[SOLVED] Stringlist.Find in Delphi & Lazarus behaves differently

[JD]

While trying to port some Delphi code to Lazarus, I discovered that TStringList Find does not behave the same way in Delphi & Lazarus.

A very simple illustration from http://docs.embarcadero.com/products/rad_studio/delphiAndcpp2009/HelpUpdate2/EN/html/delphivclwin32/Classes_TStringList_Find.html

Code: Pascal  [Select][+][-]

1. procedure TForm1.FormCreate(Sender: TObject);
2. var
3.   MyList: TStringList;
4.   Index: Integer;
5. begin
6.   MyList := TStringList.Create;
7.   try
8.     MyList.Add('Animals');
9.     MyList.Add('Flowers');
10.     MyList.Add('Cars');
11.     MyList.Sort;   { Find will only work on sorted lists! }
12.     if MyList.Find('Flowers', Index) then
13.     begin
14.       ListBox1.Items.AddStrings(MyList);
15.       Label1.Caption := 'Flowers has an index value of ' + IntToStr(Index);
16.     end;
17.   finally
18.     MyList.Free;
19.   end;
20. end;
21.  

While the example works in Delphi, it fails in Lazarus and throws an EListError exception saying Cannot use find on an unsorted list.

I need to use Find so that the stringlist can be searched using the binary search method. How do I do this in Lazarus?

Thanks,

JD

[marcov]

Read the text of the exception AND the embarcadero URL you quote.

FIND ONLY WORKS ON SORTED LISTS, so add

Code: Pascal  [Select][+][-]

1. mylist.sorted:=true;

before adding elements.

The only difference is that Delphi fails silently, and FPC with an exception. Deliberately.

[taazz]

At first I did not see the problem my self the code worked as expected in 1.4.4 then I tried on 1.6.4 and long story sort here is the difference in the code.

  if Not Sorted then
    Raise EListError.Create(SErrFindNeedsSortedList);

apparently someone things that we do not know how to handle a binary search properly. any way set the property sorted to true in lazarus or copy the tstringlist code to a custom class to make sure that this kind of thing does not happen in the future. I would say to post a bug report but I do not think it qualifies as a bug.

[taazz]

Read the text of the exception AND the embarcadero URL you quote.

FIND ONLY WORKS ON SORTED LISTS, so add

Code: Pascal  [Select][+][-]

1. mylist.sorted:=true;

before adding elements.

The only difference is that Delphi fails silently, and FPC with an exception. Deliberately.

No, the difference is that delphi does not fail and finds the item required and fpc fails with an exception.

[marcov]

No, the difference is that delphi does not fail and finds the item required and fpc fails with an exception.

Newer versions might contain fallback to indexof, iirc the older ones didn't. But then it isn't binary search, and it isn't documented

[BeniBela]

I need to use Find so that the stringlist can be searched using the binary search method. How do I do this in Lazarus?

whatever you do, do not add a widestring manager

[JD]

I need to use Find so that the stringlist can be searched using the binary search method. How do I do this in Lazarus?

whatever you do, do not add a widestring manager

Thanks for the tip. The example now works in Lazarus. And I have no need for a widestring manager in this project.

JD

[JD]

Read the text of the exception AND the embarcadero URL you quote.

FIND ONLY WORKS ON SORTED LISTS, so add

Code: Pascal  [Select][+][-]

1. mylist.sorted:=true;

before adding elements.

The only difference is that Delphi fails silently, and FPC with an exception. Deliberately.

Thanks marcov. It appears the use of the Sort method is redundant in Lazarus. Anyway it now works.

JD

[taazz]

No, the difference is that delphi does not fail and finds the item required and fpc fails with an exception.

Newer versions might contain fallback to indexof, iirc the older ones didn't. But then it isn't binary search, and it isn't documented

handle it the same way that duplicates are handled, let the end user define the behavior exception, indexof, or silent fail, you can always make the exception the default behavior.

[Thaddy]

Use IndexOf instead of find. If sorted = false it will fall back to linear search else binary search.