如何在一个通用的TList中搜索具有某个字段值的记录？ [英] How can I search a generic TList for a record with a certain field value?

问题描述

关于一般的 TList 的一切。我有这个结构：

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>;

主列表是 TExtrList ，在此列表我有所有的日期和日期所有的轮与那个日期。我想搜索一个日期是否存在。如果不存在，请从 TEstr 中的Extract子列表中添加 TExtractList 。当我从 TExtrList 中搜索时，Delphi向我询问 TEstr 类型。我只需要搜索 Date 。那么如何在通用的 TList 中搜索单个字段？

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：我已经删除了最后一个帖子，因为在这里我试图解释更好。

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

推荐答案

这里我们再去一次。

您应该使用内置的 TList< T> .BinarySearch（）函数，即使它正确地要求一个 TEstr 记录作为参数。您首先需要使用 TList< T> .Sort（）使用与搜索相同的条件对列表进行排序，然后调用 BinarySearch （）找到你的记录。

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.

这里有一个功能（排序和搜索）：

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 假定列表被排序二进制搜索的本质！）。在您的第一个通话中，您需要设置 Sort = True ，以便列表被正确排序。在以后的调用中 Sort 应该是 False 。当然，在实际使用中，您可能会有单独的例程进行搜索和排序，您可能会将它们作为从 TList< TEstr> （>使事情更容易）。我把这两个都放在同一个例程中进行解释。

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.

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