I am trying to find a savings rate (rate_approx in my code) that will result in a savings (current_savings in code) over 36 months that is within $100 of the down payment (down_payment in code).
I'm running the for loop each time within the while loop to represent the growth of current_savings over the 36 month period. Then once that savings rate is adjusted depending on whether it is too high or too low, I want it to save a new savings_rate and try that on the 36 month period. So I reset current_savings to make sure it's not starting from the current_savings from the prior for loop run and try again.
house_price = float(1000000)
down_payment = float(house_price*0.25)
high = float(1)
low = float(0)
rate_approx = float((low + high)/2)
salary = float(150000)
monthly_salary = float(salary/12)
monthly_contribution = float(monthly_salary*rate_approx)
semi_annual_raise = float(1.07)
APR = float(1.04)
current_savings = 0
while abs(current_savings - down_payment) > 100:
current_savings = 0
for n in range(35):
current_savings = float((current_savings + monthly_contribution) * APR)
if n > 0 and n%6 == 0:
monthly_salary = float(monthly_salary * semi_annual_raise)
monthly_contribution = float(monthly_salary*rate_approx)
if current_savings < down_payment + 100:
current_savings = 0
low = rate_approx
rate_approx = (low + high)/2
print("low")
print("savings rate:",rate_approx)
elif current_savings > down_payment + 100:
current_savings = 0
high = rate_approx
rate_approx = (low + high)/2
print("high")
print("savings rate:",rate_approx)
else:
break
print("savings rate:", float(rate_approx))
print("savings:", float(current_savings))
Why does the program continue outputting smaller and smaller savings rates that are giving larger and larger final current_savings values?
Related
What's different between lines 16 and 17?
#user input
annual_salary = float(input("Enter your annual salary: "))
portion_saved = float(input("Enter the percent of your salary to save, as a decimal: "))
total_cost = float(input("Enter the cost of your dream home: "))
#static vars
portion_down_payment = total_cost*.25
monthly_salary = annual_salary/12
r = .04 #annual rate of return
months_to_save = 0
current_savings = 0
investment_return = current_savings * r / 12
while current_savings < portion_down_payment:
#current_savings += (current_savings * r / 12) + (portion_saved * monthly_salary) # Line 16
current_savings += (investment_return) + (portion_saved*monthly_salary) # Line 17
months_to_save += 1
print("Number of months: ", months_to_save)
I tried running it through pythontutor and the variation happens on step 15 of execution, but I can't quite figure out what's different.
When you use current_savings += investment_return, it adds the same amount of interest to current_savings each time through the loop. That interest is equal to current_savings * r / 12 calculated before you started the loop.
But when you use current_savings += (current_savings * r / 12), you recalculate the interest each time through the loop. So the interest is calculated based on the current value of current_savings, which gets bigger each time the loop runs.
In other words, the first one calculates simple interest, and the second one calculates compound interest.
Python Learner. Working on a recurring monthly deposit, interest problem. Except I am being asked to build in a raise after every 6th month in this hypothetical. I am reaching the goal amount in fewer months than I'm supposed to.
Currently using the % function along with += function
annual_salary = float(input("What is your expected Income? "))
portion_saved = float(input("What percentage of your income you expect to save? "))
total_cost = float(input("what is the cost of your dream home? "))
semi_annual_raise = float(input("Enter your expected raise, as a decimal "))
monthly_salary = float(annual_salary/12)
monthly_savings = monthly_salary * portion_saved
down_payment= total_cost*.25
savings = 0
for i in range(300):
savings = monthly_savings*(((1+.04/12)**i) - 1)/(.04/12)
if float(savings) >= down_payment:
break
if i % 6 == 0 :
monthly_salary += monthly_salary * .03
monthly_savings = monthly_salary * portion_saved
Thanks for the advice all. My code is getting clearer and I reached correct outputs! The problem was with how and when I was calculating interest. In the case of a static contribution I successfully used the formula for interest on a recurring deposit, here, the simpler move of calculating interest at each month was needed to work with the flow of the loop.
annual_salary = float(input("What is your expected Income? "))
portion_saved = float(input("What percentage of your income you expect to save? "))
total_cost = float(input("what is the cost of your dream home? "))
semi_annual_raise = float(input("Enter your expected raise, as a decimal "))
monthly_salary = float(annual_salary/12)
monthly_savings = monthly_salary * portion_saved
down_payment = total_cost*.25
savings = 0
month = 1
while savings < down_payment :
print(savings)
savings += monthly_savings
savings = savings * (1+(.04/12))
month += 1
if month % 6 == 0 :
monthly_salary += (monthly_salary * semi_annual_raise)
monthly_savings = (monthly_salary * portion_saved)
print("")
print("it will take " + str(month) + " months to meet your savings goal.")
