Encoding Categorical Features

Encoding Categorical Features

Python code on how to transform nominal and ordinal variables to integers.

This Notebook includes:

• Ordinal Encoding with LabelEncoder, Panda’s Factorize, and Panda’s Map
• Nominal Encoding with One-Hot Encoding and Binary Encoding

Why??

• There is a great number of machine learning algorithms that can’t work with categorical data - they require numbers. Examples are SVMs, Logistic Regression, Linear Regreesion, Neural Networks.

• On the other hand tree-based algorithms like Random Forest, XGBoost, LightGBM, and Naive Bayes can work with categorical data, but their accuracy might improve with encoding.

Import Libraries

import numpy as np
import pandas as pd
import time
from sklearn.preprocessing import LabelEncoder
import category_encoders as ce

pd.set_option('display.max_columns', 500)
pd.set_option('display.max_rows', 500)
pd.set_option('max_colwidth', 100)

train = pd.read_csv('train.csv')
train.head()

	Id	MSSubClass	MSZoning	LotFrontage	LotArea	Street	Alley	LotShape	LandContour	Utilities	LotConfig	LandSlope	Neighborhood	Condition1	Condition2	BldgType	HouseStyle	OverallQual	OverallCond	YearBuilt	YearRemodAdd	RoofStyle	RoofMatl	Exterior1st	Exterior2nd	MasVnrType	MasVnrArea	ExterQual	ExterCond	Foundation	BsmtQual	BsmtCond	BsmtExposure	BsmtFinType1	BsmtFinSF1	BsmtFinType2	BsmtFinSF2	BsmtUnfSF	TotalBsmtSF	Heating	HeatingQC	CentralAir	Electrical	1stFlrSF	2ndFlrSF	LowQualFinSF	GrLivArea	BsmtFullBath	BsmtHalfBath	FullBath	HalfBath	BedroomAbvGr	KitchenAbvGr	KitchenQual	TotRmsAbvGrd	Functional	Fireplaces	FireplaceQu	GarageType	GarageYrBlt	GarageFinish	GarageCars	GarageArea	GarageQual	GarageCond	PavedDrive	WoodDeckSF	OpenPorchSF	EnclosedPorch	3SsnPorch	ScreenPorch	PoolArea	PoolQC	Fence	MiscFeature	MiscVal	MoSold	YrSold	SaleType	SaleCondition	SalePrice
0	1	60	RL	65.0	8450	Pave	NaN	Reg	Lvl	AllPub	Inside	Gtl	CollgCr	Norm	Norm	1Fam	2Story	7	5	2003	2003	Gable	CompShg	VinylSd	VinylSd	BrkFace	196.0	Gd	TA	PConc	Gd	TA	No	GLQ	706	Unf	0	150	856	GasA	Ex	Y	SBrkr	856	854	0	1710	1	0	2	1	3	1	Gd	8	Typ	0	NaN	Attchd	2003.0	RFn	2	548	TA	TA	Y	0	61	0	0	0	0	NaN	NaN	NaN	0	2	2008	WD	Normal	208500
1	2	20	RL	80.0	9600	Pave	NaN	Reg	Lvl	AllPub	FR2	Gtl	Veenker	Feedr	Norm	1Fam	1Story	6	8	1976	1976	Gable	CompShg	MetalSd	MetalSd	None	0.0	TA	TA	CBlock	Gd	TA	Gd	ALQ	978	Unf	0	284	1262	GasA	Ex	Y	SBrkr	1262	0	0	1262	0	1	2	0	3	1	TA	6	Typ	1	TA	Attchd	1976.0	RFn	2	460	TA	TA	Y	298	0	0	0	0	0	NaN	NaN	NaN	0	5	2007	WD	Normal	181500
2	3	60	RL	68.0	11250	Pave	NaN	IR1	Lvl	AllPub	Inside	Gtl	CollgCr	Norm	Norm	1Fam	2Story	7	5	2001	2002	Gable	CompShg	VinylSd	VinylSd	BrkFace	162.0	Gd	TA	PConc	Gd	TA	Mn	GLQ	486	Unf	0	434	920	GasA	Ex	Y	SBrkr	920	866	0	1786	1	0	2	1	3	1	Gd	6	Typ	1	TA	Attchd	2001.0	RFn	2	608	TA	TA	Y	0	42	0	0	0	0	NaN	NaN	NaN	0	9	2008	WD	Normal	223500
3	4	70	RL	60.0	9550	Pave	NaN	IR1	Lvl	AllPub	Corner	Gtl	Crawfor	Norm	Norm	1Fam	2Story	7	5	1915	1970	Gable	CompShg	Wd Sdng	Wd Shng	None	0.0	TA	TA	BrkTil	TA	Gd	No	ALQ	216	Unf	0	540	756	GasA	Gd	Y	SBrkr	961	756	0	1717	1	0	1	0	3	1	Gd	7	Typ	1	Gd	Detchd	1998.0	Unf	3	642	TA	TA	Y	0	35	272	0	0	0	NaN	NaN	NaN	0	2	2006	WD	Abnorml	140000
4	5	60	RL	84.0	14260	Pave	NaN	IR1	Lvl	AllPub	FR2	Gtl	NoRidge	Norm	Norm	1Fam	2Story	8	5	2000	2000	Gable	CompShg	VinylSd	VinylSd	BrkFace	350.0	Gd	TA	PConc	Gd	TA	Av	GLQ	655	Unf	0	490	1145	GasA	Ex	Y	SBrkr	1145	1053	0	2198	1	0	2	1	4	1	Gd	9	Typ	1	TA	Attchd	2000.0	RFn	3	836	TA	TA	Y	192	84	0	0	0	0	NaN	NaN	NaN	0	12	2008	WD	Normal	250000

