Project Structure

• Calc.dpr
• CalcF.pas
• CalcF.dfm

program Calc;

uses
  Forms,
  CalcF in 'CalcF.pas' {CalcForm};

{$R *.RES}

begin
  Application.Initialize;
  Application.CreateForm(TCalcForm, CalcForm);
  Application.Run;
end.

unit CalcF;

interface

uses
  SysUtils, Windows, Messages, Classes, Graphics, Controls,
  StdCtrls, Forms, DBCtrls, DB, DBGrids, DBTables, Grids, ExtCtrls;

type
  TCalcForm = class(TForm)
    DBGrid1: TDBGrid;
    DBNavigator: TDBNavigator;
    Panel1: TPanel;
    Panel2: TPanel;
    DataSource1: TDataSource;
    Table1: TTable;
    Table1PopulationDensity: TFloatField;
    Table1Area: TFloatField;
    Table1Population: TFloatField;
    Table1Name: TStringField;
    Table1Capital: TStringField;
    Table1Continent: TStringField;
    procedure FormCreate(Sender: TObject);
    procedure Table1CalcFields(DataSet: TDataset);
    procedure DBGrid1EditButtonClick(Sender: TObject);
  private
    { private declarations }
  public
    { public declarations }
  end;

var
  CalcForm: TCalcForm;

implementation

uses Dialogs;

{$R *.DFM}

procedure TCalcForm.FormCreate(Sender: TObject);
begin
  Table1.Open;
end;

procedure TCalcForm.Table1CalcFields(DataSet: TDataset);
begin
  // plain version (very dangerous)
{  Table1PopulationDensity.Value :=
    Table1Population.Value / Table1Area.Value;}

  // version based on exceptions (ok)
{  try
    Table1PopulationDensity.Value :=
      Table1Population.Value / Table1Area.Value;
  except
    on Exception do
      Table1PopulationDensity.Value := 0;
  end;}

  // definitive version
  if not Table1Area.IsNull and
      (Table1Area.Value <> 0) then
    Table1PopulationDensity.Value :=
      Table1Population.Value / Table1Area.Value
  else
      Table1PopulationDensity.Value := 0;
end;

procedure TCalcForm.DBGrid1EditButtonClick(Sender: TObject);
begin
  MessageDlg (Format (
    'The population density (%.2n)'#13 +
    'is the Population (%.0n)'#13 +
    'devided by the Area (%.0n).'#13#13 +
    'Edit these two fields to change it.',
    [Table1PopulationDensity.AsFloat,
    Table1Population.AsFloat,
    Table1Area.AsFloat]),
    mtInformation, [mbOK], 0);
end;

end.

object CalcForm: TCalcForm
  Left = 335
  Top = 304
  Width = 556
  Height = 236
  ActiveControl = Panel1
  Caption = 'Calculated Field'
  Color = clBtnFace
  Font.Charset = DEFAULT_CHARSET
  Font.Color = clBlack
  Font.Height = -11
  Font.Name = 'Arial'
  Font.Style = []
  OldCreateOrder = True
  Position = poScreenCenter
  OnCreate = FormCreate
  PixelsPerInch = 96
  TextHeight = 14
  object Panel1: TPanel
    Left = 0
    Top = 0
    Width = 548
    Height = 33
    Align = alTop
    TabOrder = 0
    object Panel2: TPanel
      Left = 406
      Top = 1
      Width = 141
      Height = 31
      Align = alRight
      BevelOuter = bvNone
      Caption = 'Panel2'
      TabOrder = 0
      object DBNavigator: TDBNavigator
        Left = 18
        Top = 6
        Width = 116
        Height = 19
        DataSource = DataSource1
        VisibleButtons = [nbFirst, nbPrior, nbNext, nbLast]
        TabOrder = 0
      end
    end
  end
  object DBGrid1: TDBGrid
    Left = 0
    Top = 33
    Width = 548
    Height = 176
    Align = alClient
    DataSource = DataSource1
    TabOrder = 1
    TitleFont.Charset = DEFAULT_CHARSET
    TitleFont.Color = clBlack
    TitleFont.Height = -11
    TitleFont.Name = 'Arial'
    TitleFont.Style = []
    OnEditButtonClick = DBGrid1EditButtonClick
    Columns = <
      item
        Expanded = False
        FieldName = 'Name'
        Visible = True
      end
      item
        Expanded = False
        FieldName = 'Capital'
        Visible = True
      end
      item
        DropDownRows = 3
        Expanded = False
        FieldName = 'Continent'
        PickList.Strings = (
          'North America'
          'South America')
        Width = 87
        Visible = True
      end
      item
        Expanded = False
        FieldName = 'Population'
        Visible = True
      end
      item
        Expanded = False
        FieldName = 'Area'
        Visible = True
      end
      item
        ButtonStyle = cbsEllipsis
        Expanded = False
        FieldName = 'Population Density'
        ReadOnly = True
        Visible = True
      end>
  end
  object DataSource1: TDataSource
    DataSet = Table1
    Left = 77
    Top = 65533
  end
  object Table1: TTable
    Active = True
    OnCalcFields = Table1CalcFields
    DatabaseName = 'DBDEMOS'
    TableName = 'country.db'
    Left = 10
    Top = 65533
    object Table1Name: TStringField
      DisplayLabel = 'Country'
      DisplayWidth = 11
      FieldName = 'Name'
      FixedChar = False
      Size = 24
    end
    object Table1Capital: TStringField
      DisplayWidth = 13
      FieldName = 'Capital'
      FixedChar = False
      Size = 24
    end
    object Table1Continent: TStringField
      FieldName = 'Continent'
      FixedChar = False
      Size = 24
    end
    object Table1Population: TFloatField
      DisplayWidth = 14
      FieldName = 'Population'
      DisplayFormat = '###,###,###'
    end
    object Table1Area: TFloatField
      DisplayWidth = 13
      FieldName = 'Area'
      DisplayFormat = '###,###,###'
    end
    object Table1PopulationDensity: TFloatField
      DisplayWidth = 16
      FieldKind = fkCalculated
      FieldName = 'Population Density'
      DisplayFormat = '###.##'
      Precision = 2
      Calculated = True
    end
  end
end