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pROBLEMS Problem 1 - Login security One important aspect of security in computer science is the concept of hashing: taking some text, and somehow converting it to a number. This is needed because many security algorithms work through math, so numbers are needed.

Another important aspect is the use of the modulo operator (%). You've seen this -- it returns the remainder portion of a division. This is useful because unlike most other math operators, modulo is one-way. That is, I can tell you that I'm thinking of a number x, and when I mod it by 5, I get 3, but from this information alone, you don't know whether x is 3 or 8 or 13 or 18, or ...

In this problem, we'll create a login screen, where the user must enter a password in order to see a secret message. We will give the user 3 chances to get the password right, and either print the secret message or a failure message (after 3 chances).

First, define a function encrypt that takes one string. It will hash the string using the built-in Python function hash (try it on the shell) and modulo the value by a prime number (e.g. 541 -this is very small in the computer science world but good enough for us). The function should then return this number. e.g. encrypt("mypassword") -> 283 (if you use 541 as the prime, for example) At the top(see of the define a2)variable _KEY to be the result, e.g. _KEY = 283. backfile, for Problem programminghomeworkhelp.com

Problem 2 - The game of Nims / Stones In this game, two players sit in front of a pile of 100 stones. They take turns, each removing between 1 and 5 stones (assuming there are at least 5 stones left in the pile). The person who removes the last stone(s) wins.

Write a program to play this game. This may seem tricky, so break it down into parts. Like many programs, we have to use nested loops (one loop inside another).

In the outermost loop, we want to keep playing until we are out of stones.

Inside that, we want to keep alternating players. You have the option of either writing two blocks of code, or keeping a variable that tracks the current player. The second way is slightly trickier since we haven't learned lists yet, but it's definitely do-able!

Finally, we might want to have an innermost loop that checks if the user's input is valid. Is it a number? Is it a valid number (e.g. between 1 and 5)? Are there enough stones in the pile to take off this many? If any of these answers are no, we should tell the user and re-ask them the question.

So, the basic outline of the program should be something like this: TOTAL = 100 MAX = 5 pile = TOTAL # all stones are in the pile to start while [pile is not empty]: while [player 1's answer is not valid]: [ask player 1] [check player 1's input... is it valid?] [same as above for player 2] Note how the important numbers 100 and 5 are stored in a single variable at the top. This is good practice -- it allows you to easily change the constants of a program. For example, for testing, you may want to start with only 15 or 20 stones.

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SOLUTIONS # login.py # Example solution for Lab 4, problem 1 # # Aseem Kishore # # 6.189 - Intro to Python # IAP 2008 - Class 3 # Some constants...

LARGE_PRIME = 541 _KEY = 171 # to get this number, I used the password "solution" MAX_FAILURES = 3 # stop when we hit this many failures # The encrypt function. Remember, functions shouldn't be asking for input or # printing their result. Any input a function needs (in this case, a string to # encrypt) should be passed in, and the output should be returned. def encrypt(text): return hash(text) % LARGE_PRIME # Main program code num_failures = 0

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# We'll keep looping until we hit the max number of failures... # We need to break out of the loop when we get it correct also, see below. while num_failures < MAX_FAILURES: login = raw_input("Please enter the password: ") if encrypt(login) == _KEY: print "Correct!" break # remember, this breaks out of the current loop else: num_failures = num_failures + 1 print "Incorrect! You have failed", num_failures, "times." # When we get here, it's either because num_failures == MAX_FAILURES, or # because we hit the break statement (i.e. we got the correct login), so... if num_failures >= MAX_FAILURES: print "Sorry, you have hit the maximum number of failures allowed." # # Mihir Kedia # Homework 3 - Nim # # Notes: This program will crash if someone enters a non-integer at the prompt. # We haven't learned how to fix this yet. # programminghomeworkhelp.com

print ' ____...----....' print ' |""--..__...------._' print ' `""--..__|.__ `.' print ' | `\ |' print ' | | |' print ' / | |' print ' __\' / |' print ' ,\' ""--..__ ,-\' |' print '|""--..__ ,\' |' print '| ""| |' print '| ; , | |' print '| \' | |"' print '| | |.-.___' print '| | | "---(=' print '|__ | __..\'' print ' ""--..__|__..--""' print print "Welcome to Nim!" print print "The rules of this game are simple. You start with a pile of stones.", \ "Each player takes turns removing between 1 and 5 stones from the pile.", \ "The player who removes the last stone wins!" print print

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#get a valid initial pile size pile_size = int(raw_input("How many stones would you like to start with?\n")) while pile_size <= 0: pile_size = int(raw_input("You need to start with at least one stone in the pile!\n")) #2 players; player 1 and player 2. Start with player 1 player = 1 #main game loop while pile_size > 0: prompt = "Player " + str(player) + ", there are " + str(pile_size) + " stones in front of you. " + \ "How many stones would you like to remove (1-5)?\n" move = int(raw_input(prompt)) if move <= 0: print "Hey! You have to remove at least one stone!" elif move > pile_size: print "There aren't even that many stones in the pile..." elif move > 5: print "You can't remove more than five stones.“ else: #if we're here, they gave a valid move pile_size = pile_size - move player = 2-(player-1) #this is kind of cute: changes 1 to 2 and 2 to 1. #If we've exited the loop, the pile size is 0. The player whose turn it is NOW just lost the game.. print "Player", 2-(player-1), "has won the game!" programminghomeworkhelp.com

# nims.py # Example solution for Lab 4, problem 2. # # Aseem Kishore # # 6.189 - Intro to Python # IAP 2008 - Class 3 TOTAL = 100 MAX = 5 pile = TOTAL # number of stones in the pile at any given time def is_valid(x): # returns True iff x is between 1 and MAX, and there's also enough stones, # otherwise returns False return (x >= 1) and (x <= MAX) and (x <= pile) while pile > 0: # player 1 turn print "Player 1's turn. There are", pile, "stones in the pile." x=0 while not is_valid(x): programminghomeworkhelp.com

# ask x = int(raw_input("Player 1, how many? ")) # check if not is_valid(x): print "That's not valid, it has to be between 1 and 5, and",\ "you can't pick up more than there are in the pile." pile = pile - x if pile == 0: # win -- do something print "Congratulations, Player 1, you win!" break # player 2 turn print "Player 2's turn. There are", pile, "stones in the pile." y=0 while not is_valid(y): y = int(raw_input("Player 2, how many? ")) if not is_valid(x): print "That's not valid, it has to be between 1 and 5, and",\ "you can't pick up more than there are in the pile." pile = pile - y if pile == 0: # win -- do something print "Congratulations, Player 2, you win!" break print "Game over." programminghomeworkhelp.com