I am new to python, I will really appreciate it if someone can help me find the errors in my code. Thank you in advance.

The problem asks us to find the best rate of savings to achieve a down payment on a house in 36 months. Since hitting this exactly is a challenge, the answer only needs to be within $100 of the required down payment.

To simplify things, assume:

1. Your semiannual raise is .07 (7%)
2. Your investments have an annual return of 0.04 (4%)
3. The down payment is 0.25 (25%) of the cost of the house
4. The cost of the house that you are saving for is $1M.

Here is my original code:

annual_salary=float(input("Enter your annual salary:"))
semi_annual_raise=0.07
total_cost=1000000
portion_down_payment = 0.25
current_saving=0
r=0.04
low=0
high=10000
guess=(low high)/2
numGuesses=0
while abs(portion_down_payment*total_cost-current_saving)>=100:
    current_saving=0
    portion_saved=guess/10000
    for i in range (36):
        current_saving =current_saving*r/12 annual_salary/12*portion_saved
        if i%6==0 and i>0:
            annual_salary =annual_salary*semi_annual_raise
    
    if portion_down_payment*total_cost-current_saving>0:
        low=guess
    else:
        high=guess
    guess=(low high)/2
    numGuesses =1   
    if numGuesses > 1000:
        break

if numGuesses>1000:
    print("It is not possible to pay the down payment in three years.")
else:
    rate=guess/10000
    print("steps in bisection search is",numGuesses,"and the best saving rate is",rate)

When 150000 was entered as annual_salary, the result is:

It is not possible to pay the down payment in three years.

Because it wouldn’t return the correct answer, I used a function to calculate the curring saving and the code is shown below:

annual_salary=float(input("Enter your annual salary:"))
semi_annual_raise=0.07
total_cost=1000000
portion_down_payment = 0.25
r=0.04
low=0
high=10000
guess=(low high)/2
numGuesses=0

def current_saving(annual_salary,guess,months):
    portion_saved=guess/10000
    saving=0
    r=0.04
    semi_annual_raise=0.07
    for i in range (months):
        saving =saving*r/12 annual_salary/12*portion_saved
        if i%6==0 and i>0:
            annual_salary =annual_salary*semi_annual_raise
    return(saving)
total=0
while abs(portion_down_payment*total_cost-total)>=100:
    total=current_saving(annual_salary,guess,36)
    if portion_down_payment*total_cost-total>0:
        low=guess
    else:
        high=guess
    guess=(low high)/2
    numGuesses =1   
    if numGuesses > 1000:
        break

if numGuesses>1000:
    print("It is not possible to pay the down payment in three years.")
else:
    rate=guess/10000
    print("steps in bisection search is",numGuesses,"and the best saving rate is",rate)
        

When 150000 was entered as annual_salary this time, the result is:

steps in bisection search is 7 and the best saving rate is 0.44140625

I feel the two versions basically do the same thing, how come the first one wouldn't work but the second do? I assume there is something wrong within the while loop, but I could not pinpoint my error. Thank you so much again for any help!

CodePudding user response：

In the version that does not work, you do not re-set annual_salary on each iteration of the while loop.

In the version that does work, annual_salary the local variable is reset each time your method is called.

to make the broken one work:

## save the original value
annual_salary_original = annual_salary

while abs(portion_down_payment * total_cost - current_saving) >= 100:
    ## reset salary on each iteration
    annual_salary = annual_salary_original
    ...