Number, Currency, Percentage Edit Control

Introduction

Simplify working with numbers, currency and percentage values in edit controls with the CNumEdit, CCurrencyEdit and CPercentageEdit classes referred to collectively in this article as NCPEdit.

Why is this helpful?

In a nutshell NCPEdit simplifies working with numbers, currency and percentage data by:

• Providing robust validation of user input. The precision and sign of the number are able to be stipulated and enforced;
• Alerting the user to invalid input as it is typed rather then when UpdateData() is called.
• Formatting the currency and percentage values appropriately once the edit control looses focus and removing the formatting when the edit control receives focus;
• Allowing you as the programmer to think in terms of numbers only. The burden of converting between strings and numbers is left to the classes to deal with.

How do you use it?

The screen shot shows how you might use NCPEdit in a simple loan repayment application. Note this example also includes basic serialization demonstrating how to get the data in and out of the NCPEdit controls.

To use the classes follow these steps:

Step 1

Add the CNumEdit, CCurrencyEdit and CPercentage source files to your project.

Step 2

Add a standard edit control to you dialog/form view.

Step 3

Use ClassWizard to add a Control member variable of type CNumEdit or CCurrencyEdit or CPercentageEdit for the edit control you previously added in Step 2. If ClassWizard will not let you do this select CEdit. If you needed to do the latter in your view class change the ClassWizard code added to either CNumEdit, CCurrencyEdit and Cpercentage as required. For example

public:
	//{{AFX_DATA(CNcpTestView)
	enum { IDD = IDD_NCPTEST_FORM };
	CCurrencyEdit	edit_loan_amount;

Step 4

Use ClassWizard to overide OnInitialUpdate for you view class. Initialise each of the NCPEdit controls. The prototypes for each of intialisation functions are self explanatory and are as follows:

void CNumEdit::Init(double fNumber, int dPrecision, bool bNegValues)
void CCurrencyEdit::Init(double	fNumber, int dPrecision,
                        bool bNeg_Values, TCHAR chCurrency_Symbol)
void CPercentageEdit::Init(double fNumber,int dPrecision, bool bNeg_Values)

For example

edit_loan_amount.Init(m_fLoan_Amount, 0, false, '$'); 

Step 5

To get and set the value in the NCPedit control call GetNumber() and SetNumber() functions.

m_fLoan_Amount =  edit_loan_amount.Get_Number();	
edit_repayment_amount.Set_Number(m_fRepayment_Amount);

That’s it!!!!

How does NCPedit work?

CCurrencyEdit and CPercentageEdit are both derived from CNumEdit. CNumEdit as you probably guessed subclasses good old CEdit. Refer to Chris Maunder’s article Create your own controls - the art of subclassing for the ins and outs of subclassing.

The flow of the control for all NCPEdit classes is relatively simple and can be summarised as follows:

When the NCPEdit control recieves focus OnSetfocus() is called and the edit control is updated with the unformated text.

void CNumEdit::OnSetfocus() 
{
	Set_EditCtrl_Text(m_strText);
}

As characters are inputed they are checked to ensure they valid.

void CNumEdit::OnChar(UINT nChar, UINT nRepCnt, UINT nFlags) 
{

// 1 - Check validity of character typed
	if(Validate_Char(nChar)!=true)
	{
		MessageBeep(0);
		return;
	}
	
// 2 - Standard OnChar processeing 
	CEdit::OnChar(nChar,nRepCnt,nFlags);
} 

Here the function Validate_Char(nChar) does most of the work.

When the control looses focus OnKillfocus() is called which does two main things. Firstly it calls Get_EditCtrl_Text() which

stores the data in its unformatted text form m_strText for use next time OnSetfocus() is called

and stores it also as a number in the m_fNumber member variable.

Secondly the function Update() is called which produces the formatted currency or percentage text storing as a (m_strCurrency or m_strPercentage) member variable. The edit control is then updated with this formatted text.

All of this isn’t too brain straining. Something which may be of interest however to programmers begining in C++ is the functions used to convert between numbers and strings and back again. These can be extremely useful and are presented below. For more information on the format string used in the format function of CString look up "format specification fields, printf" in MSDN.

double  CNumEdit::Text_To_Number(CString strText)
{
	return atof(strText);
}

CString CNumEdit::Number_To_Text(double fNumber)
{
// 1 - Prepare the format string 
//	   eg format string %.5f formats a double with the precision set 
//	   to five (5)
	CString strFormat;
	strFormat.Format("%d",m_dPrecision);
	strFormat="%."+strFormat+"f";

// 2 - Format the number to text
	CString strText;
	strText.Format(strFormat,fNumber);

	return strText;
}

What next?

Well NCPEdit may be quite simple but as always there is room for improvement. If you have any suggestions or find any bugs please let me know and I will try and correct them. Have fun!