Compound Interest

	Compound Interest

Introduction

This small application shows how to calculate the compound interest of a saving.

Prerequisites:

Dialog Boxes
Group Box
Static Text Control
Edit Box
Button

When you deposit money in a savings account, your money earns interest that is calculated every month or quarter, etc. Because this is not money you need right away, the amount accrued can be reinvested, thus boosting your interest so the next calculation would be based on the original amount plus the new added value. This is the basis of compound interest.

The compound interest can be calculated monthly or quarterly, etc based on the original amount you deposited, the interest rate, and the period you and the institution agreed upon.

Creating the Application

This application uses a dialog box equipped with the necessary controls used to perform the type or related calculated. The formula we will use to perform the calculations is as follows:

	P = Principal
	r = Annual (Interest) Rate
	m = Number of Compounding Periods per Year
	n = Total Number of Compounding Periods
	A = Amount Earned After n periods

Practical Learning: Starting the Exercise

Start Borland C++BuilderX and, on the main menu, click File -> New...
In the Object Gallery dialog box, click New GUI Application and click OK
In the New GUI Application Project Wizard - Step 1 of 3, in the Directory edit box of the Project Settings section, type the path you want. Otherwise, type
C:\Programs\Win32 Programming
In the Name edit box, type CompoundInterest
Click Next
In the New GUI Application Project Wizard - Step 2 of 3, accept the defaults and click Next
In the New GUI Application Project Wizard - Step 3 of 3, click the check box under Create
Select Untitled under the Name column header. Type Exercise to replace the name and press Tab
Click Finish
To create a resource header file, on the main menu, click File -> New File...
In the Create New File dialog box, in the Name, type resource
In the Type combo box, select h, and click OK

In the file, type:

#define IDD_MAIN_DLG                     101
#define IDC_PRINCIPAL                   1001
#define IDC_ANNUAL_RATE              1002
#define IDC_NBR_OF_PERIODS        1003
#define IDC_COMPOUND                  1004
#define IDC_QUATERLY                    1005
#define IDC_SEMIANNUALLY             1006
#define IDC_RADIO4                        1007
#define IDC_ANNUALLY                    1007
#define IDC_CALCULATE_BTN          1008
#define IDC_INTEREST_EARNED       1009
#define IDC_AMOUNT_EARNED         1010

To create a resource script, on the main menu, click File -> New File...
In the Create New File dialog box, in the Name, type CompoundInterest
In the Type combo box, select rc, and click OK

In the file, type:

#include "resource.h"

//---------------------------------------------------------------------------
// Dialog

IDD_MAIN_DLG DIALOGEX 200, 100, 329, 132
STYLE DS_SETFONT | DS_MODALFRAME | DS_FIXEDSYS | WS_POPUP | WS_CAPTION |
    WS_SYSMENU
CAPTION "Compound Interest"
FONT 8, "MS Shell Dlg" // , 400, 0, 0x1
BEGIN
    GROUPBOX        "Preparation",IDC_STATIC,6,6,156,72
    LTEXT           "Principal: ...............",IDC_STATIC,18,21,72,8
    EDITTEXT        IDC_PRINCIPAL,90,18,40,12,ES_RIGHT | ES_AUTOHSCROLL
    LTEXT           "Interest: ...............",IDC_STATIC,18,40,72,8
    EDITTEXT        IDC_ANNUAL_RATE,90,36,40,12,ES_RIGHT | ES_AUTOHSCROLL
    LTEXT           "Number of Periods: ...",IDC_STATIC,18,58,72,8
    EDITTEXT        IDC_NBR_OF_PERIODS,90,54,40,12,ES_RIGHT | ES_AUTOHSCROLL
    LTEXT           "%",IDC_STATIC,132,42,8,8
    LTEXT           "years",IDC_STATIC,132,54,19,8
    GROUPBOX        "Compound Frequency",IDC_STATIC,168,6,90,72
    CONTROL         "Monthly",IDC_COMPOUND,"Button",BS_AUTORADIOBUTTON |
                    BS_LEFTTEXT | WS_GROUP,180,20,60,10
    PUSHBUTTON      "Calculate",IDC_CALCULATE_BTN,270,9,50,14
    PUSHBUTTON      "Close",IDCANCEL,270,30,50,14
    CONTROL         "Quarterly",IDC_QUATERLY,"Button",BS_AUTORADIOBUTTON |
                    BS_LEFTTEXT,180,33,60,10
    CONTROL         "Semiannually",IDC_SEMIANNUALLY,"Button",
                    BS_AUTORADIOBUTTON | BS_LEFTTEXT,180,46,60,10
    CONTROL         "Annually",IDC_ANNUALLY,"Button",BS_AUTORADIOBUTTON |
                    BS_LEFTTEXT,180,59,60,10
    GROUPBOX        "Results",IDC_STATIC,6,84,252,36
    LTEXT           "Interest Earned:",IDC_STATIC,15,99,54,8
    EDITTEXT        IDC_INTEREST_EARNED,78,96,52,12,ES_RIGHT |
                    ES_AUTOHSCROLL
    LTEXT           "Amount Earned:",IDC_STATIC,144,99,53,8
    EDITTEXT        IDC_AMOUNT_EARNED,198,96,48,12,ES_RIGHT | ES_AUTOHSCROLL
END
//---------------------------------------------------------------------------

