We Keep Coding

Unsure why output is 0. Trying to count months to pay downpayment

print("Please enter you starting annual salary: ")
annual_salary = float(input())
monthly_salary = annual_salary/12
print("Please enter your portion of salary to be saved: ")
portion_saved = float(input())
print ("Please enter the cost of your dream home: ")
total_cost = float(input())
current_savings = 0
r = 0.04/12
n = 0
portion_down_payment = total_cost*int(.25)
if current_savings < portion_down_payment:
monthly_savings = monthly_salary*portion_saved
interest = monthly_savings*r
current_savings = current_savings + monthly_savings + interest
n =+ 1
else:
print(n)
The above is my code. I keep getting output = 0 but unsure why.
This the problem statement, I am a HS student attempting OCW coursework.
Call the cost of your dream home ​total_cost​.
Call the portion of the cost needed for a down payment ​portion_down_payment​. For simplicity, assume that portion_down_payment = 0.25 (25%).
Call the amount that you have saved thus far ​current_savings​. You start with a current savings of $0.
Assume that you invest your current savings wisely, with an annual return of ​r ​(in other words, at the end of each month, you receive an additional ​current_savings*r/12​ funds to put into your savings – the 12 is because ​r​ is an annual rate). Assume that your investments earn a return of r = 0.04 (4%).
Assume your annual salary is ​annual_salary​.
Assume you are going to dedicate a certain amount of your salary each month to saving for the down payment. Call that ​portion_saved​. This variable should be in decimal form (i.e. 0.1 for 10%).
At the end of each month, your savings will be increased by the return on your investment, plus a percentage of your ​monthly salary ​(annual salary / 12). Write a program to calculate how many months it will take you to save up enough money for a down payment. You will want your main variables to be floats, so you should cast user inputs to floats.
Your program should ask the user to enter the following variables:
The starting annual salary (annual_salary)
The portion of salary to be saved (portion_saved)
The cost of your dream home (total_cost)
Test Case 1
Enter your annual salary:​ 120000 Enter the percent of your salary to save, as a decimal:​ .10 Enter the cost of your dream home:​ 1000000 Number of months:​ 183

You have n =+ 1 but I think you mean n += 1
Also int(.25) evaluates to 0, I think you want int(total_cost*.25). As your code is, the if statement will always evaluate to False because current_savings == 0 and portion_down_payment == 0
More generally, when your code isn't working as expected, you should put in either print() or assert statements to narrow down where your code is deviating from what you expect. For example, before the if statement you could have it print the two values you are comparing.

future value of monthly investments calculator with user input - Python

I am trying to write a program that will calculate the future value of a monthly investment. Here is what I have so far:
def get_number(prompt, low, high):
while True:
number = float(input(prompt))
if number > low and number <= high:
is_valid = True
return number
else:
print("Entry must be greater than", low,
"and less than or equal to", high,
"Please try again.")
def get_integer(prompt, low, high):
while True:
number = int(input(prompt))
if number > low and number <= high:
is_valid = True
return number
else:
print("Entry must be greater than", low,
"and less than or equal to", high,
"Please try again.")
def calculate_future_value(monthly_investment, yearly_interest, years):
# convert yearly values to monthly values
monthly_interest_rate = ((yearly_interest / 100) + 1) ** (1 / 12)
months = years * 12
# calculate future value
future_value = 0.0
for i in range(1, months):
future_value += monthly_investment
monthly_interest = (future_value * monthly_interest_rate)-future_value
future_value += monthly_interest
return future_value
def main():
choice = "y"
while choice.lower() == "y":
# get input from the user
monthly_investment = get_number("Enter monthly investment:\t", 0, 1000)
yearly_interest_rate = get_number("Enter yearly interest rate:\t", 0, 15)
years = get_integer("Enter number of years:\t\t", 0, 50)
# get and display future value
future_value = calculate_future_value(
monthly_investment, yearly_interest_rate, years)
print("Future value:\t\t\t" + str(round(future_value, 2)))
# see if the user wants to continue
choice = input("Continue? (y/n): ")
print("Bye!")
if __name__ == "__main__":
main()
Everything in the program is working fine except for the def calculate_future_value(monthly_investment, yearly_interest, years): section I believe I have a logic error but I can't find exactly what's going wrong.
The output should look like this
Enter monthly investment: 350
Enter yearly interest rate: 10
Enter number of years: 36
Future value: 1484636.15
Continue? (y/n): n
Bye!
But im getting
Enter monthly investment: 350
Enter yearly interest rate: 10
Enter number of years: 36
Future value: 1312573.73
Continue? (y/n): no
Bye!

In testing out your code, I believe you really only need to correct one formula. The following statement does not appear to be correct.
monthly_interest_rate = ((yearly_interest / 100) + 1) ** (1 / 12)
That would appear to raise your yearly interest rate to the "1/12th" power and not divide it into 1/12th of the yearly interest. I revised that formula as follows:
monthly_interest_rate = ((yearly_interest / 12 / 100) + 1)
When I ran the program the value came out close to what you noted in your issue.
#Dev:~/Python_Programs/Investment$ python3 Invest.py
Enter monthly investment: 350
Enter yearly interest rate: 10
Enter number of years: 36
Future value: 1472016.43
Continue? (y/n): n
Bye!
The difference between this value and the value you stated might be attributable to actually having the interest compounded daily each month.
But you might give this tweak a try and see if it meets the spirit of your project.

