Re: Selector speed

From: Philippe Canal <pcanal_at_fnal.gov>
Date: Wed, 18 Jan 2012 16:08:49 -0600


Hi Art,

In order to achieve similar speed that TTree::Draw, you need to call branch->GetEntry (for each of the branch you need) rather than fChain->GetEntry (which means read all the data).

Cheers,
Philippe.

On 1/13/12 3:00 PM, Arthur E. Snyder wrote:
>
> I've noticed that my selector run much slower than a single variable |Draw|. There are lots of variables (and lots of events).
>
> Would it speed it up to not do |SetBranch|, etc. for variables I'm not using?
>
> -Art
>
>
> A.E. Snyder, The Former Group C (TFC) \!c*p?/
> SLAC Mail Stop #95 ((. .))
> Box 4349 |
> Stanford, Ca, USA, 94309 '\|/`
> e-mail:snyder_at_slac.stanford.edu o
> phone:650-926-2701 _
> http://www.slac.stanford.edu/~snyder BaBar
> FAX:707-313-0250 Collaboration
> &
> Fermi/GLAST
>
>
>
> On Tue, 13 Sep 2011, Philip Rodrigues wrote:
>
>> Here is my favourite example of the confusing behaviour of TH1::GetMean():
>>
>> root [0] TH1F* h2=new TH1F("foo2", "bar", 10, 0, 5)
>>
>> root [1] h2->Fill(2.2)
>>
>> root [2] h2->GetMean()
>> (const Double_t)2.20000000000000018e+00
>>
>> // This excludes a zero bin, so shouldn't affect the mean:
>> root [3] h2->GetXaxis()->SetRangeUser(-1, 3.8)
>>
>> root [4] h2->GetMean()
>> (const Double_t)2.25000000000000000e+00
>>
>> On Tuesday, September 13, 2011 04:16:05 PM Arthur E. Snyder wrote:
>>> Stefan and Aammer:
>>>
>>>
>>> What would be nice would be an option, e.g., GetMean("binned") vs.
>>> GetMean("unbinned") with default being what folks are used to.
>>>
>>> I suspect that problem occurs not just for subtracted historgrams but for
>>> weighted ones.
>>>
>>> On the other hand sometimes you want the unbinned mean. If you've made big
>>> bins to make the plot look nice, the mean may be seriously over estimated
>>> by using middle of the bin ...
>>>
>>> Maybe there should be a warning when |h->Sumw2()| has been involked ...
>>>
>>> -Art S.
>>>
>>> A.E. Snyder, The Former Group C (TFC) \!c*p?/
>>> SLAC Mail Stop #95 ((. .))
>>> Box 4349 |
>>> Stanford, Ca, USA, 94309 '\|/`
>>> e-mail:snyder_at_slac.stanford.edu o
>>> phone:650-926-2701 _
>>> http://www.slac.stanford.edu/~snyder BaBar
>>> FAX:707-313-0250 Collaboration
>>> &
>>> Fermi/GLAST
>>>
>>> On Tue, 13 Sep 2011, Stefan Piperov wrote:
>>>> This issue - that ROOT reports histogram's momenta based on unbinned
>>>> data - has been discussed several times now, but without much
>>>> consequences...
>>>>
>>>> It is plain wrong, of course, to report quantities related to the
>>>> initial dataset as belonging to the histogram, but this is how ROOT was
>>>> designed from the very beginning, so it's too late now to change, I
>>>> guess. What we can do, though, is to spread the word, so that at least
>>>> the users know of this problem, and do not rely on TH1::GetMean() to get
>>>> the mean of the binned data.
>>>>
>>>> Stefan.
>>>>
>>>> On Tue, 13 Sep 2011, Arthur E. Snyder wrote:
>>>>> Aamer,
>>>>>
>>>>> It doesn't work on subtracted histograms. I had to 'roll-my-own' to do
>>>>> that (though there might be something existing that does this that I
>>>>> just didn't find).
>>>>>
>>>>> As I recall |root] does unbinned mean, so you get the same result
>>>>> regardless of binning. It's not clear what it does the case of
>>>>> subtracted histograms, but if the histograms is asymmetric the result
>>>>> can be wildly wrong.
>>>>>
>>>>> -Art S.
>>>>>
>>>>> A.E. Snyder, The Former Group C (TFC) \!c*p?/
>>>>> SLAC Mail Stop #95 ((. .))
>>>>> Box 4349 |
>>>>> Stanford, Ca, USA, 94309 '\|/`
>>>>> e-mail:snyder_at_slac.stanford.edu o
>>>>> phone:650-926-2701 _
>>>>> http://www.slac.stanford.edu/~snyder BaBar
>>>>> FAX:707-313-0250 Collaboration
>>>>>
>>>>> &
>>>>>
>>>>> Fermi/GLAST
>>>>>
>>>>> On Tue, 13 Sep 2011, Aamer Wali Rauf wrote:
>>>>>> Hi,
>>>>>> I have always assumed (and thus used) that the TH1::GetMean(1) method
>>>>>> gives out the weighted mean value
>>>>>> of the x-axis of the histogram. Visibly it looks to me that way but is
>>>>>> it really so? Can someone
>>>>>> comment on that please?
>>>>>>
>>>>>> Thanks in advance,
>>>>>> Aamer
>>>>
>>>> *---------------------------------------------------------------------*
>>>>
>>>> Stefan Piperov Mail: FNAL P.O.Box 500, MS 205, Batavia, IL-60510
>>>> Phone: (630) 840-5176 E-Mail: piperov_at_fnal.gov
>>>>
>>>> *---------------------------------------------------------------------*
>>>> "Give a skeptic an inch... and he'll measure it."
>>
>>
>
Received on Wed Jan 18 2012 - 23:08:56 CET

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