Making slices for TH2F

ROOT
1

Hi,

I have a TH2F, and I want to make Y projections for the different X bins. I can do this by issuing commands like,

h2->ProjectionY(“bin1”,Xbin1,Xbin1),
h2->ProjectionY(“bin2”,Xbin2,Xbin2), etc.

but is there a method which I can invoke at the command line and make all the projection in one shot. We have FitSlicesY, which fits the Y slices, but no method which just makes them.

vivek

1 Like
2

If you know how to make an individual projection, couldn’t you just make a rootscript which loops over all the bins and fits all the projections?

james

3

The main problem is not a function returning all the slices, but where?
It is quite trivial to achieve what you want to do with something like:

TH1 *hbins[100] ;//or whatever you need, ie nbins for (int i=0;i<nbins;i++) { hbins[i] = h2->ProjectionY(Form("bin%d",i+1),i+1,i+2); }
Rene

4

Hi Rene,

Well, you could return them to the same place where you return the results of FitSlicesY. For instance, if I fit a histogram h with a Gaus, I get h_0, h_1, h_2, and h_chi2.

In the same way, you could return h2_1, h2_2, …, as the result of something like h->SlicesY()

vivek

p.s. thanks for the tip about how to do it in a loop. Didn’t know that you could do that.

5

Hi Vivek,

check the most complex THStack constructor - it does what you want. You can query the histograms via THStack::GetHists() afterwards.

Cheers, Axel.

1 Like
6

HI Axel,

Maybe I am not understanding something, but I do the following,

THStack hs(“tag_IPinfo_weight2D_trkPt_u”,“Y”,“YProj”,“Y Projections for X Slices”,1,50,"","") (I also tried it without the trailing ,"","")

where I want to get Y projections for the TH2F tag_IPinfo_weight2D_trkPt_u

I get an error message that no definition of the constructor matches my arguments.

Error: Can’t call THStack::THStack(“tag_IPinfo_weight2D_trkPt_u”,“Y”,“YProj”,“Y Projections for X Slices”,1,50,"","") in current scope (tmpfile):1:
Possible candidates are…
filename line:size busy function type and name (in THStack)
…
(compiled) 0:0 0 public: THStack THStack(const TH1* hist,Option_t* axis=x,const char* name=0,const char* title=0,Int_t firstbin=-1,Int_t lastbin=-1,Int_t firstbin2=-1,Int_t lastbin2=-1,Option_t* proj_option=,Option_t* draw_option=);

However, at root.cern.ch/root/html/THStack.h … ck:THStack

I see - hist: the histogram used for the projections. Can be an object deriving from TH2 or TH3.

7

Hi,

you cannot skip parameters “in between” by passing nothing surrounded by commas. That works for some languages (I forgot which one - perl? but definitely not C++.

Also you will need to pass the histogram to the THStack constructor, not a string - even if that string is the histogram’s name.

THStack* hs = new THStack(tag_IPinfo_weight2D_trkPt_u,"Y")

should do it (if your TH2 is in a variable called tag_IPinfo_weight2D_trkPt_u).

Cheers, Axel.

1 Like
8

thanks a lot. axel.

this is exactly what I wanted…

vivek

9

Hello Axel,

When I do this now, I get the following,

THStack* hs = new THStack(“TopMassCut3”,“Y”)

root [2] hs->GetHists()
(const class TList*)0x0

And I don’t get slices. I am using Root version 5.18.00d