# How to Deal with Categorical Data for Machine Learning

**In this blog we will explore and implement:**

- One-hot Encoding using:
- Python’s category_encoding library
- Scikit-learn preprocessing
- Pandas’ get_dummies

- Binary Encoding
- Frequency Encoding
- Label Encoding
- Ordinal Encoding

Categorical data is a type of data that is used to group information with similar characteristics, while numerical data is a type of data that expresses information in the form of numbers.

Example of categorical data: **gender**

**Why do we need encoding?**

- Most machine learning algorithms cannot handle categorical variables unless we convert them to numerical values
- Many algorithm’s performances even vary based upon how the categorical variables are encoded

**Categorical variables can be divided into two categories:**

- Nominal: no particular order
- Ordinal: there is some order between values

We will also refer to a cheat sheet that shows when to use which type of encoding.

category_encoders is an amazing Python library that provides 15 different encoding schemes.

**Here is the list of the 15 types of encoding the library supports:**

- One-hot Encoding
- Label Encoding
- Ordinal Encoding
- Helmert Encoding
- Binary Encoding
- Frequency Encoding
- Mean Encoding
- Weight of Evidence Encoding
- Probability Ratio Encoding
- Hashing Encoding
- Backward Difference Encoding
- Leave One Out Encoding
- James-Stein Encoding
- M-estimator Encoding
- Thermometer Encoder

Importing libraries:

import pandas as pd import sklearn pip install category_encoders import category_encoders as ce

Creating a dataframe:

data = pd.DataFrame({ 'gender' : ['Male', 'Female', 'Male', 'Female', 'Female'], 'class' : ['A','B','C','D','A'], 'city' : ['Delhi','Gurugram','Delhi','Delhi','Gurugram'] }) data.head()

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**Implementing one-hot encoding through category_encoder**

In this method, each category is mapped to a vector that contains 1 and 0 denoting the presence or absence of the feature. The number of vectors depends on the number of categories for features.

Create an object of the one-hot encoder:

ce_OHE = ce.OneHotEncoder(cols=['gender','city']) data1 = ce_OHE.fit_transform(data) data1.head()

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**Binary Encoding**

Binary encoding converts a category into binary digits. Each binary digit creates one feature column.

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ce_be = ce.BinaryEncoder(cols=['class']); # transform the data data_binary = ce_be.fit_transform(data["class"]); data_binary

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Similarly, there are another 14 types of encoding provided by this library.

pd.get_dummies(data,columns=["gender","city"])

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We can assign a prefix if we want to, if we do not want the encoding to use the default.

pd.get_dummies(data,prefix=["gen","city"],columns=["gender","city"])

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Scikit-learn also has 15 different types of built-in encoders, which can be accessed from sklearn.preprocessing.

## Scikit-learn One-hot Encoding

Let’s first get the list of categorical variables from our data:

s = (data.dtypes == 'object') cols = list(s[s].index) from sklearn.preprocessing import OneHotEncoder ohe = OneHotEncoder(handle_unknown='ignore',sparse=False)

Applying on the gender column:

data_gender = pd.DataFrame(ohe.fit_transform(data[["gender"]])) data_gender

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Applying on the city column:

data_city = pd.DataFrame(ohe.fit_transform(data[["city"]])) data_city

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Applying on the class column:

data_class = pd.DataFrame(ohe.fit_transform(data[["class"]])) data_class

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This is because the class column has 4 unique values.

Applying to the list of categorical variables:

data_cols = pd.DataFrame(ohe.fit_transform(data[cols])) data_cols

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Here the first 2 columns represent gender, the next 4 columns represent class, and the remaining 2 represent city.

## Scikit-learn Label Encoding

In label encoding, each category is assigned a value from 1 through N where N is the number of categories for the feature. There is no relation or order between these assignments.

from sklearn.preprocessing import LabelEncoder le = LabelEncoder() Label encoder takes no arguments le_class = le.fit_transform(data[["class"]])

**Comparing with one-hot encoding**

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## Ordinal Encoding

Ordinal encoding’s encoded variables retain the ordinal (ordered) nature of the variable. It looks similar to label encoding, the only difference being that label coding doesn’t consider whether a variable is ordinal or not; it will then assign a sequence of integers.

**Example: Ordinal encoding will assign values as Very Good(1) < Good(2) < Bad(3) < Worse(4)**

First, we need to assign the original order of the variable through a dictionary.

temp = {'temperature' :['very cold', 'cold', 'warm', 'hot', 'very hot']} df=pd.DataFrame(temp,columns=["temperature"]) temp_dict = {'very cold': 1,'cold': 2,'warm': 3,'hot': 4,"very hot":5} df

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Then we can map each row for the variable as per the dictionary.

df["temp_ordinal"] = df.temperature.map(temp_dict) df

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## Frequency Encoding

The category is assigned as per the frequency of values in its total lot.

data_freq = pd.DataFrame({'class' : ['A','B','C','D','A',"B","E","E","D","C","C","C","E","A","A"]})

Grouping by class column:

fe = data_freq.groupby("class").size()

Dividing by length:

Mapping and rounding off:

data_freq["data_fe"] = data_freq["class"].map(fe_).round(2) data_freq

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In this article, we saw 5 types of encoding schemes. Similarly, there are 10 other types of encoding which we have not looked at:

- Helmert Encoding
- Mean Encoding
- Weight of Evidence Encoding
- Probability Ratio Encoding
- Hashing Encoding
- Backward Difference Encoding
- Leave One Out Encoding
- James-Stein Encoding
- M-estimator Encoding
- Thermometer Encoder

There is no single method that works best for every problem or dataset. I personally think that the get_dummies method has an advantage in its ability to be implemented very easily.

If you want to read about all 15 types of encoding, here is a very good article to refer to.

Here is a cheat sheet on when to use what type of encoding:

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## References:

- https://towardsdatascience.com/all-about-categorical-variable-encoding-305f3361fd02
- https://pypi.org/project/category-encoders/
- https://pandas.pydata.org/docs/reference/api/pandas.get_dummies.html

**Shelvi Garg** is a Data Scientist at Spinny