Generics Collections TDictionary (Delphi)

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Description

This example demonstrates the usage of the main methods and properties in TDictionary.

Code

type
  TCity = class
    Country : String;
    Latitude : Double;
    Longitude : Double;
  end;
 
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;
  City := TCity.Create;
  { 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. }
  writeln('Number of pairs in the dictionary: ' +
  IntToStr(Dictionary.Count));
 
  // Try looking up "Iasi".
  if (Dictionary.TryGetValue('Iasi', City) = True) then
  begin
    writeln(
    'Iasi is located in ' + City.Country +
    ' with latitude = ' + FloatToStrF(City.Latitude, ffFixed, 4, 2) +
    ' and longitude = ' + FloatToStrF(City.Longitude, ffFixed, 4, 2)
    );
  end
  else
    writeln('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 number of entries. }
  Dictionary.TrimExcess;
 
  { Test if "Iasi" is a key in the dictionary. }
  if Dictionary.ContainsKey('Iasi') then
    writeln('The key "Iasi" is in the dictionary.')
  else
    writeln('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
    writeln('The value (United Kingdom, 51.5, -0.17) is in the dictionary.')
  else
    writeln('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;
  writeln(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;
  writeln(temp);
 
  { Clear all entries in the dictionary. }
  Dictionary.Clear;
 
  { There should be no entries at this point. }
  writeln('Number of key-value pairs in the dictionary: ' + IntToStr(Dictionary.Count));
 
  { Free the memory allocated for the dictionary. }
  Dictionary.Free;
  readln;
end.

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