train['ExterQual'].value_counts()

TA    906
Gd    488
Ex     52
Fa     14
Name: ExterQual, dtype: int64

Ordinal Encoding - LabelEncoder and Panda’s Factorize

Each unique categorical value becomes a unique number. With LabelEncoder and Panda’s Factorize, each unique instance gets a value between 0 and N-1, where N is the total number of unique values.

The result will have 1 dimension.

After encoding, the feature is considered continuous and that might be a problem. So after encoding nominal features with LabelEncoder, an one-hot encoding should be used.

The main difference between pandas and scikit-learn encoders is that scikit-learn encoders are made to be used in scikit-learn pipelines with fit and transform methods.

train1 = train.copy()

lbl = LabelEncoder()
lbl.fit((train1['ExterQual']))
train1['ExterQual'] = lbl.transform(train1['ExterQual'])

train1['ExterQual'].value_counts()

3    906
2    488
0     52
1     14
Name: ExterQual, dtype: int64

train2 = train.copy()

train2['ExterQual'], indexer = pd.factorize(train2['ExterQual'])

train2['ExterQual'].value_counts()

1    906
0    488
2     52
3     14
Name: ExterQual, dtype: int64

Ordinal Encoding - Pandas map

LabelEncoding converts categorical features to numbers depending on the order that it finds them in the column.

For ordinal columns that might be a problem. For example having a column with unique values ['TA','Fa','Gd','Ex'] like the above example, LabelEncoder encodes them to [3,1,2,0] which is not the right order (considering ‘Fa’ is Fair, ‘Gd’ is Good, and ‘Ex’ is Excellent).

So, pandas .map() can be used to give are own mapping to the column values. Having too many unique values, this becomes impractical though.

train3 = train.copy()

train3['ExterQual'] = train3['ExterQual'].map({'TA':0,'Fa':1,'Gd':2,'Ex':3})

train3['ExterQual'].value_counts()

0    906
2    488
3     52
1     14
Name: ExterQual, dtype: int64

Nominal Encoding - One-hot Encoding with Panda’s get_dummies

It converts a categorical column to N columns, where N is the number of unique values of this column. Each new column gets a 0, if the row does not contain the column value, and 1 if it does.

This encoding can create a huge number of features, if there are high cardinality features and can result in memoryproblems.

train4 = train.copy()

train4 = pd.concat([train4, pd.get_dummies(train4['ExterQual'])], axis=1)
#train4.drop(['ExterQual'], axis=1, inplace=True) # Uncomment this

train4[['ExterQual','Ex','Fa','Gd','TA']].head()

	ExterQual	Ex	Fa	Gd	TA
0	Gd	0	0	1	0
1	TA	0	0	0	1
2	Gd	0	0	1	0
3	TA	0	0	0	1
4	Gd	0	0	1	0

Nominal Encoding - Binary Encoding with category_encoders

This technique is not as intuitive as the previous ones. In this technique, first the categories are encoded as ordinal, then those integers are converted into binary code, then the digits from that binary string are split into separate columns. This encodes the data in fewer dimensions than one-hot.

train5 = train.copy()

encoder = ce.BinaryEncoder(cols=['ExterQual'])
train5 = encoder.fit_transform(train5)

train5.head()

