TDictionary.Clear Method

Clear all data.

Pascal

procedure Clear;

C++

__fastcall Clear();

Description

Clear removes all keys and values from the dictionary. The Count property is set to 0. The capacity is also set to 0. This operation requires O(n) time, where n is Count, the number of dictionary entries.

Note: Clear does not free the items as they are removed. If you need to free them, use the OnKeyNotify event and the OnValueNotify event, which occur for every entry removed and provides the removed items.

Delphi Examples:

Copy Code

{
This example demonstrates the usage of the main methods and properties in TDictionary.
It uses the ShowMessage procedure and requires no visual components on the form.
}
procedure TForm1.KNotify(Sender: TObject; const Item: String;
  Action: TCollectionNotification);
begin
  { Show a message each time a key is added or removed }
  ShowMessage('The key "' + Item + '" was added/removed.');
end;

procedure TForm1.VNotify(Sender: TObject; const Item: TCity;
  Action: TCollectionNotification);
begin
  { Show a message each time a value is added or removed }
  ShowMessage('The value "' + Item.Country + '" was added/removed.');
end;

procedure TForm1.FormCreate(Sender: TObject);
var
  Dictionary: TDictionary<String, TCity>;
  City, Value: TCity;
  Key: String;
  temp: String;
  pair: TEnumerator<TPair<String, TCity>>;

begin
  { Create the dictionary }
  Dictionary := TDictionary<String, TCity>.Create;
  Dictionary.OnKeyNotify := KNotify;
  Dictionary.OnValueNotify := VNotify;

  { Add some key-value pairs to the dictionary }
  City.Country := 'Romania';
  City.Latitude := 47.16;
  City.Longitude := 27.58;
  Dictionary.Add('Iasi', City);

  City.Country := 'United Kingdom';
  City.Latitude := 51.5;
  City.Longitude := -0.17;
  Dictionary.Add('London', City);

  City.Country := 'Argentina';
  { Notice the wrong coordinates }
  City.Latitude := 0;
  City.Longitude := 0;
  Dictionary.Add('Buenos Aires', City);

  { Display the current number of key-value entries }
  ShowMessage('Number of pairs in the dictionary: ' +
  IntToStr(Dictionary.Count));

  // Try looking up "Iasi"
  if (Dictionary.TryGetValue('Iasi', City) = True) then
  begin
    ShowMessage(
    'Iasi is located in ' + City.Country +
    ' with latitude = ' + FloatToStrF(City.Latitude, ffFixed, 4, 2) +
    ' and longitude = ' + FloatToStrF(City.Longitude, ffFixed, 4, 2)
    );
  end
  else
    ShowMessage('Could not find Iasi in the dictionary');

  { Remove the "Iasi" key from dictionary }
  Dictionary.Remove('Iasi');

  { Make sure the dictionary's capacity is set to the no. of entries }
  Dictionary.TrimExcess;

  { Test if "Iasi" is a key in the dictionary }
  if Dictionary.ContainsKey('Iasi') then
    ShowMessage('The key "Iasi" is in the dictionary.')
  else
    ShowMessage('The key "Iasi" is not in the dictionary.');

  { Test if (Argentina, 0, 0) is a value in the dictionary }
  City.Country := 'United Kingdom';
  City.Latitude := 51.5;
  City.Longitude := -0.17;
  if Dictionary.ContainsValue(City) then
    ShowMessage('The value (United Kingdom, 51.5, -0.17) is in the dictionary.')
  else
    ShowMessage('The value (United Kingdom, 51.5, -0.17) is not in the dictionary.');

  { Update the coordinates to the correct ones }
  City.Country := 'Argentina';
  City.Latitude := -34.6;
  City.Longitude := -58.45;
  Dictionary.AddOrSetValue('Buenos Aires', City);

  { Generate the exception "Duplicates not allowed" }
  try
    Dictionary.Add('Buenos Aires', City);
  except
    on Exception do
      { ShowMessage('Could not add entry. Duplicates are not allowed.'); }
  end;

  { Display all countries }
  temp := 'All countries:' + #13;
  for Value in Dictionary.Values do
    temp := temp + #13 + Value.Country;
  ShowMessage(temp);

  { Iterate through all keys in the dictionary and display their coordinates }
  temp := 'All cities and their coordinates:' + #13;
  for Key in Dictionary.Keys do
    temp := temp + #13 + Key + ': ' + #13 +
    FloatToStrF(Dictionary.Items[Key].Latitude, ffFixed, 4, 2) + ', ' +
    FloatToStrF(Dictionary.Items[Key].Longitude, ffFixed, 4, 2) + #13;
  ShowMessage(temp);

  { Clear all entries in the dictionary }
  Dictionary.Clear;

