Problem with changing the order of elements within a TList

ROOT
1

Hi,

I’ve got the following simple problem:
I fill a TList, but afterwards I want to change the order. But I fail in doing some simple add and remove.

example:

i fill a tlist with some strings:

TList *t = new TList()
t->Add( new TObjString( "1" ) )
t->Add( new TObjString( "2" ) )
t->Add( new TObjString( "3" ) )
t->Add( new TObjString( "4" ) )

if i print the list, i get:

t->Print()
TObjString = 1
TObjString = 2
TObjString = 3
TObjString = 4

… which is fine.
Now I add the last element as the first (thus, I got element “4” twice, … at position 0 and 4)

t->AddFirst( t->Last() )
t->Print()
TObjString = 4
TObjString = 1
TObjString = 2
TObjString = 3
TObjString = 4

… next step: I remove the last element.

t->RemoveLast()
t->Print()
TObjString = 1
TObjString = 2
TObjString = 3
TObjString = 4

… but actually the first element (which was the last one added) is removed. Why?

How is changing the order of elements supposed to be done with a TList?

thanks,
Peter

2

Hehe! That’s because RemoveLast is implemented as
Remove(Last()); so it traverses the list and removes the first object, “equal” to Last.

3

Lol, even RemoveAt will not work as you want. Really interesting logic of TList. It first will obtain pointer At(idx) and after that it’ll call Remove(ptr). :laughing:

4

:slight_smile:

ok, … now that I know how the logic is working I know how to work with it.

I simply have to get the “Last()” element and store it outside the list. Then I do “RemoveLast()” and afterwards I “AddFirst()” the stored element.

thanks!

  • peter
5

Yes, the this can be improved in case of duplicates in a sequential collection (where first and last makes sense).

Cheers, Fons.

6

Well, I was a bit incorrect: RemoveAt/RemoveLast/etc. come from TSeqCollection, but as I can see from TList header, TList has its own methods to work directly with TList (for your specific case only :frowning: - I mean only the last object in TList).
For TList you can use TObjLink object instead of TObject * pointer.
For example,

TList a;
a.Add(new TObjString(“1”));
a.Add(new TObjString(“2”));
a.Add(new TObjString(“3”));
a.Add(new TObjString(“4”));
a.Print();
a.AddFirst(a.Last());
a.Print();
//Now, remove the last element:
a.Remove(a.LinkAt(a.LastLink()));
a.Print();

Or you can obtain link for first object : a.FirstLink(), after that, “increment” such a link : lnk->Next() (or lnk->Prev()), so you can iterate
a list as if you use std::list::iterator (not exactly, but similar).
And you can remove pointed object a.Remove(lnk);

And do not forget about memory management - it’s up to you to free memory correctly.