Machine Learning
Code Snippets

In questo post, che aggiornerò con il tempo, riporterò snippets di codice con le classi e metodi (di librerie come scikit-learn, numpy, matplotlib, etc.) più utili per progetti di ML. Poiché, non utilizzando per certi periodi alcune di queste funzioni la mia memoria decide di formattare tutto, ho deciso di mantenere traccia di questi importanti strumenti in questa pagina. Spero possa essere d’aiuto a qualcuno quanto lo è per me…

Data Pre-Processing

> Import a Dataset from 'csv' file

import numpy as np
import pandas as pd

dataset = pd.read_csv('name_file.csv')
X = dataset.iloc[:,:-1].values
y = dataset.iloc[:,[-1]].values

Il metodo “read_csv” ha molte funzioni, tra cui, headerdelimiterquoting, etc. 

> Missing Data (data munging)

from sklearn.preprocessing import Imputer

imputer = Imputer(missing_values='NaN', strategy='mean', axis=0)
X[:,m] = imputer.fit_transform(X[:,m])

> Encode Categorical Data

from sklearn.preprocessing import LabelEncoder

labelencoder_X = LabelEncoder()
X[:,m] = labelencoder_X.fit_transform(X[:,m])

Crea delle etichettature numeriche. Ex: collina, montagna e mare —> 0, 1, 2

from sklearn.preprocessing import LabelBinarizer

labelbinarizer_X = LabelBinarizer()
X[:,m] = labelbinarizer_X.fit_transform(X[:,m])

Crea un vettore “OneHotEncoded”. Ex: collina, montagna e mare –> [[1,0,0], [0, 1, 0], [0, 1, 0]]

from sklearn.preprocessing import OneHotEncoder

onehotencoder = OneHotEncoder(categorical_features=[m])
X = onehotencoder.fit_transform(X).toarray()

Come “LabelBinarizer” crea un vettore “OneHotEncoded”, ma funziona solo con dati multi-colonna. Si seleziona la colonna da trasformare durante la creazione dell’istanza e poi si passa direttamente tutta la matrice di dati. Dummy Encoding: eliminare sempre una colonna di features. X = X[:,1:].

> Splitting the Dataset

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)

> Feature Scaling

from sklearn.preprocessing import StandardScaler

sc_X = StandardScaler()
X = sc_X.fit_transform(X)
standard_scaling_formulas
from sklearn.preprocessing import Normalizer

nr_X = Normalizer()
X = nr_X.fit_transform()
normalize_formula

Plotting Data

> Scatter Plot with Second Order Regression Line

import matplotlib.pyplot as plt

# create a canvas
plt.figure(figsize=(8,8))
# scatter plot
plt.scatter(X, y, s=45, marker='o', c='blue')
# fit a second order 
z = np.polyfit(X, y, 2)
# create a function
p = np.poly1d(z)
# create a number of points
x = np.linspace(0, 1.0, 100)
# plot the line
plt.plot(x,p(x),c = 'r', linewidth=1, dashes=[6, 2])
plt.title('test')
plt.xlabel('xlabel')
plt.ylabel('y_kabel')
plt.text(0, 1, '$y = 0.44x^{2} + 0.17x + 0.22$', fontsize=12)
plt.text(0, 0.95, 'r = 0.81', fontsize=12)
# limit x and y axis
plt.xlim((0,1))
plt.ylim((0,1))
# save the plot with a png file
plt.savefig('scatter_test_set.png', dpi=300, transparent=True)
# show plot
plt.legend()
plt.show()

> Different Types of Plots

  • Histograms
plt.hist(X, bins=20, color='red')
  • Boxplot
import seaborn as sns

sns.boxplot(X)
  • Distribution and Histogram
sns.distplot(X)
  • Violin Plot
sns.violinplot(X)
  • Cumulative Plot
sns.kdeplot(X, cumulative=True)

> Plot Images

  • Display a single figure
plt.imshow(image)
  • Display a Multiple Figures (first method)
n = 0
nrows = 5
ncols = 5
# create a plot with a grid of 5x5 sub-plots (ax)
fig, ax = plt.subplots(nrows,ncols,sharex=True,sharey=True, figsize=(18,18))
for row in range(nrows):
    for col in range(ncols):
        # use indexes to select sub-plots rows and columns
        ax[row,col].imshow(img[n])
        # show grid
        ax.grid()
        ax[row,col].set_title("title_number {}".format(n))
        n += 1
  • Display a Multiple Figures (second method)
n = 20  # how many images we are gonna display

plt.figure(figsize=(20, 4))
for i in range(n):
    # iterate on sub-plots
    ax = plt.subplot(2, n/2, i + 1)
    plt.imshow(X_test[i].reshape(28, 28))
    plt.gray()
    ax.get_xaxis().set_visible(True)
    ax.get_yaxis().set_visible(False)
    
plt.show()
  • Display a Multiple Figures (third method)
fig, axes = plt.subplots(r, c, figsize=(15,15))
axes = axes.flatten()
for img_batch, label_batch, ax in zip(images_batch[0], images_batch[1], axes):
    ax.imshow(img_batch)
    ax.grid()
    ax.set_title('Class: {}'.format(classes[np.argmax(label_batch)]))
plt.tight_layout()
plt.show()