	ExterQual_0	ExterQual_1	ExterQual_2	Id	MSSubClass	MSZoning	LotFrontage	LotArea	Street	Alley	LotShape	LandContour	Utilities	LotConfig	LandSlope	Neighborhood	Condition1	Condition2	BldgType	HouseStyle	OverallQual	OverallCond	YearBuilt	YearRemodAdd	RoofStyle	RoofMatl	Exterior1st	Exterior2nd	MasVnrType	MasVnrArea	ExterCond	Foundation	BsmtQual	BsmtCond	BsmtExposure	BsmtFinType1	BsmtFinSF1	BsmtFinType2	BsmtFinSF2	BsmtUnfSF	TotalBsmtSF	Heating	HeatingQC	CentralAir	Electrical	1stFlrSF	2ndFlrSF	LowQualFinSF	GrLivArea	BsmtFullBath	BsmtHalfBath	FullBath	HalfBath	BedroomAbvGr	KitchenAbvGr	KitchenQual	TotRmsAbvGrd	Functional	Fireplaces	FireplaceQu	GarageType	GarageYrBlt	GarageFinish	GarageCars	GarageArea	GarageQual	GarageCond	PavedDrive	WoodDeckSF	OpenPorchSF	EnclosedPorch	3SsnPorch	ScreenPorch	PoolArea	PoolQC	Fence	MiscFeature	MiscVal	MoSold	YrSold	SaleType	SaleCondition	SalePrice
0	0	0	1	1	60	RL	65.0	8450	Pave	NaN	Reg	Lvl	AllPub	Inside	Gtl	CollgCr	Norm	Norm	1Fam	2Story	7	5	2003	2003	Gable	CompShg	VinylSd	VinylSd	BrkFace	196.0	TA	PConc	Gd	TA	No	GLQ	706	Unf	0	150	856	GasA	Ex	Y	SBrkr	856	854	0	1710	1	0	2	1	3	1	Gd	8	Typ	0	NaN	Attchd	2003.0	RFn	2	548	TA	TA	Y	0	61	0	0	0	0	NaN	NaN	NaN	0	2	2008	WD	Normal	208500
1	0	1	0	2	20	RL	80.0	9600	Pave	NaN	Reg	Lvl	AllPub	FR2	Gtl	Veenker	Feedr	Norm	1Fam	1Story	6	8	1976	1976	Gable	CompShg	MetalSd	MetalSd	None	0.0	TA	CBlock	Gd	TA	Gd	ALQ	978	Unf	0	284	1262	GasA	Ex	Y	SBrkr	1262	0	0	1262	0	1	2	0	3	1	TA	6	Typ	1	TA	Attchd	1976.0	RFn	2	460	TA	TA	Y	298	0	0	0	0	0	NaN	NaN	NaN	0	5	2007	WD	Normal	181500
2	0	0	1	3	60	RL	68.0	11250	Pave	NaN	IR1	Lvl	AllPub	Inside	Gtl	CollgCr	Norm	Norm	1Fam	2Story	7	5	2001	2002	Gable	CompShg	VinylSd	VinylSd	BrkFace	162.0	TA	PConc	Gd	TA	Mn	GLQ	486	Unf	0	434	920	GasA	Ex	Y	SBrkr	920	866	0	1786	1	0	2	1	3	1	Gd	6	Typ	1	TA	Attchd	2001.0	RFn	2	608	TA	TA	Y	0	42	0	0	0	0	NaN	NaN	NaN	0	9	2008	WD	Normal	223500
3	0	1	0	4	70	RL	60.0	9550	Pave	NaN	IR1	Lvl	AllPub	Corner	Gtl	Crawfor	Norm	Norm	1Fam	2Story	7	5	1915	1970	Gable	CompShg	Wd Sdng	Wd Shng	None	0.0	TA	BrkTil	TA	Gd	No	ALQ	216	Unf	0	540	756	GasA	Gd	Y	SBrkr	961	756	0	1717	1	0	1	0	3	1	Gd	7	Typ	1	Gd	Detchd	1998.0	Unf	3	642	TA	TA	Y	0	35	272	0	0	0	NaN	NaN	NaN	0	2	2006	WD	Abnorml	140000
4	0	0	1	5	60	RL	84.0	14260	Pave	NaN	IR1	Lvl	AllPub	FR2	Gtl	NoRidge	Norm	Norm	1Fam	2Story	8	5	2000	2000	Gable	CompShg	VinylSd	VinylSd	BrkFace	350.0	TA	PConc	Gd	TA	Av	GLQ	655	Unf	0	490	1145	GasA	Ex	Y	SBrkr	1145	1053	0	2198	1	0	2	1	4	1	Gd	9	Typ	1	TA	Attchd	2000.0	RFn	3	836	TA	TA	Y	192	84	0	0	0	0	NaN	NaN	NaN	0	12	2008	WD	Normal	250000

We can see that 3 columns were created.