import torch
from ....Exceptions import NotFittedError
[docs]
class MultinomialNaiveBayes:
"""
The MultinomialNaiveBayes classifier model. Applies the Bayes theorem to classify samples with positive integer features.
Attributes:
n_features (int): The number of features. Available after fitting.
n_classes (int): The number of classes. Available after fitting.
"""
[docs]
def fit(self, X, y, alpha=1):
"""
Fits the MultinomialNaiveBayes model to the input data by calculating the prior probabilities and likelihoods.
Args:
X (torch.Tensor of shape (n_samples, n_features)): The input data, where each row is a sample and each column is a feature. Must contain only non-negative integers.
y (torch.Tensor of shape (n_samples,)): The labels corresponding to each sample. Every element must be in [0, ..., n_classes - 1].
alpha (float | int): Laplacian smoothing parameter. Must be non-negative. For no smoothing, alpha is set to zero. Defaults to 1.
Returns:
None
Raises:
TypeError: If the input matrix or the label matrix is not a PyTorch tensor.
ValueError: If the input matrix or the label matrix is not the correct shape.
"""
if not isinstance(X, torch.Tensor) or not isinstance(y, torch.Tensor):
raise TypeError("The input matrix and the label matrix must be PyTorch tensors.")
if X.ndim != 2:
raise ValueError("The input matrix must be a 2 dimensional tensor.")
if y.ndim != 1 or y.shape[0] != X.shape[0]:
raise ValueError("The labels must be 1 dimensional with the same number of samples as the input data")
vals = torch.unique(y).numpy()
if set(vals) != {*range(len(vals))}:
raise ValueError("y must only contain the values in [0, ..., n_classes - 1].")
if (X.dtype == torch.int or X.dtype == torch.int8 or X.dtype == torch.int16 or X.dtype == torch.int32 or X.dtype == torch.int64) and torch.all(X >= 0):
raise ValueError("The features must be non-negative integers. If your features are not non-negative integers, consider other NaiveBayes models.")
if not isinstance(alpha, int | float) or alpha < 0:
raise ValueError("alpha must be a non-negative real number.")
self.n_classes = len(vals)
self.n_features = X.shape[1]
self.priors = torch.zeros((self.n_classes,), dtype=torch.float32)
self.likelihoods = torch.zeros((self.n_classes, X.shape[1]), dtype=torch.float32)
for i in range(self.n_classes):
X_cls = X[y == i]
self.priors[i] = len(X_cls) / len(y)
self.likelihoods[i] = (X_cls.sum(dim=0) + alpha) / (X_cls.sum() + alpha * X.shape[1]) # laplace smoothing
[docs]
def predict(self, X):
"""
Applies the fitted MultinomialNaiveBayes model to the input data, predicting the labels.
Args:
X (torch.Tensor of shape (n_samples, n_features)): The input data to be classified.
Returns:
labels (torch.Tensor of shape (n_samples,)): The predicted target values corresponding to each sample.
Raises:
NotFittedError: If the MultinomialNaiveBayes model has not been fitted before predicting.
TypeError: If the input matrix is not a PyTorch tensor.
ValueError: If the input matrix is not the correct shape.
"""
if not hasattr(self, "priors"):
raise NotFittedError("MultinomialNaiveBayes.fit() must be called before predicting.")
if not isinstance(X, torch.Tensor):
raise TypeError("The input matrix must be a PyTorch tensor.")
if X.ndim != 2 or X.shape[1] != self.n_features:
raise ValueError("The input matrix must be a 2 dimensional tensor with the same number of features as the fitted tensor.")
if (X.dtype == torch.int or X.dtype == torch.int8 or X.dtype == torch.int16 or X.dtype == torch.int32 or X.dtype == torch.int64) and torch.all(X >= 0):
raise ValueError("The features must be non-negative integers. If your features are not non-negative integers, consider other NaiveBayes models.")
posteriors = torch.zeros((self.n_classes, len(X)), dtype=torch.float32)
for i in range(self.n_classes):
prior = torch.log(self.priors[i])
posterior = (X * torch.log(self.likelihoods[i])).sum(dim=1) + prior
posteriors[i] = posterior
return torch.argmax(posteriors, dim=0)
[docs]
def predict_proba(self, X):
"""
Applies the fitted MultinomialNaiveBayes model to the input data, predicting the labels.
Args:
X (torch.Tensor of shape (n_samples, n_features)): The input data to be classified.
Returns:
labels (torch.Tensor of shape (n_samples,)): The predicted target values corresponding to each sample.
Raises:
NotFittedError: If the MultinomialNaiveBayes model has not been fitted before predicting.
TypeError: If the input matrix is not a PyTorch tensor.
ValueError: If the input matrix is not the correct shape.
"""
if not hasattr(self, "priors"):
raise NotFittedError("MultinomialNaiveBayes.fit() must be called before predicting.")
if not isinstance(X, torch.Tensor):
raise TypeError("The input matrix must be a PyTorch tensor.")
if X.ndim != 2 or X.shape[1] != self.n_features:
raise ValueError("The input matrix must be a 2 dimensional tensor with the same number of features as the fitted tensor.")
if (X.dtype == torch.int or X.dtype == torch.int8 or X.dtype == torch.int16 or X.dtype == torch.int32 or X.dtype == torch.int64) and torch.all(X >= 0):
raise ValueError("The features must be non-negative integers. If your features are not non-negative integers, consider other NaiveBayes models.")
posteriors = torch.zeros((self.n_classes, len(X)), dtype=torch.float32)
for i in range(self.n_classes):
prior = torch.log(self.priors[i])
posterior = (X * torch.log(self.likelihoods[i])).sum(dim=1) + prior
posteriors[i] = posterior
prob_normalizers = torch.logsumexp(posteriors, dim=0)
log_probs = posteriors - prob_normalizers
probs = torch.exp(log_probs).T
if self.n_classes == 2: # binary classification
probs = probs[:, 1]
return probs