# Regression models in Python for Machine Learning and Data Sciense cheat sheets.

Python programming language and its libraries combined together form a powerful tool for solving Regression analysis tasks.

Regression study is a predictive modelling method that analyzes the relation between the target or dependent variable and features or independent variables in a dataset. The different types of regression analysis methods are used when the target and independent features described by a linear or non-linear relationships between each other, and the target variable contains continuous values. The regression technique gets used mainly to determine the predictor strength, forecast trends, time series, and sometimes in case of cause & effect relation.

Regression analysis is the basic technique to solve the regression problems in machine learning ML using data models. It consists of determining the best fit line, which is a line that passes through all the data points in such a way that distance of the line from each data point is optimal/minimized.

## Data Preprocessing Template for Jupiter Notebook or Google Colab.

### Importing the libraries

import numpy as np
import matplotlib.pyplot as plt
import pandas as pd

### Importing the dataset

X = dataset.iloc[:, :-1].valuest
y = dataset.iloc[:, -1].values

### Splitting the dataset into the Training set and Test set

from sklearn.model_selection import train_test_split
X_train, X_test, y_train, y_test = train_test_split(X, y, test_size = 0.2, random_state = 0)

### Taking care of missing data

from sklearn.impute import SimpleImputer
imputer = SimpleImputer(missing_values=np.nan, strategy='mean')
imputer.fit(X[:, 1:3])
X[:, 1:3] = imputer.transform(X[:, 1:3])

### Encoding categorical data

#### Encoding the Independent Variable

from sklearn.compose import ColumnTransformer
from sklearn.preprocessing import OneHotEncoder
ct = ColumnTransformer(transformers=[('encoder', OneHotEncoder(), )], remainder='passthrough')
X = np.array(ct.fit_transform(X))

#### Encoding the Dependent Variable

from sklearn.preprocessing import LabelEncoder
le = LabelEncoder()
y = le.fit_transform(y)

### Feature Scaling

from sklearn.preprocessing import StandardScaler
sc = StandardScaler()
X_train[:, 3:] = sc.fit_transform(X_train[:, 3:])
X_test[:, 3:] = sc.transform(X_test[:, 3:])

### Training the Simple Linear Regression model on the Training set for Jupiter Notebook or Google Colab

from sklearn.linear_model import LinearRegression
regressor = LinearRegression()
regressor.fit(X_train, y_train)

### Predicting the Test set results

y_pred = regressor.predict(X_test)

### Visualising the Training set results

plt.scatter(X_train, y_train, color = 'red')
plt.plot(X_train, regressor.predict(X_train), color = 'blue')
plt.title('Target vs Feature (Training set)')
plt.xlabel('Feature')
plt.ylabel('Target')
plt.show()

### Visualising the Test set results

plt.scatter(X_test, y_test, color = 'red')
plt.plot(X_train, regressor.predict(X_train), color = 'blue')
plt.title('Target vs Feature (Test set)')
plt.xlabel('Feature')
plt.ylabel('Target')
plt.show()

### Concatenating predictions and actaul values

np.set_printoptions(precision=2)
print(np.concatenate((y_pred.reshape(len(y_pred),1), y_test.reshape(len(y_test),1)),1))

### Training the Polynomial Regression model on the whole dataset

from sklearn.linear_model import LinearRegression
lin_reg = LinearRegression()
lin_reg.fit(X, y)
from sklearn.preprocessing import PolynomialFeatures
poly_reg = PolynomialFeatures(degree = 4)
X_poly = poly_reg.fit_transform(X)
lin_reg_2 = LinearRegression()
lin_reg_2.fit(X_poly, y)

### Predicting a new single result with Linear Regression - single feature

lin_reg.predict([[put_here_any_value]])

### Predicting a new single result with Polynomial Regression - single feature

lin_reg_2.predict(poly_reg.fit_transform([[put_here_any_value]]))

## Support Vector Regression (SVR) in Python.

#Importing the dataset and reshaping target
X = dataset.iloc[:, :-1].values
y = dataset.iloc[:, -1].values
y = y.reshape(len(y),1)

#Feature Scaling
from sklearn.preprocessing import StandardScaler
sc_X = StandardScaler()
sc_y = StandardScaler()
X = sc_X.fit_transform(X)
y = sc_y.fit_transform(y)

#Training the SVR model on the whole datase (without split)
from sklearn.svm import SVR
regressor = SVR(kernel = 'rbf')
regressor.fit(X, y)

#Predicting a new result (SVR) - single feature
sc_y.inverse_transform(regressor.predict(sc_X.transform([[put_here_any_value]])))

#Visualising the SVR results
plt.scatter(sc_X.inverse_transform(X), sc_y.inverse_transform(y), color = 'red')
plt.plot(sc_X.inverse_transform(X), sc_y.inverse_transform(regressor.predict(X)), color = 'blue')
plt.title('Target vs Feature (SVR)')
plt.xlabel('Feature')
plt.ylabel('Target')
plt.show()

## Decision Tree Regression in Python.

#Importing the dataset
X = dataset.iloc[:, :-1].values
y = dataset.iloc[:, -1].values
#Training the Decision Tree Regression model on the whole datase (without split)
from sklearn.tree import DecisionTreeRegressor
regressor = DecisionTreeRegressor(random_state = 0)
regressor.fit(X, y)

#Predicting a new result (Decision Tree) - single feature
regressor.predict([[put_here_any_value]])

#Visualising the Decision Tree Regression results in higher resolution
X_grid = np.arange(min(X), max(X), 0.01)
X_grid = X_grid.reshape((len(X_grid), 1))
plt.scatter(X, y, color = 'red')
plt.plot(X_grid, regressor.predict(X_grid), color = 'blue')
plt.title('Target vs Feature (Decision Tree Regression)')
plt.xlabel('Feature')
plt.ylabel('Target')
plt.show()

## Random Forest Regression model in Python.

#The only difference in code from Decision Tree Regression above is Training the Random Forest Regression model, on the whole datase (without split) in this example
from sklearn.ensemble import RandomForestRegressor
regressor = RandomForestRegressor(n_estimators = 10, random_state = 0)
regressor.fit(X, y)

## Regression models tips and features.

Model: Linear Regression.
Pros: Works on any size of dataset, gives informations about relevance of features.
Cons: Linear Regression Assumptions.

Model: Polynomial Regression.
Pros: Works on any size of dataset, works very well on non linear problems.
Cons: Needed to choose the right polynomial degree for a good bias/variance tradeoff.

Model: SVR.
Pros: Easily adaptable, works very well on non linear problems, not biased by outliers.
Cons: Compulsory to apply feature scaling, difficult interpretations.

Model: Decision Tree Regression.
Pros: Interpretability, no need for feature scaling, works on both linear / nonlinear problems.
Cons: Poor results on too small datasets, overfitting can easily occur.

Model: Random Forest Regression.
Pros: Powerful and accurate, good performance on many problems, including non linear.
Cons: No interpretability, overfitting can easily occur, needed to choose the number of trees.