Re: proj ntuple into predefined histogram

From: Stephen Bailey (bailey@physics.harvard.edu)
Date: Wed Aug 19 1998 - 15:56:40 MEST


On Wed, 19 Aug 1998, Rene Brun wrote:

> Stephen Bailey wrote:
> > 
> > Hi.
> > 
> > I'm trying to project an ntuple into a histogram which I
> > defined so that I can control the number of bins used.
> > e.g.
> > 
> >    TH1F h1 = new TH1F("blat", "quat", 20, 4.5, 6);
> >    ntuple->Draw("m>>+h1", "(4.5<m)&&(m<6.0)");
> >    h1->Draw();
> > 
> > I also tryed m>>h1, but the ntuple->Draw command always
> > picks its own choice for number of bins and h1 always
> > seems to remain empty.  If I don't predefine h1 then it
> > gets created and properly filled but I don't have control
> > over the number of bins.
> > 
> > Is there a way to do this easily (i.e. without looping
> > trough the ntuple and filling the histogram event by event)?
> 
> Replace:
>  TH1F h1 = new TH1F(..
> by
>  TH1F *h1 = new TH1F(..
> 
> The TTree::Draw function (in the form "m>>+h1") fills an histogram
> with the name "h1". In your case h1 is the name of the pointer,
> not the name of the histogram ("blat").
> It is a good practice to have the pointer_name = histogram_name.
> Your program will work with:
>    TH1F *h1 = new TH1F("blat", "quat", 20, 4.5, 6);
>    ntuple->Draw("m>>+blat", "(4.5<m)&&(m<6.0)");
>    h1->Draw();
> or
>    TH1F *blat = new TH1F("blat", "quat", 20, 4.5, 6);
>    ntuple->Draw("m>>+blat", "(4.5<m)&&(m<6.0)");
>    blat->Draw();
> 
> Rene Brun

The difference in the example is whether the histogram_name ==
variable_name.  If they are *not* equal, CINT apparently lets me
get away with the sloppy usage of a non-pointer as a pointer,
whereas it didn't work if the histogram and its variable had the
same name.  But if I correctly type them as pointers as you suggest,
then both cases work.  Thanks for the help.

Stephen



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