Hi,
Tree::Draw can help you with that 
. Do something like:mytree->SetEstimate(end_index-begin_index);
mytree->Draw("X","","goff",end_index,begin_index);
XX = mytree->GetV1(); // returns an array of double
Cheers,
Philippe.
Hi Philippe,
Thanks for your hint.
I used your bit of code in order to generate a pointer to an entire column, like that
tree.Draw("t","","goff",N,0)
df_t=tree.GetV1()
Now, type (df_t) is given as
<type ‘buffer’>
I know that this is a c pointer to my data, but I was not able to find out how to cast that thing to a scipy array without looping over it. Is that not possible using ctypes?
Maybe there is a simple way I did not find…
One more observation:
using the following code
[code]tree.Draw(“t”,"",“goff”,N,0)
df_t=tree.GetV1()
tree.Draw(“X”,"",“goff”,N,0)
df_X=tree.GetV1()[/code]
will result in df_t and df_X containing the exact same data (that of X, so apparaently the GetV1 function gives back a kind of pointer which is the same for every call… ) Is that supposed to be that way?
Thanks, Bjoern
You must copy the array returned by tree.GetV1(). This array is refilled by each call to tree.Draw().
Rene
Yes, I figured that much.
I was just astonished because the C++ equivalent does not do the same thing.
In python, I solved the problem with
tree.Draw("t","","goff",N,0)
df_t=tree.GetV1()
t=copy.deepcopy(scipy.frombuffer(buffer=df_t,dtype='double',count=N))
tree.Draw("X","","goff",N,0)
df_X=tree.GetV1();
X=copy.deepcopy(scipy.frombuffer(buffer=df_X,dtype='double',count=N))
tree.Draw("Xa","","goff",N,0)
df_Xa=tree.GetV1();
Xa=copy.deepcopy(scipy.frombuffer(buffer=df_Xa,dtype='double',count=N))It is necessary to perform the copy.deepcopy operation AND to perform this operation directly after the call to Draw()
In C++,
the following does the trick:
Double_t * t_persistent = new Double_t [nentries] ;
Double_t * X_persistent = new Double_t [nentries] ;
Double_t * Xa_persistent = new Double_t [nentries] ;
t1->Draw("t","","goff",nentries,0);
t = t1->GetV1(); // returns an array of double
t1->Draw("X","","goff",nentries,0);
X = t1->GetV1(); // returns an array of double
t1->Draw("Xa","","goff",nentries,0);
Xa = t1->GetV1(); // returns an array of double
for (int i=1;i<nentries;i++)
{
t_persistent[i]=t[i];
X_persistent[i]=X[i];
Xa_persistent[i]=Xa[i];
}
In the C++ case, the “copy” operation takes place in the loop; here, it is seemingly not necessary to perfom the copy operation directly after the call to Draw(). This difference confuses me.
I am now down to approximately 1/2 the speed of C++ using my python approach. This is already quite good. One final step would be to swig the whole reading application and then map it back on a scipy array. I wrote the swig application which returns a
<Swig Object of type ‘Double_t *’ at 0x2ac741fdd010>
I am now hoping that someone has a nice and simple solution to map this thing onto a scipy array, as my previous solution (which worked with the
<type ‘buffer’> object and scipy.frombuffer) does not work anymore…
Cheers,
Philippe
Bjoern,
[quote]<Swig Object of type ‘Double_t *’ at 0x2ac741fdd010>
I am now hoping that someone has a nice and simple solution to map this thing onto a scipy array[/quote]
likely both scipy and Swig allow conversion from and to CObjects, so I’d search for that to find a way of casting one into the other.
Cheers,
Wim