Regression Models

> Simple/Multiple Linear Regression

from sklearn.linear_model import LinearRegression

regressor = LinearRegression()
regreessor.fit(X_train, y_train)

y_pred = regressor.predict(X_test)

È praticamente equivalente ad utilizzare np.polyfit(X, y, 1); utilizzano entrambi la minimizzazione ai minimi quadrati per trovare “m” e “q”.

Se si utilizza un vettore X multidimensionale (più di una feature) si otterrà una multiple linear regression —> y = b0 + b1*x1 + b2 * x2 + … + bn * xn

> Polynomial Regression

from sklearn.linear_model import LinearRegression
from sklearn.preprocessing import PolynomialFeatures

m = 3

regressor = LinearRegression()
poly_reg = PolynomialFeatures(degree = m)
X_poly = poly_reg.fit_transform(X)
regressor.fit(X_poly, y_train)
regressor.predict(poly_reg.fit_transform(X_test))

Possiamo semplicemente riutilizzare una regressione lineare andando a aggiungere al vettore X di input le sue potenze (grado “m”). La funzione infatti ha forma —> y = b0 + b1 * x1 + b2 * x1^2 + b3 * x1^3 + … + bm * x1^m

> Support Vector Regression

from sklearn.svm import SVR

regressor = SVR(kernel = 'rbf')
regressor.fit(X, y)
y_pred = regressor.predict(sc_X.transform(np.array([[n]])
y_pred = sc_y.inverse_transform(y_pred)

Richiede lo scalamento delle variabili. Quindi dopo la predizione bisogna tornare alla range iniziale.

> Decision Tree Regression

from sklearn.tree import DecisionTreeRegressor

regressor = DecisionTreeRegressor()
regressor.fit(X, y)

> Random Forest Regression

from sklearn.ensemble import RandomForestRegressor

n = 100

regressor = RandomForeestRegressor(n_estimatores = n)
regressor.fit(X, y)

Classification Models

Tutti i modelli richiedono uno scaling iniziale. Però , a differenza della regressione non dobbiamo trasformare le predizioni.

> Logistic Regression

from sklearn.linear_model iimport LogisticRegression

classifier = LogistcRegression()
classifier.fit(X_tran, y_train)
y_pred = classifier.predict(X_test)

> K-Nearest Neighbors

from sklearn.neighbors import KNeighborsClassifier

classifier = KNeighborsClassifier(n_neighbors=n, 
metric='minkowiski', p=2)
classifier.fit(X_train, y_train)
y_pred = classifier.predict(X_test)

Con K-Neareest Neighbors bisogna scegliere quanti vicini prendere in considerazione, “n_neighbors”, e che metrica utilizzare, “p” e “metric”.

> Support Vector Machine (SVM)

from sklearn.svm import SVC

classifier = SVC(kernel = 'linear')
classifier.fit(X_train, y_train)
y_pred = classifier.predict(X_test)

> Kernel SVM

from sklearn.svm import SVC

classifier = SVC(kernel = 'rbf')
classifier.fit(X_train, y_train)
y_pred = classifier.predict(X_test)

Possiamo facilmente utilizzare diversi tipi di Kernel attraverso l’attributo “kernel”.

> Naive Bayes

from sklearn.naive_bayes import GaussianNB

classifier = GaussianNB()
classifier.fit(X_train, y_train)

> Decision Tree Classifier

from sklearn.tree import DecisionTreeClassifier

classifier = DecisionTreeClassifier(criterion = 'entropy')
classifier.fit(X_train, y_train)

> Random Forest Classifier

from sklearn.ensemble import RandomForestClassifier

n = 100

classifier = RandomForestClassifier(n_estimators=n, criterion='entropy')
classifier.fit(X_train, y_train)

> Metrics

from sklearn.metrics import confusion_matrix, f1_score, precision_score, accuracy_score, recall_score, classification_report

cm = confusion_matrix(y_test, y_pred)
f1 = f1_score(y_test, y_pred)
precision = precision_score(y_test, y_pred)
accuracy = accuracy_score(y_test, y_pred)
recall = recall_score(y_test, y_pred)
report = classification_report(y_test, y_pred)