Determine size of std::vector stored in TTree

[Peter_Thomassen]

Hi,

When I store an std::vector in a TTree, Scan() outputs an instance for each item of the vector.

Now, is there a way to determine the size of such a stored vector, without retrieving the full vector as an object? I found that Draw(“vec.size”) or Draw(“vec->size()”) doesn’t work.

Best,
Peter

[Danilo]

Hi Peter,

your post kind of escaped my attention. Sorry for the late answer.
The strategy to achieve what you want is documented here:
root.cern.ch/root/html/TTree.html#TTree:Draw@2

Also
tree->Draw(“event.fTracks.size()”);
would plot the result of the member function Track::size() for each
Track object inside the collection.
To access information about the collection itself, TTree::Draw support
the ‘@’ notation. If a variable which points to a collection is prefixed
or postfixed with ‘@’, the next part of the expression will pertain to
the collection object. For example:
tree->Draw(“event.@fTracks.size()”);
will plot the size of the collection referred to by fTracks (i.e the number
of Track objects).

Cheers,
Danilo