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VCL Controls:
The Database Radio Group Control

Introduction

When creating a database object such as a table, you may provide a column that allows a user to set a single value from a small list of values. For example, you can create a column that would be used to specify the gender of an applicant. Instead of letting the user enter either Female or Male, you can create the options yourself and then let the user only select the desired one. To implement this scenario in a Windows application, you can use radio buttons.

To support radio buttons in a database, the VCL provides the DBRadioGroup control. The DBRadioGroup control primarily behaves like a RadioGroup control. In fact, both descend from the TCustomRadioGroup class. To use a DBRadioGroup in your application, you can click it from the Data Controls section of the Component Palette and click the desired container.

Like the RadioGroup control, the radio buttons of a DBRadioGroup object are represented by the Items property that is a list based on the TStrings class. This also means that, when creating a table for your application, you can create a string-based column and you would later on associate it to a DBRadioGroup object. You probably already know that if you create a text-based column, it can have just any value. If you intend to associated such a column to a DBRadioGroup control, it is your responsibility to make sure that the list of possible strings for this column should (must) be limited. In fact, when planning your table, you should make sure that the user will not be allowed to create new options. For example, if you create a column that you anticipate to have a fixed list of strings, such as Female and Male, keep in mind that you may not allow the user to add new strings such as Unknown or Hermaphrodite.

Practical Learning Practical Learning: Introducing the DB Radio Group Control
  1. To start a database application, on the main menu, click Tools -> Database Desktop
  2. On the main menu of the Database Desktop, click File -> New -> Table...
  3. Accept the Paradox 7 option and click OK
  4. Create the fields as follows:
     
    Field Name Type Size Key
    LoanPrepID +   *
    PreparedBy A 50  
    PreparedFor A 50  
    Description M 240  
    Principal A 12  
    InterestRate A 12  
    Periods A 10  
    CompoundFrequency A 20  
    InterestEarned A 12  
    AmountPaid A 12  
     
  5. To save the table, click the Save As button
  6. Set the name to LoanPreparation and select the BCDEMOS alias
     
  7. Click Save
  8. Close the Database Desktop
  9. In Borland C++ Builder, create a new Application
  10. Set the form's Caption to Watts A Loan
  11. Save the application in a new folder named WattsALoan1
  12. Save the unit as Exercise and the project as WattsALoan
  13. In the BDE section of the Component Palette, click Table and click the form
  14. In the Object Inspector, set its properties as follows:
    DatabaseName: BCDEMOS
    Name: tblLoanPrep
    TableName: LoanPreparation
  15. In the Data Access section of the Component Palette, click DataSource and click the form
  16. In the Object Inspector, set its properties as follows:
    DataSet: tblLoanPrep
    Name: dsLoanPrep
  17. Design of the form as follows:
     
     
    Control Name Caption Other Properties
    Form frmMain Watts A Loan BorderIcons: [biSystemMenu,biMinimize]
    Position: poScreenCenter
    ShowHint: true
    GroupBox   Preparation  
    Label   Prepared By:  
    DBEdit dbePreparedBy   DataSource: dsLoanPrep
    DataField: PreparedBy
    Label   Prepared For:  
    DBEdit dbePreparedFor   DataSource: dsLoanPrep
    DataField: PreparedFor
    Label   Description:  
    DBEdit dbeDescription   DataSource: dsLoanPrep
    DataField: Description
    GroupBox   Values  
    Label   Principal  
    DBEdit dbePrincipal   DataSource: dsLoanPrep
    DataField: Principal
    Label   Interest Rate:  
    DBEdit dbeInterestRate   DataSource: dsLoanPrep
    DataField: InterestRate
    Label   %  
    Label   Periods:  
    DBEdit dbePeriods   DataSource: dsLoanPrep
    DataField: Periods
  18. Save all

Characteristics of the DB Radio Group Control

If you add a DBRadioGroup control to your application, at design time, you should create a list of the available options yourself. To do this, display the String List Editor from double-clicking the Strings field of the Items property in the Object Inspector.

As described above, each item of the radio buttons of a DBRadioGroup control is a member of a TStrings collection and therefore is of type AnsiString. Such an item is represented by the Value property of this control. This means that, when the user selects a radio button, its Value, not its TStrings::ItemIndex property, gets stored in the corresponding column of the table. This also implies the the value of the record is saved as a string. This value is the caption of the radio button, as you would have created it using the Items property. It is important to know that you can use the properties of the TStrings class to identity the radio button that the user had clicked but when the record is saved, it is the Value, which is a string of type AnsiString, which is the caption of the radio button, that is saved in the field, not the ItemIndex.

While each radio button holds Value string, the group of values of the radio buttons is stored in the Values property of the DBRadioGroup control.

 

Practical Learning Practical Learning: Using a DB Radio Group Control
  1. In the Component Palette, click the DBRadioGroup control and click the empty lower-right section of the form
  2. While the new control is still selected, in the Object Inspector, click Items and click its ellipsis button
  3. Complete the list with Monthly, Quarterly, Semiannually, and Annually
     
  4. Click OK
  5. Complete the design of the form as follows:
     
    Watts A Loan - Form Design
     
    Control Name Caption Other Properties
    DBRadioGroup grpFrequency Compound Frequency DataSource: dsLoanPrep
    DataField: CompoundFrequency
    GroupBox   Results  
    Label   Interest Earned:  
    DBEdit dbeInterestEarned   DataSource: dsLoanPrep
    DataField: InterestEarned
    Label   Amount Paid:  
    DBEdit dbeAmountEarned    
    BitBtn btnCalculate Calculate  
    DBNavigator     DataSource: dsLoanPrep
    BitBtn     Kind: bkClose
  6. Double-click the Calculate button and implement its event as follows:
      
    //---------------------------------------------------------------------------
#include <vcl.h>
#include <math.h>
#pragma hdrstop

#include "Exercise.h"
//---------------------------------------------------------------------------
#pragma package(smart_init)
#pragma resource "*.dfm"
TfrmMain *frmMain;
//---------------------------------------------------------------------------
__fastcall TfrmMain::TfrmMain(TComponent* Owner)
    : TForm(Owner)
{
}
//---------------------------------------------------------------------------
void __fastcall TfrmMain::btnCalculateClick(TObject *Sender)
{
    double Principal, InterestRate, InterestEarned,
           AmountPaid;
    int Periods, CompoundType;

    Principal = this->dbePrincipal->Text.ToDouble();
    InterestRate = this->dbeInterestRate->Text.ToDouble() / 100;
    switch( this->grpFrequency->ItemIndex )
    {
    case 0:
        CompoundType = 12;
        break;
    case 1:
        CompoundType = 4;
        break;
    case 2:
        CompoundType = 2;
        break;
    case 3:
        CompoundType = 1;
        break;
    }

    Periods = this->dbePeriods->Text.ToInt();
	double i = InterestRate / CompoundType;
	int n = CompoundType * Periods;

	AmountPaid     = Principal * pow(1 + i, n);
	InterestEarned = AmountPaid - Principal;

	this->dbeInterestEarned->Text = FloatToStrF(InterestEarned, ffFixed, 8, 2);
	this->dbeAmountPaid->Text     = FloatToStrF(AmountPaid, ffFixed, 8, 2);
}
//---------------------------------------------------------------------------
  7. Return to the form.
    Set the table's Active property to true
     
  8. Execute the application and create a loan as follows and click the Calculate button:
     
  9. Save the Post Edit button
  10. Click the Close button
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