Does something like this work for you? Typically, we want to use while loops over for loops when we don't know how many iterations the loop will ultimately need.
monthly_savings = 1.1 # saving 10% each month
monthly_salary = 5000
down_payment = 2500
interest = .02
savings = 0
months = 0
while savings < goal:
print(savings)
savings = (monthly_salary * monthly_savings) + (savings * interest)
months += 1
if months % 6 == 0 :
monthly_salary += monthly_salary * .03
print("Took " + str(months) + " to save enough")
I'm trying to write a program that uses bisection to find a fixed monthly rate that will pay off a balance with a month. This is what I have at the moment and what I get is an infinite loop but I'm not sure why.
balance = 3329
tempB = balance
annualInterestRate = 0.2
monthlyIntrestRate = (annualInterestRate/12)
low = balance / 12
high = (balance * (1 + monthlyIntrestRate)**12)/12
avg = (high + low)/2
epsilon = 0.01
while abs(balance - epsilon) >= 0.01:
avg = (high + low)/2
for i in range(12):
monthlyUnpaidBalance = balance - avg
updatedBalanceMonth = monthlyUnpaidBalance + (monthlyIntrestRate * monthlyUnpaidBalance)
balance = updatedBalanceMonth
if abs(balance - epsilon) <= 0.01:
print(avg)
break
else:
balance = tempB
if abs(updatedBalanceMonth) > epsilon:
low = avg
elif abs(updatedBalanceMonth) < epsilon:
high = avg
I have a much simpler code that does the same thing but it is inefficient the code to it is
MinPay = 10
balance = 3329
tempB = balance
annualInterestRate = 0.2
monthlyIntrestRate = (annualInterestRate/12)
while balance >= 0:
for i in range(12):
monthlyUnpaidBalance = balance - MinPay
updatedBalanceMonth = monthlyUnpaidBalance + (monthlyIntrestRate * monthlyUnpaidBalance)
balance = updatedBalanceMonth
if balance <= 0:
print(MinPay)
break
else:
MinPay += 10
balance = tempB
In general, if you have infinite loop, I would suggest you run your code under pdb and do step trace to find why the loop doesn't end.
In your case, I think the issue is on these lines:
if abs(updatedBalanceMonth) > epsilon:
low = avg
elif abs(updatedBalanceMonth) < epsilon:
high = avg
which abs() should generally be greater than epsilon. I see your intention is that if your monthly pay is too much so the resulting balance is negative then you should set high to avg so you try with a lower pay on next search iteration. So you should remove abs() in these conditions.
But I would suggest you make your code easier to read as follows, especially that's intended for finance application:
balance = 3329
rate = 0.2 # 20% p.a.
lopay = 1
hipay = balance
epsilon = 0.01
monthly = 100 # search start
pv = sum([monthly/((1+rate/12)**i) for i in range(1,13)])
while abs(balance - pv) >= epsilon:
if pv > balance:
hipay = monthly
else:
lopay = monthly
monthly = (hipay + lopay) / 2
pv = sum([monthly/((1+rate/12)**i) for i in range(1,13)])
which you simply compute the present value of the 12 monthly payments and compare that to the starting balance.
It gets stuck an infinite loop because your expression:
abs(balance - episolon)
when it is evaluated in the while loop exit condition never actually changes.
If you get stuck in a while loop, you can try little modifications to code like this:
balance = 3329
tempB = balance
annualInterestRate = 0.2
monthlyIntrestRate = (annualInterestRate/12)
low = balance / 12
high = (balance * (1 + monthlyIntrestRate)**12)/12
avg = (high + low)/2
epsilon = 0.01
count = 1
while abs(balance - epsilon) >= 0.01 and count <= 100:
print(abs(balance - epsilon))
avg = (high + low)/2
for i in range(12):
monthlyUnpaidBalance = balance - avg
updatedBalanceMonth = monthlyUnpaidBalance + (monthlyIntrestRate * monthlyUnpaidBalance)
balance = updatedBalanceMonth
if abs(balance - epsilon) <= 0.01:
print(avg)
break
else:
balance = tempB
if abs(updatedBalanceMonth) > epsilon:
low = avg
elif abs(updatedBalanceMonth) < epsilon:
high = avg
count += 1
In this case, the iterations are capped and you can see the value that you are evaluating in your exit condition as:
3328.99
3328.99
3328.99
3328.99
3328.99
3328.99
3328.99
3328.99
3328.99
3328.99
(continuing)
Your going to need to refine your logic so that you can actually hit an exit condition.
