Finding yearly interest out of compound interest

steven6706 (1)
When I try to find out my yearly interest, I get the wrong answer. The first year calculation works just fine but for the next year interest doesn't print out the right answer or goes off by 0.01. What is the matter?

// Compound Interest
#include <iostream>
#include <string>
#include <iomanip>
using namespace std;

void get_user_input(float &principal, float &interest_rate, int &first_year, int &first_month, int &total_months)
{
cout << "Enter the starting principal: ";
cin >> principal;
cout << "Enter the current interest rate (Ex. 0.09 for 9%): ";
cin >> interest_rate;
do
{
cout << "Enter the first year: ";
cin >> first_year;
if ((first_year < 1900) || (first_year > 2050))
{
cout << "Invalid year.\n";
cin.clear();
cin.ignore(100, '\n');
}
} while ((first_year < 1900) || (first_year > 2050));

do
{
cout << "Enter the first month (1 for Jan., 2 for Feb., etc...): ";
cin >> first_month;
if ((first_month < 1) || (first_month > 12))
{
cout << "Invalid month.\n";
cin.clear();
cin.ignore(80, '\n');
}
} while ((first_month < 1) || (first_month > 12));

cout << "Enter the total number of months: ";
cin >> total_months;
cin.ignore(80, '\n');
}

void print_date(int year, int month)
{
switch (month)
{
case 1:
cout << "Jan";
break;
case 2:
cout << "Feb";
break;
case 3:
cout << "mar";
break;
case 4:
cout << "Apr";
break;
case 5:
cout << "May";
break;
case 6:
cout << "Jun";
break;
case 7:
cout << "Jul";
break;
case 8:
cout << "Aug";
break;
case 9:
cout << "Sep";
break;
case 10:
cout << "Oct";
break;
case 11:
cout << "Nov";
break;
case 12:
cout << "Dec";
break;
}
cout << " " << year << ": ";
}

void calculate_principal(float &principal, float interest_rate)
{
principal = principal + (principal * (interest_rate / 12));
}
void interest(float first_principal, float principal, float &interest_earned)
{
interest_earned = principal - first_principal;
}

int main()
{
float first_principal, next_principal = 0.0 , principal, interest_rate, interest_earned, total_interest = 0.0, year_interest = 0.0;
int first_year, first_month, total_months, month_count, current_year, current_month;
char WaitTemp[2];

get_user_input(principal, interest_rate, first_year, first_month, total_months);


current_year = first_year;
current_month = first_month;
first_principal = principal;
next_principal = principal;


cout.setf(ios::fixed);
cout.setf(ios::showpoint);
for (month_count = 0; month_count < total_months; month_count++)
{
if (current_month != first_month)
{
calculate_principal(principal, interest_rate);
first_month = 0;
}
interest(next_principal, principal, interest_earned);
print_date(current_year, current_month);
cout << "$" << setprecision(2) << principal << " with the $" << setprecision(2) << interest_earned << " earned this month. \n";

if (current_month < 12)
{
current_month++;
next_principal = principal;
total_interest += interest_earned;
year_interest += interest_earned;
}


else
{
total_interest += interest_earned;
year_interest += interest_earned;
cout << "The yearly interest earned in is $" << year_interest << endl;
cout << "\nPress Enter to Continue... \n";
cin.get(WaitTemp, 0);
cin.ignore(80, '\n');
current_month = 1;
current_year++;
next_principal = principal;
first_principal = principal;
year_interest *= 0;
cin.clear();
cin.ignore(80, '\n');
}
}
cout << "The total interest that you earned in " << month_count << "months is/are : $" << total_interest << endl;
return 0;
}
Last edited on
salem c (3700)
Use [code][/code] tags when you post code (it's the <> format icon on the right).
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#include <iostream>
#include <string>
#include <iomanip>
using namespace std;

void get_user_input(float &principal, float &interest_rate, int &first_year,
                    int &first_month, int &total_months)
{
  cout << "Enter the starting principal: ";
  cin >> principal;
  cout << "Enter the current interest rate (Ex. 0.09 for 9%): ";
  cin >> interest_rate;
  do {
    cout << "Enter the first year: ";
    cin >> first_year;
    if ((first_year < 1900) || (first_year > 2050)) {
      cout << "Invalid year.\n";
      cin.clear();
      cin.ignore(100, '\n');
    }
  } while ((first_year < 1900) || (first_year > 2050));

  do {
    cout << "Enter the first month (1 for Jan., 2 for Feb., etc...): ";
    cin >> first_month;
    if ((first_month < 1) || (first_month > 12)) {
      cout << "Invalid month.\n";
      cin.clear();
      cin.ignore(80, '\n');
    }
  } while ((first_month < 1) || (first_month > 12));

  cout << "Enter the total number of months: ";
  cin >> total_months;
  cin.ignore(80, '\n');
}

void print_date(int year, int month)
{
  switch (month) {
  case 1:
    cout << "Jan";
    break;
  case 2:
    cout << "Feb";
    break;
  case 3:
    cout << "mar";
    break;
  case 4:
    cout << "Apr";
    break;
  case 5:
    cout << "May";
    break;
  case 6:
    cout << "Jun";
    break;
  case 7:
    cout << "Jul";
    break;
  case 8:
    cout << "Aug";
    break;
  case 9:
    cout << "Sep";
    break;
  case 10:
    cout << "Oct";
    break;
  case 11:
    cout << "Nov";
    break;
  case 12:
    cout << "Dec";
    break;
  }
  cout << " " << year << ": ";
}

void calculate_principal(float &principal, float interest_rate)
{
  principal = principal + (principal * (interest_rate / 12));
}

void interest(float first_principal, float principal, float &interest_earned)
{
  interest_earned = principal - first_principal;
}

int main()
{
  float first_principal, next_principal =
      0.0, principal, interest_rate, interest_earned, total_interest =
      0.0, year_interest = 0.0;
  int first_year, first_month, total_months, month_count, current_year,
      current_month;
  char WaitTemp[2];

  get_user_input(principal, interest_rate, first_year, first_month,
                 total_months);


  current_year = first_year;
  current_month = first_month;
  first_principal = principal;
  next_principal = principal;


  cout.setf(ios::fixed);
  cout.setf(ios::showpoint);
  for (month_count = 0; month_count < total_months; month_count++) {
    if (current_month != first_month) {
      calculate_principal(principal, interest_rate);
      first_month = 0;
    }
    interest(next_principal, principal, interest_earned);
    print_date(current_year, current_month);
    cout << "$" << setprecision(2) << principal << " with the $" <<
        setprecision(2) << interest_earned << " earned this month. \n";
    if (current_month < 12) {
      current_month++;
      next_principal = principal;
      total_interest += interest_earned;
      year_interest += interest_earned;
    }
    else {
      total_interest += interest_earned;
      year_interest += interest_earned;
      cout << "The yearly interest earned in is $" << year_interest << endl;
      cout << "\nPress Enter to Continue... \n";
      cin.get(WaitTemp, 0);
      cin.ignore(80, '\n');
      current_month = 1;
      current_year++;
      next_principal = principal;
      first_principal = principal;
      year_interest *= 0;
      cin.clear();
      cin.ignore(80, '\n');
    }
  }
  cout << "The total interest that you earned in " << month_count <<
      "months is/are : $" << total_interest << endl;
  return 0;
}


You might want to look at line 138.
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