# CSCI 150, Spring 2013
# Solution to Interest Calculator
# Author: Michael Goldwasser

from random import sample, randint

#---------------------------------------------------------------------------------

def greeting():
    print """This interest calculator will ask you to select an interest rate,
followed by a principal value.  It will then calculate and display
the principal, interest rate, and balance after one year.  You will
then be invited to execute the process again or terminate.
"""
#---------------------------------------------------------------------------------

def indexToChar(k):
    """Utility that converts an index, such as 3, into the appropriate uppercase letter, such as 'D'."""
    return chr(ord('A') + k)

#---------------------------------------------------------------------------------

def getRate(choices):
    n = len(choices)
    for k in range(n):
        print indexToChar(k) + ') ' + str(choices[k]) + '%'
    valid = False
    while not valid:
        ans = raw_input('Enter A-' + indexToChar(n-1) + ': ').upper()
        if len(ans) == 1 and 'A' <= ans <= indexToChar(n-1):
            valid = True
        else:
            print 'That is not a valid selection.'
    return choices[ord(ans) - ord('A')]/100.0

#---------------------------------------------------------------------------------

def getPrincipal(limit):
    valid = False
    while not valid:
        print
        response = raw_input('Enter the principal: ')
        if response.startswith('$'):
            response = response[1:]       # remove one leading $ symbol

        if 0 <= response.find('.') < len(response) - 3:
            # there are too many characters after a decimal point
            print 'The principal must be specified in dollars and cents'
        else:
            try:
                principal = float(response)
                if principal < 0:
                    print 'You must enter a positive amount.'
                elif principal >= limit:
                    print 'The principal must be less than $' + str(limit) + '.'
                else:
                    valid = True    # Hooray!!!
            except ValueError:
                print 'Please enter a number.'
    return principal

#---------------------------------------------------------------------------------

def computeBalance(principal, rate):
    return principal * (1+rate)

#---------------------------------------------------------------------------------

def displayResult(principal, rate, balance):
    print
    print 'Initial Principal    Interest Rate    End of Year Balance'
    print '========================================================='
    print '$%-19.2f %-16.2f $%.2f' % (principal, rate, balance)
    print

#---------------------------------------------------------------------------------

def yesNo(prompt):
    valid = False
    while not valid:
        ans = raw_input(prompt).lower()
        if ans.startswith('y') or ans.startswith('n'):
            valid = True
    return ans.startswith('y')

#---------------------------------------------------------------------------------

def main():
  greeting()

  again = True
  while again:
      choices = sorted(sample(range(1,20), randint(2,6)))
      rate = getRate(choices)
      prin = getPrincipal(1000000)
      balance = computeBalance(prin, rate)
      displayResult(prin, rate, balance)
      again = yesNo('Another Computation [y/n]? ')
      print

  print "Quitting program. Bye."

#---------------------------------------------------------------------------------

# let's get things started...
main()