delphi - How can I search a generic TList for a record with a certain field value?

Question

Everything about generic TList. I have this structure:

Type
  TExtract = record
    Wheel: string;
    Extract: array [1..5] of Byte;
  end;

  TExtractList = TList<TExtract>

  TEstr = record
    Date: TDate;
    Extract: TExtractList;
  end;

  TEstrList = TList<TEstr>;

The main list is TExtrList and in this list I have all dates and for date all wheel with that date. I want to search if a date exists or not. If not exist I add in sublist TExtractList of Extract from TEstr the info. When I search from TExtrList Delphi asks me about TEstr type. I need to search for Date only. So how can I search for single field in a generic TList?

PS: I have deleted last post because here I have tried to explain better.

Answer 1

Here we go again.

You should use the built-in TList<T>.BinarySearch() function, even if it rightfully asks for a TEstr record as a parameter. You'll first need to use TList<T>.Sort() to sort the list using the same criteria as for the search, then call BinarySearch() to find your record.

Here's a function that does both (sort and search):

uses Generics.Defaults; // this provides TDelegatedComparer
uses Math; // this provides Sign()

function SearchList(Date:TDate; Sort:Boolean; List:TList<TEstr>): Integer;
var Comparer: IComparer<TEstr>;
    Dummy: TEstr;
begin
  // Prepare a custom comparer that'll be used to sort the list
  // based on Date alone, and later to BinarySearch the list using
  // date alone.
  Comparer := TDelegatedComparer<TEstr>.Construct(
    function (const L, R: TEstr): Integer
    begin
      Result := Sign(L.Date - R.Date);
    end
  );

  // If the list is not sorted, sort it. We don't know if it's sorted or not,
  // so we rely on the "Sort" parameter
  if Sort then List.Sort(Comparer);

  // Prepare a Dummy TEstr record we'll use for searching
  Dummy.Date := Date;

  // Call BinarySearch() to look up the record based on Date alone
  if not List.BinarySearch(Dummy, Result, Comparer) then
    Result := -1;
end;

BinarySearch assumes the list is sorted (that's the essence of binary searching!). On your first call you need to set Sort=True so the list is properly sorted. On subsequent calls Sort should be False. Of course, in actual use you'd probably have separate routines for searching and sorting, and you'd probably have them as methods of a class descending from TList<TEstr> (to make things easyer). I places both in the same routine for dempnstration purposes.