Added Gradient Boosting Classifier (TheAlgorithms#10944)

* Added Gradient Boosting Classifier

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* Update gradient_boosting_classifier.py

* Update gradient_boosting_classifier.py

* Update gradient_boosting_classifier.py

* Update gradient_boosting_classifier.py

* [pre-commit.ci] auto fixes from pre-commit.com hooks

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Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

Author: SannketNikam

machine_learning/gradient_boosting_classifier.py

@@ -0,0 +1,118 @@
+import numpy as np
+from sklearn.datasets import load_iris
+from sklearn.metrics import accuracy_score
+from sklearn.model_selection import train_test_split
+from sklearn.tree import DecisionTreeRegressor
+
+
+class GradientBoostingClassifier:
+    def __init__(self, n_estimators: int = 100, learning_rate: float = 0.1) -> None:
+        """
+        Initialize a GradientBoostingClassifier.
+
+        Parameters:
+        - n_estimators (int): The number of weak learners to train.
+        - learning_rate (float): The learning rate for updating the model.
+
+        Attributes:
+        - n_estimators (int): The number of weak learners.
+        - learning_rate (float): The learning rate.
+        - models (list): A list to store the trained weak learners.
+        """
+        self.n_estimators = n_estimators
+        self.learning_rate = learning_rate
+        self.models: list[tuple[DecisionTreeRegressor, float]] = []
+
+    def fit(self, features: np.ndarray, target: np.ndarray) -> None:
+        """
+        Fit the GradientBoostingClassifier to the training data.
+
+        Parameters:
+        - features (np.ndarray): The training features.
+        - target (np.ndarray): The target values.
+
+        Returns:
+        None
+
+        >>> import numpy as np
+        >>> from sklearn.datasets import load_iris
+        >>> clf = GradientBoostingClassifier(n_estimators=100, learning_rate=0.1)
+        >>> iris = load_iris()
+        >>> X, y = iris.data, iris.target
+        >>> clf.fit(X, y)
+        >>> # Check if the model is trained
+        >>> len(clf.models) == 100
+        True
+        """
+        for _ in range(self.n_estimators):
+            # Calculate the pseudo-residuals
+            residuals = -self.gradient(target, self.predict(features))
+            # Fit a weak learner (e.g., decision tree) to the residuals
+            model = DecisionTreeRegressor(max_depth=1)
+            model.fit(features, residuals)
+            # Update the model by adding the weak learner with a learning rate
+            self.models.append((model, self.learning_rate))
+
+    def predict(self, features: np.ndarray) -> np.ndarray:
+        """
+        Make predictions on input data.
+
+        Parameters:
+        - features (np.ndarray): The input data for making predictions.
+
+        Returns:
+        - np.ndarray: An array of binary predictions (-1 or 1).
+
+        >>> import numpy as np
+        >>> from sklearn.datasets import load_iris
+        >>> clf = GradientBoostingClassifier(n_estimators=100, learning_rate=0.1)
+        >>> iris = load_iris()
+        >>> X, y = iris.data, iris.target
+        >>> clf.fit(X, y)
+        >>> y_pred = clf.predict(X)
+        >>> # Check if the predictions have the correct shape
+        >>> y_pred.shape == y.shape
+        True
+        """
+        # Initialize predictions with zeros
+        predictions = np.zeros(features.shape[0])
+        for model, learning_rate in self.models:
+            predictions += learning_rate * model.predict(features)
+        return np.sign(predictions)  # Convert to binary predictions (-1 or 1)
+
+    def gradient(self, target: np.ndarray, y_pred: np.ndarray) -> np.ndarray:
+        """
+        Calculate the negative gradient (pseudo-residuals) for logistic loss.
+
+        Parameters:
+        - target (np.ndarray): The target values.
+        - y_pred (np.ndarray): The predicted values.
+
+        Returns:
+        - np.ndarray: An array of pseudo-residuals.
+
+        >>> import numpy as np
+        >>> clf = GradientBoostingClassifier(n_estimators=100, learning_rate=0.1)
+        >>> target = np.array([0, 1, 0, 1])
+        >>> y_pred = np.array([0.2, 0.8, 0.3, 0.7])
+        >>> residuals = clf.gradient(target, y_pred)
+        >>> # Check if residuals have the correct shape
+        >>> residuals.shape == target.shape
+        True
+        """
+        return -target / (1 + np.exp(target * y_pred))
+
+
+if __name__ == "__main__":
+    iris = load_iris()
+    X, y = iris.data, iris.target
+    X_train, X_test, y_train, y_test = train_test_split(
+        X, y, test_size=0.2, random_state=42
+    )
+
+    clf = GradientBoostingClassifier(n_estimators=100, learning_rate=0.1)
+    clf.fit(X_train, y_train)
+
+    y_pred = clf.predict(X_test)
+    accuracy = accuracy_score(y_test, y_pred)
+    print(f"Accuracy: {accuracy:.2f}")