Modification of Future Value

For this you have to add the annual contribution to the beginning of the year (the principal total) before computing interest for that year.
I am stuck and need help. This is what I have so far:
def main():
print("Future Value Program - Version 2")
print()
principal = eval(input("Enter Initial Principal:"))
contribution = eval(input("Enter Annual Contribution:"))
apr = eval(input("Enter Annual Percentage Rate (decimal):"))
yrs = eval(input("Enter Number of Years:"))
for k in range (1, yrs):
principal= principal * (1 + apr)
print()
print( yrs,) ": Amount $", int(principal * 100 + 0.5)/100)
main()
It is supposed to look like this:
Future Value Program - Version 2
Enter Initial Principal: 1000.00
Enter Annual Contribution: 100.00
Enter Annual Percentage Rate (decimal): 0.034
Enter Number of Years: 5
Year 1: Amount $ 1137.4
Year 2: Amount $ 1279.47
Year 3: Amount $ 1426.37
Year 4: Amount $ 1578.27
Year 5: Amount $ 1735.33
The value in 5 years is $ 1735.33

Here's a working example that produces the expected output:
def main():
print("Future Value Program - Version 2")
print()
principal = float(input("Enter Initial Principal: "))
contribution = float(input("Enter Annual Contribution: "))
apr = float(input("Enter Annual Percentage Rate (decimal): "))
yrs = int(input("Enter Number of Years: "))
print()
for yr in range(1, yrs + 1):
principal += contribution
principal = int(((principal * (1 + apr)) * 100) + 0.5) / 100
print("Year {0}: Amount $ {1}".format(yr, principal))
print()
print("The value in {0} years is $ {1}".format(yrs, principal))
if __name__ == '__main__':
main()
There were a few problems with the example in the question:
A syntax error in the print statement on line 12. Calling print with parens means all the arguments should be enclosed inside the parens. Python interpreted the errant paren as the end of arguments passed to print.
As noted by others, you shouldn't call eval on the inputs. Call float for floating point numbers, int for integers.
The range operator had an off by one error.
As noted by others, print is called outside of the loop, so intermediate states of the principal aren't output.
As far as basic maths, it seems as though adding the contribution was left out.
As per the expected output, the final print was missing.

Calculating Compound Interest using Python functions

Write a function that computes the balance of a bank account with a given initial balance and interest rate, after a given number of years. Assume interest is compounded yearly.
I am having the error " ValueError: unsupported format character 'I' (0x49) at index 28"
Here is my code so far.
def BankBalance():
InputB = 1000
return InputB
print("Your initial balance is $1000")
def Interest():
InputI = 0.05
return InputI
print("The rate of interest is 5%")
def CountNumber():
InputN = float(input("Please enter the number of times per year you would like your interest to be compounded: "))
return InputN
def Time():
InputT = float(input("Please enter the number of years you need to compund interest for:"))
return InputT
def Compount_Interest(InputB, InputI, InputT, InputN):
Cinterest = (InputB*(1+(InputI % InputN))**(InputN * InputT))
print("The compound interest for %.InputT years is %.Cinterest" %Cinterest)
B = BankBalance()
I = Interest()
N = CountNumber()
T = Time()
Compount_Interest(B, I, N, T)

Here is how you would do it.
def main():
# Getting input for Balance
balance = float(input("Balance: $ "))
# Getting input for Interest Rate
intRate = float(input("Interest Rate (%) : "))
# Getting input for Number of Years
years = int(input("Years: "))
newBalance = calcBalance(balance, intRate, years)
print ("New baance: $%.2f" %(newBalance))
def calcBalance(bal, int, yrs):
newBal = bal
for i in range(yrs):
newBal = newBal + newBal * int/100
return newBal
# Program run
main()

You are trying to use your variable as a function.
Try this instead :
Cinterest = (InputB * (1+(InputI % InputN))**(InputN * InputT))

Python, and most other programming languages, don't assume that two adjacent mathematical expressions with no operator between them means multiplication. You are missing a multiplication operator (*) between InputB and the rest of the expression:
Cinterest = (InputB * (1+(InputI % InputN))**(InputN * InputT))
# Here -------------^

Python Bisection search overshoots target

I have to create a code to find the exact payment, to the cent, needed to pay off a loan in 12 months using bisection. The code I created for this works but it overshoots it's target. The loan will be payed off within the 12 months but after making 12 payments the final balance should be around 0. However it is a way bigger negative number.
The code I am using is:
StartBalance = float(raw_input('Credit Balance in $: '))
AnnualRate = float(raw_input('Annual interest rate in decimals: '))
MonthlyRate = AnnualRate / 12.0
MinPaymentLow = StartBalance / 12.0
MinPaymentHigh = (StartBalance*(1+MonthlyRate)**12.0)/12.0
cent = 0.01
Payment = (MinPaymentHigh+MinPaymentLow)/2.0
while (Payment*12-StartBalance) >= cent:
for month in range(0, 12):
Balance = (StartBalance-Payment)/10*(1+MonthlyRate)
if Balance < 0:
MinPaymentLow = Payment
elif Balance > 0:
MinPaymentHigh = Payment
Payment = (MinPaymentHigh + MinPaymentLow)/ 2.0
print 'RESULT'
print 'Number of months needed: 12'
print 'Montly pay: $', round(Balance,2)

It looks like these lines:
for month in range(0, 12):
Balance = (StartBalance-Payment)/10*(1+MonthlyRate)
Should be:
Balance = StartBalance
for month in range(0, 12):
Balance = (Balance-Payment) * (1 + MonthlyRate)
Or something similar, depending on how you want to calculate interest, and whether you consider payments occurring at the start or end of the month.

Your code seemed to work fine for me, but if you're getting results that are "way off" it's probably because you're using the float datatype. Float is untrustable, because it rounds on every operation. Given enough iterations and you've rounded off a lot of money. Try using decimal instead. This module keeps track of decimals as indexed integer values.