Variable not changing after its modified in C++ - c++

Question

I am trying to run balN with the variable N changing as I change it in the program but balN is only running it with the value that was assigned with N at the very beginning. How can I make it so balN will change as the value of N changes? I am new to programming so any help will be appreciated. Thanks!
const double Monintrate = 0.09 / 12.0;
const double Totnum = 36.0;
const double Payment = 165.25;
int main ()
{
double N = 1.0;
double balN = Payment * (1 - pow(1 + Monintrate, N - Totnum))/Monintrate;
cout << fixed;
cout << setprecision(2) << "Monthly Payment: $" << Payment << endl;
cout << "Annual Interest rate: 9%" << endl;
cout << "Total Number of Payments: " << Totnum << endl << endl;
cout << "Balance after Payment:" << endl;
cout << "1: $" << balN << endl;
N++;
cout << "2: $" << balN << endl;
N++;
cout << "3: $" << balN << endl;
}

Answer 1

Because in your program, you only modified the value of N, not balN, you need to call
balN = Payment * (1 - pow(1 + Monintrate, N - Totnum))/Monintrate;
every time you modified N.
cout << "1: $" << balN << endl;
N++;
balN = Payment * (1 - pow(1 + Monintrate, N - Totnum))/Monintrate;
cout << "2: $" << balN << endl;
N++;
balN = Payment * (1 - pow(1 + Monintrate, N - Totnum))/Monintrate;
cout << "3: $" << balN << endl;

Answer 2

You never reassign balN with the new values depending on N.
You should reassign it every time you change N:
cout << "1: $" << balN << endl;
N++;
balN = Payment * (1 - pow(1 + Monintrate, N - Totnum))/Monintrate;
cout << "2: $" << balN << endl;
N++;
balN = Payment * (1 - pow(1 + Monintrate, N - Totnum))/Monintrate;
cout << "3: $" << balN << endl;
Maybe a loop here can be a solution to avoid code repetition:
for ( int i = 1; i <= 3; i++ )
{
balN = Payment * (1 - pow(1 + Monintrate, N - Totnum))/Monintrate;
cout << i << ": $" << balN << endl;
N++;
}
Or Use a function to do that:
double calcBalN( double N )
{
return Payment * (1 - pow(1 + Monintrate, N - Totnum))/Monintrate;
}
int main ()
{
// ...
cout << "1: $" << calcBalN( N ) << endl;
N++;
cout << "2: $" << calcBalN( N ) << endl;
N++;
cout << "3: $" << calcBalN( N ) << endl;
//...
}

Answer 3

double balN = Payment * (1 - pow(1 + Monintrate, N - Totnum))/Monintrate;
The expression at the right side of the assignment will be evaluated and the result will be store in balN.
The value of balN won't be modified if you don't reassign a new value to it, even if the value of N is modified.
What you could do is define your calculation in a function and update balN with it :
double process(double N) {
const double Monintrate = 0.09 / 12.0;
const double Totnum = 36.0;
const double Payment = 165.25;
return Payment * (1 - pow(1 + Monintrate, N - Totnum))/Monintrate;;
}
int main ()
{
double N = 1.0;
double balN = process(N);
cout << fixed;
cout << setprecision(2) << "Monthly Payment: $" << Payment << endl;
cout << "Annual Interest rate: 9%" << endl;
cout << "Total Number of Payments: " << Totnum << endl << endl;
cout << "Balance after Payment:" << endl;
cout << "1: $" << balN << endl;
N++;
balN = process(N);
cout << "2: $" << balN << endl;
N++;
balN = process(N);
cout << "3: $" << balN << endl;
}

Answer 4

Make balN a function. C language does not support reactive programming otherwise you could have benefitted with your code. Following code will help.
const double Monintrate = 0.09 / 12.0;
const double Totnum = 36.0;
const double Payment = 165.25;
double balN (double N)·
{
double bal = Payment * (1 - pow(1 + Monintrate, N - Totnum))/Monintrate;
return bal;·
}
int main ()
{
double N = 1.0;
cout << fixed;
cout << setprecision(2) << "Monthly Payment: $" << Payment << endl;
cout << "Annual Interest rate: 9%" << endl;
cout << "Total Number of Payments: " << Totnum << endl << endl;
cout << "Balance after Payment:" << endl;
cout << "1: $" << balN() << endl;
N++;
cout << "2: $" << balN() << endl;
N++;
cout << "3: $" << balN() << endl;
}