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Question: I'm creating a database application with Delphi and need a way to let my users enter percentage values easily. What can I do?

Answer: Use the TField's OnGetText() and OnSetText() events to convert the percentage value as it's being read from and saved to the underlying table, as shown in the following code sample:

procedure TForm1.tblItemsDiscountGetText(
   Sender: TField; var Text: String; DisplayText: Boolean );
begin
   Text := formatFloat( '0.0###%', Sender.AsFloat * 100.0 );
end;


procedure TForm1.tblItemsDiscountSetText(
   Sender: TField; const Text: String );
var
   dblValue : Double;
   strValue : String;
begin

   if rightStr( Text, 1 ) = '%' then
      strValue := trim( leftStr( Text, length( Text ) - 1 ) )
   else
      strValue := trim( Text );

   dblValue := strToFloat( strValue );

   if ( dblValue > 1.0 ) or ( dblValue < 0.01 ) then
      dblValue := dblValue / 100.0;

   Sender.AsFloat := dblValue;

end;

Note: This example assumes you're using Delphi 7 or later. If you're using an older version, you may need to use different functions from the StrUtils or SysUtils units. For more information, search Delphi Help for the "String handling routines" topic.

Understanding the Problem

From a design perspective, it's natural and appropriate to store percentages as floating point values; that is, a value of 20% is best stored as 0.20.

From a user's perspective, however, it can be difficult to remember to mentally convert the percentage to a decimal representation.

In this example, OnSetText() automatically converts string percentage values to their decimal equivalents. It does this by:

  1. Detecting and removing any trailing percentage symbols.

  2. Removing any white space that could trigger a conversion error.

  3. Determining if the resulting value is greater than 1.0 (100%) or less than 0.01 (1%). If so, the user's value is divided by 100.

As a result, the table contains a floating point value that accurately reflects the percentage value entered by the user. In turn, this lets your users focus on the values in front of them (20%) and not on their internal representations (0.20).

If your application requires percentages larger than 100%, you'll need to adjust OnSetText() accordingly. This may be more difficult than it seems at first glance; use care to ensure accuracy.

The preceding OnGetText() event supports the conversion process by converting the internal value to a percentage and then appending a percentage symbol. This reminds reminds users to enter percentages as numbers, e.g. as 20%, not 0.20.

Remember to remove the percentage symbols; otherwise, Delphi raises an EConvertError exception when trying to save the value.

Because this code is placed on the field object, no additional code is needed for your data entry forms.

By managing the conversion between the value entered by the user and the number actually stored in your database tables, you can create applications that are easier to use. In turn, this ensures your applications will be used more frequently and with less frustration.