  { There should be no entries at this point }
  ShowMessage('Number of key-value pairs in the dictionary: ' + IntToStr(Dictionary.Count));

  { Free the memory allocated for the dictionary }
  Dictionary.Destroy;
end;

Copy Code

{
This example demonstrates the usage of the main methods and properties in TDictionary.
It uses the ShowMessage procedure and requires no visual components on the form.
}
procedure TForm1.KNotify(Sender: TObject; const Item: String;
  Action: TCollectionNotification);
begin
  { Show a message each time a key is added or removed }
  ShowMessage('The key "' + Item + '" was added/removed.');
end;

procedure TForm1.VNotify(Sender: TObject; const Item: TCity;
  Action: TCollectionNotification);
begin
  { Show a message each time a value is added or removed }
  ShowMessage('The value "' + Item.Country + '" was added/removed.');
end;

procedure TForm1.FormCreate(Sender: TObject);
var
  Dictionary: TDictionary<String, TCity>;
  City, Value: TCity;
  Key: String;
  temp: String;
  pair: TEnumerator<TPair<String, TCity>>;

begin
  { Create the dictionary }
  Dictionary := TDictionary<String, TCity>.Create;
  Dictionary.OnKeyNotify := KNotify;
  Dictionary.OnValueNotify := VNotify;

  { Add some key-value pairs to the dictionary }
  City.Country := 'Romania';
  City.Latitude := 47.16;
  City.Longitude := 27.58;
  Dictionary.Add('Iasi', City);

  City.Country := 'United Kingdom';
  City.Latitude := 51.5;
  City.Longitude := -0.17;
  Dictionary.Add('London', City);

  City.Country := 'Argentina';
  { Notice the wrong coordinates }
  City.Latitude := 0;
  City.Longitude := 0;
  Dictionary.Add('Buenos Aires', City);

  { Display the current number of key-value entries }
  ShowMessage('Number of pairs in the dictionary: ' +
  IntToStr(Dictionary.Count));

  // Try looking up "Iasi"
  if (Dictionary.TryGetValue('Iasi', City) = True) then
  begin
    ShowMessage(
    'Iasi is located in ' + City.Country +
    ' with latitude = ' + FloatToStrF(City.Latitude, ffFixed, 4, 2) +
    ' and longitude = ' + FloatToStrF(City.Longitude, ffFixed, 4, 2)
    );
  end
  else
    ShowMessage('Could not find Iasi in the dictionary');

  { Remove the "Iasi" key from dictionary }
  Dictionary.Remove('Iasi');

  { Make sure the dictionary's capacity is set to the no. of entries }
  Dictionary.TrimExcess;

  { Test if "Iasi" is a key in the dictionary }
  if Dictionary.ContainsKey('Iasi') then
    ShowMessage('The key "Iasi" is in the dictionary.')
  else
    ShowMessage('The key "Iasi" is not in the dictionary.');

  { Test if (Argentina, 0, 0) is a value in the dictionary }
  City.Country := 'United Kingdom';
  City.Latitude := 51.5;
  City.Longitude := -0.17;
  if Dictionary.ContainsValue(City) then
    ShowMessage('The value (United Kingdom, 51.5, -0.17) is in the dictionary.')
  else
    ShowMessage('The value (United Kingdom, 51.5, -0.17) is not in the dictionary.');

  { Update the coordinates to the correct ones }
  City.Country := 'Argentina';
  City.Latitude := -34.6;
  City.Longitude := -58.45;
  Dictionary.AddOrSetValue('Buenos Aires', City);

  { Generate the exception "Duplicates not allowed" }
  try
    Dictionary.Add('Buenos Aires', City);
  except
    on Exception do
      { ShowMessage('Could not add entry. Duplicates are not allowed.'); }
  end;

  { Display all countries }
  temp := 'All countries:' + #13;
  for Value in Dictionary.Values do
    temp := temp + #13 + Value.Country;
  ShowMessage(temp);

  { Iterate through all keys in the dictionary and display their coordinates }
  temp := 'All cities and their coordinates:' + #13;
  for Key in Dictionary.Keys do
    temp := temp + #13 + Key + ': ' + #13 +
    FloatToStrF(Dictionary.Items[Key].Latitude, ffFixed, 4, 2) + ', ' +
    FloatToStrF(Dictionary.Items[Key].Longitude, ffFixed, 4, 2) + #13;
  ShowMessage(temp);

  { Clear all entries in the dictionary }
  Dictionary.Clear;

  { There should be no entries at this point }
  ShowMessage('Number of key-value pairs in the dictionary: ' + IntToStr(Dictionary.Count));

  { Free the memory allocated for the dictionary }
  Dictionary.Destroy;
end;