Code WORKS Math DOESN'T - c++

Question

My code compiles nicely, but the math formulas that I am using aren't providing the right outcome. I need to calculate the balance, withdrawn, and interest for all 3 months. I am also required to validate user's input. For these purposes I am using nested loops. Please let me know if you spot my mistake. Thank you lovely people!
cout << "Please enter the starting balance: ";
cin >> startBalance;
cout << "Please enter the annual interest rate: ";
cin >> annualInterest;
for (int month = 1; month <= 3; month++) {
do {
cout << setprecision(5);
cout << "Please enter the total amount deposited on month " << month << ": ";
cin >> balance;
if (balance <0) {
goodChoice = false;
cout << "\n\t***ERROR " << balance << " ***";
cout << "*** Choice must be positive***\n" << endl;
}
else {
goodChoice = true;
}
startBalance += balance; //need to calculate the balance for all 3 months
} while (!goodChoice);
do {
cout << setprecision(5);
cout << "Please enter the total amount withdrawn on " << month << ": ";
cin >> withdrawn;
if ( (withdrawn <0) || (withdrawn > startBalance) ) {
goodChoice = false;
cout << "***ERROR " << withdrawn << " ***";
cout << "*** Choice must be positive or greater than the balance***" << endl;
}
else {
goodChoice = true;
}
finalWithdrawn += withdrawn; // the total amount of withdrawn
finalBalance = startBalance - withdrawn;
monthInterest = ((startBalance + finalBalance) / 2) * (annualInterest / 12);
totalInterest += monthInterest; //total interest for all 3 months
endBalance = monthInterest + finalBalance;
} while (!goodChoice);
}
cout << "Total Deposit: " << endBalance << endl;
cout << "Total Withdrawn: " << finalWithdrawn << endl;
cout << "Total Interest: " << totalInterest << endl;
cout << "Final Balance: " << endBalance << endl;

Answer 1

You have a lot of extra variables defined which aren't needed. Also, your interest rate may have been in percentage instead of a decimal number, i.e. 10% = 0.1. Also, your monthInterest is taking an average then applying interest. I wrote this up and it seems to work.
#include <iostream>
#include <iomanip>
using namespace std;
int main()
{
float totalDeposit = 0.0f;
float totalWithdrawn = 0.0f;
float totalInterest = 0.0f;
float totalBalance = 0.0f;
float interestRate = 0.0f;
setprecision(5); //?
cout << "Enter starting balance: ";
cin >> totalBalance;
cout << "Enter annual interest rate (%): ";
cin >> interestRate;
// Convert to monthly and non-percent; i.e. 10% = 0.1 = 10 / 100
interestRate = interestRate / 100.0f / 12.0f;
for (int month = 1; month <= 3; month++)
{
float deposit = -1.0; // Default to an error state
while (deposit < 0.0)
{
cout << "Enter total deposited in month " << month << ": ";
cin >> deposit;
if (deposit < 0.0f)
{
cout << "ERROR: Invalid amount" << endl;
continue;
}
}
float withdrawn = -1.0f; // Default to an error state
while (withdrawn < 0.0f)
{
cout << "Enter total withdrawn in month " << month << ": ";
cin >> withdrawn;
if (withdrawn < 0.0f)
{
cout << "ERROR: Invalid amount" << endl;
continue;
}
}
totalDeposit += deposit;
totalWithdrawn += withdrawn;
totalBalance = totalBalance + deposit - withdrawn;
totalBalance += totalBalance * interestRate;
totalInterest += totalBalance * interestRate;
}
cout << "Total Deposit: " << totalDeposit << endl;
cout << "Total Withdrawn: " << totalWithdrawn << endl;
cout << "Total Interest: " << totalInterest << endl;
cout << "Final Balance: " << totalBalance << endl;
int wait; // Pause so console window doesn't close. Delete this line.
cin >> wait;
return 0;
}