07 Neural Networks

Last updated on 2025-07-10 | Edit this page

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Neural Network (MLPClassifier) with Breast Cancer Dataset

In this notebook, we will use a simple Multi-layer Perceptron (MLP) neural network to classify breast tumors.

What is an MLP?

An MLP is a type of feedforward neural network consisting of one or more hidden layers. Each neuron computes a weighted sum of its inputs and passes the result through a nonlinear activation function.

MLPs are suitable for classification tasks and are trained using backpropagation to minimize loss.

Step 1: Load the Breast Cancer Dataset

PYTHON 

from sklearn.datasets import load_breast_cancer
from sklearn.model_selection import train_test_split
from sklearn.preprocessing import StandardScaler
import pandas as pd

# Load dataset
data = load_breast_cancer()
X = data.data
y = data.target

# Split dataset
X_train, X_test, y_train, y_test = train_test_split(
    X, y, test_size=0.3, random_state=31
)

# Normalize (Standardize) features
scaler = StandardScaler()
X_train = scaler.fit_transform(X_train)
X_test = scaler.transform(X_test)

Step 3: Train an MLPClassifier

We use MLPClassifier from Scikit-Learn with one hidden layer.

PYTHON 

from sklearn.neural_network import MLPClassifier

model = MLPClassifier(hidden_layer_sizes=(50,), max_iter=2000, random_state=42)
model.fit(X_train, y_train)

Step 4: Evaluate the Model

PYTHON 

from sklearn.metrics import (
    accuracy_score, precision_score, recall_score, f1_score,
    ConfusionMatrixDisplay, classification_report, roc_curve, auc
)

y_pred = model.predict(X_test)
print("Accuracy:", accuracy_score(y_test, y_pred))
print("Precision:", precision_score(y_test, y_pred))
print("Recall:", recall_score(y_test, y_pred))
print("F1 Score:", f1_score(y_test, y_pred))
print("\nClassification Report:\n", classification_report(y_test, y_pred))

SH 

Accuracy: 0.9824561403508771
Precision: 0.9732142857142857
Recall: 1.0
F1 Score: 0.9864253393665159

Classification Report:
               precision    recall  f1-score   support

           0       1.00      0.95      0.98        62
           1       0.97      1.00      0.99       109

    accuracy                           0.98       171
   macro avg       0.99      0.98      0.98       171
weighted avg       0.98      0.98      0.98       171

What is a Confusion Matrix?

A confusion matrix shows how well the model distinguishes between classes:

Predicted Positive Predicted Negative
Actual Positive True Positive (TP) False Negative (FN)
Actual Negative False Positive (FP) True Negative (TN)

This matrix lets us compute metrics like accuracy, precision, recall, and F1 score.

PYTHON 

import matplotlib.pyplot as plt
import seaborn as sns

ConfusionMatrixDisplay.from_estimator(model, X_test, y_test)
plt.title('Neural Network Confusion Matrix')
plt.show()
png
png

What is an ROC Curve?

The ROC Curve shows the trade-off between True Positive Rate and False Positive Rate. AUC quantifies this performance. Closer to 1.0 = better classifier.

PYTHON 

y_proba = model.predict_proba(X_test)[:, 1]
fpr, tpr, _ = roc_curve(y_test, y_proba)
roc_auc = auc(fpr, tpr)

plt.plot(fpr, tpr, label='AUC = ' + str(round(roc_auc, 2)))
plt.plot([0, 1], [0, 1], 'k--')
plt.xlabel('False Positive Rate')
plt.ylabel('True Positive Rate')
plt.title('Neural Network ROC Curve')
plt.legend()
plt.show()
png
png