problem 3

gistfile1.py

balance = float (raw_input ('What is your current balance? '))
interest = float (raw_input ('What is your interest rate? '))

monthly_interest_rate = interest / 12.0

# Monthly payment lower bound = Balance / 12.0
# Monthly payment upper bound = (Balance * (1 + (Annual interest rate / 12.0)) ** 12.0) / 12.0
lower_bound = balance / 12.0
upper_bound = ( balance * ( 1 + (monthly_interest_rate)) ** 12.0 ) / 12.0

print upper_bound, lower_bound
exit
# Here you want to have two loops.
# One loop on the outside which keeps trying until it finds a good value
amount_paid_off_over_12_months = 0

# While we're still looking for the answer
still_looking_for_answer = True

while still_looking_for_answer:
  # Now we'll keep looping and trying different calculations

  # Set this to a 0 to 'start over' for this try
  amount_paid_off_over_12_months = 0

  # This is the monthly payment we'll aim for, lower_bound + our delta.

  # Here is how much more than the minimum payment we'll try for on this run
  # Start with 1/2 the range (pick the number in the middle)
  monthly_payment = ((upper_bound - lower_bound) / 2.0) + lower_bound

  # Make a copy of balance so we don't stomp on it and change it
  # as we calculate how much we'd pay off with this monthly payment
  this_runs_balance = balance
  print "Guessing ", monthly_payment, " this time"
  # THis is the same code as problem1 !!
  # Calculates how much $$ was paid off in a year at this monthly_payment amount
  for month in range(0,12):
    interest_paid = monthly_interest_rate * this_runs_balance
    principal_paid = monthly_payment - interest_paid
    this_runs_balance = this_runs_balance - principal_paid

  # Now save this value  -- it's how much we'd pay off with this monthly payment
  amount_paid_off_over_12_months = balance - this_runs_balance

  # So now we finished our calculation.
  # If everything went ok, we won't repeat this (the while loop above will not continue)
  # If everything didn't go ok, we'll repeat.

  #1 / 0
  # Cool stuff -- keep making this number smaller (even 0, and you'll just get closer and closer
  # And it'll take longer and longer!
  if abs( amount_paid_off_over_12_months - balance) < 1:
    # If we paid off the right amount.
    still_looking_for_answer = False
    # Print out the monthly payment value here
    # And the amount_paid_off_over_12_months
    print ('Monthly payment for pay off in 1 year '), (round(monthly_payment, 2))
    print ('Remaining balance:'), (balance - amount_paid_off_over_12_months)
  else:
    # Things didn't go so well.
    # So we'll adjust the range and lower_bound.
    #
    if amount_paid_off_over_12_months > balance:
      #  means we paid too much.
      #  make the upper bound now our current guess
      upper_bound = monthly_payment
      print "We guessed too much, trying less -- upper is now", upper_bound
    else:
      # We paid too little
      # Bring up the lower bound, squeezing the right number in the middle
      lower_bound = monthly_payment
      print "We guessed too little, trying more -- lower is now" , lower_bound