pmlpp/mlpp/multinomial_nb/multinomial_nb.cpp

//
//  MultinomialNB.cpp
//
//  Created by Marc Melikyan on 1/17/21.
//

#include "multinomial_nb.h"
#include "../lin_alg/lin_alg.h"
#include "../utilities/utilities.h"

#include <algorithm>
#include <iostream>
#include <random>


MultinomialNB::MultinomialNB(std::vector<std::vector<double>> inputSet, std::vector<double> outputSet, int class_num) :
		inputSet(inputSet), outputSet(outputSet), class_num(class_num) {
	y_hat.resize(outputSet.size());
	Evaluate();
}

std::vector<double> MultinomialNB::modelSetTest(std::vector<std::vector<double>> X) {
	std::vector<double> y_hat;
	for (int i = 0; i < X.size(); i++) {
		y_hat.push_back(modelTest(X[i]));
	}
	return y_hat;
}

double MultinomialNB::modelTest(std::vector<double> x) {
	double score[class_num];
	computeTheta();

	for (int j = 0; j < x.size(); j++) {
		for (int k = 0; k < vocab.size(); k++) {
			if (x[j] == vocab[k]) {
				for (int p = class_num - 1; p >= 0; p--) {
					score[p] += std::log(theta[p][vocab[k]]);
				}
			}
		}
	}

	for (int i = 0; i < priors.size(); i++) {
		score[i] += std::log(priors[i]);
	}

	return std::distance(score, std::max_element(score, score + sizeof(score) / sizeof(double)));
}

double MultinomialNB::score() {
	Utilities util;
	return util.performance(y_hat, outputSet);
}

void MultinomialNB::computeTheta() {
	// Resizing theta for the sake of ease & proper access of the elements.
	theta.resize(class_num);

	// Setting all values in the hasmap by default to 0.
	for (int i = class_num - 1; i >= 0; i--) {
		for (int j = 0; j < vocab.size(); j++) {
			theta[i][vocab[j]] = 0;
		}
	}

	for (int i = 0; i < inputSet.size(); i++) {
		for (int j = 0; j < inputSet[0].size(); j++) {
			theta[outputSet[i]][inputSet[i][j]]++;
		}
	}

	for (int i = 0; i < theta.size(); i++) {
		for (int j = 0; j < theta[i].size(); j++) {
			theta[i][j] /= priors[i] * y_hat.size();
		}
	}
}

void MultinomialNB::Evaluate() {
	LinAlg alg;
	for (int i = 0; i < outputSet.size(); i++) {
		// Pr(B | A) * Pr(A)
		double score[class_num];

		// Easy computation of priors, i.e. Pr(C_k)
		priors.resize(class_num);
		for (int i = 0; i < outputSet.size(); i++) {
			priors[int(outputSet[i])]++;
		}
		priors = alg.scalarMultiply(double(1) / double(outputSet.size()), priors);

		// Evaluating Theta...
		computeTheta();

		for (int j = 0; j < inputSet.size(); j++) {
			for (int k = 0; k < vocab.size(); k++) {
				if (inputSet[i][j] == vocab[k]) {
					for (int p = class_num - 1; p >= 0; p--) {
						score[p] += std::log(theta[i][vocab[k]]);
					}
				}
			}
		}

		for (int i = 0; i < priors.size(); i++) {
			score[i] += std::log(priors[i]);
			score[i] = exp(score[i]);
		}

		for (int i = 0; i < 2; i++) {
			std::cout << score[i] << std::endl;
		}

		// Assigning the traning example's y_hat to a class
		y_hat[i] = std::distance(score, std::max_element(score, score + sizeof(score) / sizeof(double)));
	}
}