Academic Project · C++

Loan Repayment Calculator

Console app that takes a principal, term, and interest rate — then spits out a full monthly schedule, classifies the loan type, and totals up everything you'll actually pay.

C++ Console App Math Logic Loops & I/O

What It Does

💰Takes principal, term & interest rate as input

📊Generates a full monthly repayment schedule

🏷️Classifies loan as Micro, Standard or Premium

✅Calculates total interest & service fee

Bugs Found & Fixed

FIXED No input validation — entering 0 months or a negative rate would cause a division by zero or infinite loop. Added guard checks before the loop.

FIXED Remaining balance goes negative — floating point rounding on the last month caused rb to show e.g. -0.000003. Fixed by clamping the final balance to 0.

FIXED No decimal formatting — output printed raw doubles like 1234.5 instead of 1234.50. Added fixed << setprecision(2) to all money outputs.

NOTE Simple interest model — the formula uses flat principal/months for each payment rather than the standard amortization formula. Correct for the assignment scope, but worth knowing if you extend it.

Source Code

loan_calculator.cpp

#include <iostream>
#include <string>
#include <iomanip>
using namespace std;

int main()
{
    double pa, air, mir, pp, rb, tip, tm, ip;
    string lt;
    int sf;
    tip = 0.0;

    cout << "Enter principal loan amount: ";
    cin >> pa;
    cout << "Enter your total Number of Monthly Payments: ";
    cin >> tm;
    cout << "Enter your annual interest rate: ";
    cin >> air;

    // BUG FIX: validate inputs before doing any math
    if (pa <= 0 || tm <= 0 || air < 0) {
        cout << "Error: please enter valid positive values.\n";
        return 1;
    }

    // BUG FIX: use fixed precision for all money output
    cout << fixed << setprecision(2);

    mir = (air / 100) / 12;
    pp  = pa / tm;
    rb  = pa;

    cout << "_____Repayment Schedule_____" << endl;
    cout << setw(8)  << left  << "Month"
         << setw(15) << right << "Interest"
         << setw(15) << right << "Principal"
         << setw(15) << right << "Total"
         << setw(15) << right << "Remaining" << endl;

    for (int m = 1; m <= tm; m++)
    {
        ip         = rb * mir;
        double tmp = pp + ip;
        rb         = rb - pp;
        tip        = tip + ip;

        // BUG FIX: clamp last-month rounding error to zero
        if (rb < 0.001 && rb > -0.001) rb = 0.0;

        cout << setw(8)  << left  << m
             << setw(15) << right << ip
             << setw(15) << right << pp
             << setw(15) << right << tmp
             << setw(15) << right << rb << endl;
    }

    cout << "\n_____Final Loan Summary_____" << endl;

    if (pa < 50000)
    {
        lt = "Micro Loan";
        sf = 100;
    }
    else if (pa <= 1000000)
    {
        lt = "Standard Loan";
        sf = 500;
    }
    else
    {
        lt = "Premium Loan";
        sf = 1000;
    }

    double tamountp = pa + tip + sf;

    cout << "Loan Classification:            " << lt       << endl;
    cout << "Mandatory Service Fee:          RS " << sf       << endl;
    cout << "Total Interest Paid:            RS " << tip      << endl;
    cout << "Total Amount Needed to Settle:  RS " << tamountp << endl;
}

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