How to improve the performance of a key search?

Question

In view of the fact that TDictionary exposes only ways to recover value through the key, I have the following algorithm to fetch a key from its value:

var
    Table: TDictionary<Int64, Extended>;

function KeyOf(Value: Extended): Int64; inline;
var
    Key: Int64;
begin
    Result := -1;
    if not Table.ContainsValue(Value) then
        Exit;
    for Key in Table.Keys do
        if Table.Items[Key] = Value then
        begin
            Result := Key;
            Exit;
        end;
end;

• I’m calling this function in a loop that runs at least 50,000 times.

• I need to keep indexing by integers that do not repeat.

• During execution the structure will hold on average 50 items.

• It would also be valid to replace this structure with any other.

And then my question: is there a way to accomplish the same task with better performance as to the speed of execution?

Answer 1

Updated 26/06/2016

Guill, you can use a TPair to browse and find the value you need most efficiently.

function KeyOf(Value: Extended): Int64; inline;
var
  vIterator: TPair<Int64, Extended>;
begin
  Result := -1;
  if not Table.ContainsValue(Value) then
    Exit;

  for vIterator in Table do
  begin
    if vIterator.Value = Value then
    begin
      Result := vIterator.Key;
      break;
    end;
  end;
end;

You can test with your records and check if there is an improvement in the search.

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Take a read on this link too, contains a more detailed explanation: Delphi Tdictionary iteration