C++: Building functions -

i having trouble understanding why function won't work. i've looked @ while loop several times, don't see why program isn't executing correctly. how stop while loop going off infinitely?

i trying create program tells user how long it'll take pay off loan , needs paid on last month. user types in loan being borrowed, interest , money s/he intends pay each month.

for example, borrowed $100 @ 12% annual interest. first month have pay $100*0.01 = $1. let pay $50 per month new balance 100 + 1 - 50 = $51. have pay 1% of $0.51. new balance 51 + 0.51 - 50 = $1.51 , keep going until it's paid off.

this code looks like:

#include <iostream> using namespace std; void get_input(double &principal, double &interest, double &payment);  int main() {     double money, i, pay;       cout << "how want borrow ?";     cin >> money;     cout << "what annual interest rate expressed percent?";     cin >> i;     cout << "what monthly payment amount?";     cin >> pay;      = i/100;       get_input(money, i, pay);  }  void get_input(double &principal, double &interest, double &payment) {     double totalpayment, add = 0;     int months = 1;  while(principal>0) {     add = principal*interest;     principal = principal + add - payment;     months++; }      totalpayment = payment+principal;     cout << "the last month debt paid off is: " << months << endl;     cout << "your final payment is: " << totalpayment << endl;   } 

you need divide percentages 100 before use them:

while(principal>0) {     add = principal*interest;//this multiplies number 18 instead of .18     principal = principal + add - payment;     months++; } 

you need multiply percentage - , divide 100.

add=principal*interest/100.0; 

additionally, if pay 18% monthly interest on $1000 dollar loan, better pay more $180 month if ever want pay off. try input makes sense (like example) or add code test whether remaining debt has gone down.