Display the Exercise.cpp file and change it as follows:

#include <windows.h>
#include <cstdio>
#include <cmath>
#include "Resource.h"

//---------------------------------------------------------------------------
HWND hWnd;
LRESULT CALLBACK DlgProc(HWND hWnd, UINT Msg, WPARAM wParam, LPARAM lParam);
//---------------------------------------------------------------------------
INT WINAPI WinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance,
				   LPSTR lpCmdLine, int nCmdShow)
{
	DialogBox(hInstance, MAKEINTRESOURCE(IDD_MAIN_DLG),
	          hWnd, reinterpret_cast<DLGPROC>(DlgProc));

	return FALSE;
}
//---------------------------------------------------------------------------
LRESULT CALLBACK DlgProc(HWND hWndDlg, UINT Msg, WPARAM wParam, LPARAM lParam)
{
	// These variables will carry the values in the text boxes
	LPTSTR strPrincipal = new char[20],
		  strInterest = new char[20], strPeriods = new char[20],
		   strInterestEarned = new char[20], strAmountEarned = new char[20];
	// These are handled for the various controls
	HWND hWndPrincipal, hWndInterest, hWndPeriods, hWndCompound,
		 hWndInterestEarned, hWndAmountEarned, hWndCalculate;

	double Principal, AnnualRate, InterestEarned;
	double FutureValue, RatePerPeriod;
	int    NumberOfPeriods, CompoundType;
	double i;
	int n;

	hWndPrincipal      = GetDlgItem(hWndDlg, IDC_PRINCIPAL);
	hWndInterest       = GetDlgItem(hWndDlg, IDC_ANNUAL_RATE);
	hWndPeriods        = GetDlgItem(hWndDlg, IDC_NBR_OF_PERIODS);
	hWndCompound       = GetDlgItem(hWndDlg, IDC_COMPOUND);
	hWndInterestEarned = GetDlgItem(hWndDlg, IDC_INTEREST_EARNED);
	hWndAmountEarned   = GetDlgItem(hWndDlg, IDC_AMOUNT_EARNED);
	hWndCalculate      = GetDlgItem(hWndDlg, IDC_CALCULATE_BTN);

	switch(Msg)
	{
	case WM_INITDIALOG:
		// Identify each control
		SetWindowText(hWndPrincipal, "0.00");
		SetWindowText(hWndInterest, "7.55");
		SetWindowText(hWndPeriods, "0");
		SetWindowText(hWndInterestEarned, "0.00");
		SetWindowText(hWndAmountEarned, "0.00");

		CheckRadioButton(hWndDlg, IDC_COMPOUND, IDC_ANNUALLY, IDC_COMPOUND);
		return TRUE;

	case WM_COMMAND:
		switch(wParam)
		{
		case IDC_CALCULATE_BTN:
			GetWindowText(hWndPrincipal, strPrincipal, 20);
			GetWindowText(hWndInterest,  strInterest,  10);
			GetWindowText(hWndPeriods,   strPeriods,    8);

			Principal = atof(strPrincipal);
			AnnualRate = atof(strInterest) / 100;

			if( IsDlgButtonChecked(hWndDlg, IDC_COMPOUND) == BST_CHECKED )
				CompoundType = 12;
			else if( IsDlgButtonChecked(hWndDlg, IDC_QUATERLY) == BST_CHECKED )
				CompoundType = 4;
			else if( IsDlgButtonChecked(hWndDlg, IDC_SEMIANNUALLY) == BST_CHECKED )
				CompoundType = 2;
			else // if( IsDlgButtonChecked(hWndDlg, IDC_ANNUALLY) == BST_CHECKED )
				CompoundType = 1;

			NumberOfPeriods = atoi(strPeriods);
			i = AnnualRate / CompoundType;
			n = CompoundType * NumberOfPeriods;

			RatePerPeriod = AnnualRate / NumberOfPeriods;
			FutureValue = Principal * pow(1 + i, n);
			InterestEarned = FutureValue - Principal;

			sprintf(strInterestEarned, "$%.2f", InterestEarned);
			sprintf(strAmountEarned,   "$%.2f", FutureValue);

			SetWindowText(hWndInterestEarned, strInterestEarned);
			SetWindowText(hWndAmountEarned,   strAmountEarned);
			return TRUE;

		case IDCANCEL:
			EndDialog(hWndDlg, 0);
			return TRUE;
		}
		break;
	}

	return FALSE;
}
//---------------------------------------------------------------------------

Test the application then close the dialog box


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