FlipPredictor.py

#FlipPredictor
#A coin is drawn at random from a bag of coins of varying probabilities
#Each coin has the same chance of being drawn
#Your class FlipPredictor will be initialized with a list of the probability of 
#heads for each coin. This list of probabilities can be accessed as self.coins 
#in the functions you must write. The function update will be called after every 
#flip to enable you to update your estimate of the probability of each coin being 
#the selected coin. The function pheads may be called and any time and will 
#return your best estimate of the next flip landing on heads.

#Given a bag with coins, where each coin has a different
# probablilty of being selected, we take one coin and
# we flip it repeatedly. On each flip, return the
#probablility of the coin begin Heads on the next flip.
from __future__ import division # make division normal way of dividing
class FlipPredictor(object):
    def __init__(self,coins): #set of coins in the bag
        self.coins=coins # python list
        n=len(coins) # nbr of coins in the bag
        self.probs=[1/n]*n #python list with the same nbr of items as coins in the bags, given the prob. of a coin being the coin selected
        # the first time the coin is selected, the prob is 1/n, being n the total nbr of coins in the bag
    def pheads(self):
        #Write a function that returns 
        #the probability of the next flip being heads 
        # prob given the current prob, what is the prob of coming up with heads on the next flip
        # compute based on self.coin and self.probs
        # P(H) = ((1/n)* p(H) Coin1)+((1/n)* p(H) Coin2)+((1/n)* p(H) Coin3)+ ...+ ((1/n)* p(H) CoinN)


#Sol:
        return sum(pcoin*p for pcoin,p in zip(self.coins,self.probs))   

#        probHeads = 0
#        for coin in coins:
#            probHeads = probHeads + (self.probs * self.coins)
#        return probHeads
        
    def update(self,result):
        #Write a function that updates the probabilities of flipping each coin
        #this function takes a "result" as input (H, or T) and based on that, updates self.prob so that pheads returns the correct value in the future
#Sol:
        pheads=self.pheads()
        if result=='H':
            self.probs=[pcoin*p/pheads for pcoin,p in zip(self.coins,self.probs)]
        else:
            self.probs=[(1-pcoin)*p/(1-pheads) for pcoin,p in zip(self.coins,self.probs)]

#        pheads=self.pheads()
#        if result == 'H':
#            probHeads = 0
#            for coin in coins:
#                probHeads = probHeads + (self.probs * self.coins)
#            self.probs = probHeads * 1 /pheads
#        else:
#            probHeads = 0
#            for coin in coins:
#                probHeads = probHeads + (self.probs * self.coins)
#            self.probs = probHeads * 1 /(1-pheads)

    
#The code below this line tests your implementation. 
#You need not change it
#You may add additional test cases or otherwise modify if desired
def test(coins,flips):        
    f=FlipPredictor(coins)
    guesses=[]
    for flip in flips:
        f.update(flip)
        guesses.append(f.pheads())
    return guesses   
        
def maxdiff(l1,l2):
    return max([abs(x-y) for x,y in zip(l1,l2)])

testcases=[
(([0.5,0.4,0.3],'HHTH'),[0.4166666666666667, 0.432, 0.42183098591549295, 0.43639398998330553]),
(([0.14,0.32,0.42,0.81,0.21],'HHHTTTHHH'),[0.5255789473684211, 0.6512136991788505, 0.7295055220497553, 0.6187139453483192, 0.4823974597714815, 0.3895729901052968, 0.46081730193074644, 0.5444108434105802, 0.6297110187222278]),
(([0.14,0.32,0.42,0.81,0.21],'TTTHHHHHH'),[0.2907741935483871, 0.25157009005730924, 0.23136284577678012, 0.2766575695593804, 0.3296000585271367, 0.38957299010529806, 0.4608173019307465, 0.5444108434105804, 0.6297110187222278]),
(([0.12,0.45,0.23,0.99,0.35,0.36],'THHTHTTH'),[0.28514285714285714, 0.3378256513026052, 0.380956725493104, 0.3518717367468537, 0.37500429586037076, 0.36528605387582497, 0.3555106542906013, 0.37479179323540324]),
(([0.03,0.32,0.59,0.53,0.55,0.42,0.65],'HHTHTTHTHHT'),[0.528705501618123, 0.5522060353798126, 0.5337142767315369, 0.5521920592821695, 0.5348391689038525, 0.5152373451083692, 0.535385450497415, 0.5168208803156963, 0.5357708613431963, 0.5510509656933194, 0.536055356823069])]

for inputs,output in testcases:
    if maxdiff(test(*inputs),output)<0.001:
        print ('Correct')
    else: print ('Incorrect')