formulas

[pnwadike99]

how would you translate this formula to work in c++ and can u explain why you did what you did because I am new to this.

(p * (1 + r) n * r) / ((1 + r) n - 1)

this is what I tried so far. do I need to split it into two variables?
p is the amount borrowed
• r is the monthly decimal interest rate
• n is the number of monthly payments in the payback period

  int amtBorrowed = 270000; //$
  double rateInterest = 5.125; //%
  int payPeriod = 30; //years
  double mustPay = (amtBorrowed * (1 + rateInterest)) payPeriod * rateInterest)) / (1 + rateInterest) payPeriod - 1);

  cout << fixed << setprecision(2) << mustPay <<endl;
  

[Repeater]

(amtBorrowed * (1 + rateInterest)) HERE payPeriod * rateInterest))
See where I've written HERE in that line? There's something missing there. I'm guessing you want that to multiply, so it should perhaps be:
(amtBorrowed * (1 + rateInterest)) * payPeriod * rateInterest))


Likewise (1 + rateInterest) HERE payPeriod - 1)


Basically, just write it out much more explicitly.


(p * (1 + r) * n * r) / (((1 + r) * n) - 1)

[zapshe]

The formula you have has an assumption of multiplication that C++ doesn't follow:

(1 + 2)(5 + 19)

In math, this is simply 3*24. In C++, this is an error. C++ does not recognize that you want to multiple the sum of 1+2 and 5+19. You need to do this:

(1 + 2)*(5 + 19)


Now, here's my interpretation of your formula in code. I've changed the variable names so it's easier:

int main()
{
	int p = 270000; //$
	double r = 5.125; //%
	int n = 30; //years
	double mustPay = (p * (1 + r) * n * r) / ((1 + r) * n - 1);

	cout << fixed << setprecision(2) << mustPay << endl;
}

I've only added a couple new '*' to make this work.


In your code, you'd added many parenthesizes that are unmatched.

[jonnin]

if you were really uptight you could try to rearrange it to do less work, eg tmp = n+n*r and then (p*tmp*r)/(tmp -1)
but this is not saving enough computation to be worth a lot of over-thinking it. Alternately if the equation has a 'standard form' that people recognize, use that form.

[pnwadike99]

Thank you very much!!

[pnwadike99]

mortgageCalculator1.cpp:34:103: error: expected ')' before 'payPeriod'
34 | double mustPay = (amtBorrowed * (1 + rateInterest) * payPeriod * rateInterest) / ((1 + rateInterest) payPeriod - 1));
| ~ ^~~~~~~~~~
|


the compiler is not recognizing the second part of the equation, (I've tried adding more parentheses but just the same error with different characters.)

[Ganado]

It's still the same issue that the others mentioned.

You have :
((1 + rateInterest) payPeriod - 1))

You want:
((1 + rateInterest) * payPeriod - 1))

Multiplication is not implicit in C++.

[pnwadike99]

YUP I just saw that I need to pay more attention to detail! thank you again.

[pnwadike99]

when i did the math on paper i got a different answer than my program, how can i find out why

int amtBorrowed = 270000; //$
float rateInterest = 5.125; //%
int payPeriod = 30; //years
double mustPay = (amtBorrowed * (1 + rateInterest) * payPeriod * rateInterest) / (1 + rateInterest) * payPeriod - 1;

the answer I got was = 1470.11
but my console gets = 1245374976.00

+++++++ i just needed to use "pow" for the exponents to get the math right.

[zapshe]

You probably followed a different order of operations compared to how you coded it.

[Ganado]

Interest rate is in %. You probably want to divide by 100.

Also, your current formula doesn't match your first post.
Hint: a / b + c == (a / b) + c, not a / (b + c).

[pnwadike99]

its almost there but

with this, i am getting 17814.95 but i want 1470.11

int amtBorrowed = 270000; //$
float rateInterest = 5.125 / 100; //%
int payPeriod = 30; //years
double mustPay = (amtBorrowed * pow(1 + rateInterest, payPeriod) * rateInterest) / (pow(1 + rateInterest, payPeriod) - 1);

including the pow function helped but
im not sure what to do from here, to get where i need to get next

[Ganado]

n is "the number of monthly payments in the payback period".
If the payback period is 30 years, and you do monthly payments, that is 30 * 12 = 360 payments.

And your interest rate is the monthly interest rate. The assignment's wording isn't exactly being clear here, so I'll just say you need to divide by 12.

[pnwadike99]

I was using the wrong conversions for my interest rate and pay period
the code works perfectly otherwise.

float rateInterest = 5.125 / 100; //%
int payPeriod = 30; //years

vs

float rateInterest = .05125 / 12; //monthly rate%
int payPeriod = 360; // (12 * 30) //(12 months per year for 30 years)