.

fit(self, X, y=None)[source]

Learns the numbers that should be used to replace the labels in each variable.

  • X (pandas dataframe of shape = [n_samples, n_features]) – The training input samples. Can be the entire dataframe, not just seleted variables.
  • y (Target. Can be None if selecting encoding_method = 'arbitrary'.) –
  • needs to be passed when fitting the transformer. (Otherwise,) –