The problem is from the MIT edX Python Course 6.00.1 Problem Set 1 Part C. Here are the problems. Scroll to part C. I'm aware that there are countless questions asked about the edX course but none of them have really helped me. Whenever I run my bisection search code, nothing happens. No error message, nothing. Could someone help me find the issue in my code? Sorry if code is horribly inefficient, very new to python and programming.
#Python script for finding optimal saving rate of monthly salary in order to purchase 1M house in 36 months
salary = float(input("Enter salary: "))
total_cost = 1000000
salary_raise = 0.07 #semiannual raise rate
down = 0.25 * total_cost #downpayment
steps = 0
r = 0.04 #annual investments returns
low = 0 #low parameter for bisection search
high = 10000 #high parameter
current_savings = 0
while (current_savings <= (down - 100)) or (current_savings >= (down + 100)):
current_savings = 0
monthly_salary = salary/12
guess_raw = (high + low)/2
guess = guess_raw/10000.0
months = 0
steps += 1
while months < 36: #Finds end amount of money after 36 months based on guess
months += 1
multiple = months%6
monthly_savings = monthly_salary * guess
current_savings = current_savings + monthly_savings + current_savings*r/12
if multiple == 0:
monthly_salary += salary_raise * monthly_salary
if (current_savings >= (down - 100)) and (current_savings <= (down + 100)): #If the guess is close enough, print the rate and number of steps taken
print ("Best savings rate: ",guess)
print ("Steps in bisection search: ",steps)
break
elif current_savings < (down - 100): #If the guess is low, set the low bound to the guess
if guess == 9999:
print ("It is not possible to pay the down payment in three years.")
break
else:
low = guess
elif current_savings > (down + 100): #If the guess is high, set the high bound to the guess
high = guess
I am creating a program that will compute the lowest monthly payment to be paid for a credit card, given a certain credit card balance and interest rate. The time frame for the payoff is 12 months and the monthly payment has to be accurate down to the nearest penny, using bisection search.
I was able to get the answer, the problem is that I couldn't get my while loop to quit once the monthly payment was calculated to the nearest cent, so I had to make an infinite while loop that has a elif statement at the bottom of the while loop that quits for me. I was wondering if anyone can figure out what condition to give the while loop so it will quit on its own. Also I just started learning python a week ago, and want some advice on how good/bad my code is. Any ideas?
# random balance
balance = 999999
# random interest rate
annualInterestRate = 0.18
# assign balance to another variable that will undergo the testing
balance_tested = balance
# bounds of bisection search
low = (balance / 12.0)
high = ((balance * (1 + (annualInterestRate/12.0))**12)/12.0)
# start month
month = 1
monthlyPayment = (low + high) / 2.0 #Averages out the bounds to meet in the middle
while abs(balance_tested != 0): #While loop that I can't get right, just made it to run infinitely
balance_tested = balance #Resets balance being tested back to original balance
monthlyPayment = (low + high) / 2.0 #Bisection search recalculates
month = 1 #Month reset back to 1
while month <= 12: #Loops through all 12 months with the payments being made and interested getting added
balance_tested = (balance_tested - monthlyPayment)
balance_tested += (balance_tested * (annualInterestRate/12))
month += 1
print "Balance Remaining: %.20f" % balance_tested
if balance_tested < 0: #If the bisection search guesses to high, decreases the high bound
high = monthlyPayment
elif balance_tested <= 0.01: #Conditional statement that stops the testing if the balance gets paid off to the cent
break
else: #If bisection search guesses to low, increases low bound
low = monthlyPayment
print "Monthly Payment: %.2f" % monthlyPayment
print "Lowest Payment: %.2f" % monthlyPayment
Is there any reason you're using the break statement instead of just putting that as the condition for your while loop?
balance = 999999
annualInterestRate = 0.18
balance_tested = balance
low = (balance / 12.0) #Lower bound of bisection search
high = ((balance * (1 + (annualInterestRate/12.0))**12)/12.0)
month = 1
monthlyPayment = (low + high) / 2.0
while not (balance_tested <= 0.01):
balance_tested = balance
monthlyPayment = (low + high) / 2.0
month = 1
while month <= 12:
balance_tested = (balance_tested - monthlyPayment)
balance_tested += (balance_tested * (annualInterestRate/12))
month += 1
print "Balance Remaining: %.20f" % balance_tested
if balance_tested < 0:
high = monthlyPayment
else:
low = monthlyPayment
print "Monthly Payment: %.2f" % monthlyPayment
print "Lowest Payment: %.2f" % monthlyPayment
You already have the condition, why not put that for the while statement instead?
while not balance_tested <= 0.01:
# etc.
while abs(balance_tested != 0):
will stay in the loop if the balance_tested becomes negative. Make this
while balance_tested >= 0.1:
This will automatically handle the rounding when it drops below a penny.