#!/usr/bin/env python3
# Assignment n 1, given in lesson 03 on 01/10/19
# Author: Leonardo Tamiano

import math
import numpy as np

# DESCRIPTION
# 
# This function computes the entropy for a given discrete random
# variable X. 
#
# ARGUMENTS:
#
# 1) pmf is a numpy array describing the probability mass function of
#    a discrete random variable. In particular we have,
# 
#                 pmf[i] = P(X = x_i)
# 
def entropy(pmf):
    N_x = pmf.shape[0]
    result = 0

    for i in range(0, N_x):
        if pmf[i] != 0:
            result += pmf[i] * math.log(pmf[i], 2)
    return -result