Hi Walter,
The version in CVS clears the current pad in case the number
of selected entries is null.
Rene Brun
On Sun, 20 Apr
2003, Walter F.J. Mueller wrote:
> Dear ROOTer's using 3.05/03
>
> it seems the canvas isn't updated when TTree::Draw() is used with an
> empty selection. This gives potentially confusing visual results.
> Assume a tree has a variable 'num', than a session like
>
> file = new TFile("xxx.root");
> tree->Draw("num>>h(100,0.,50000.)");
>
> produces some nice histogram. Now give as third command
>
> tree->Draw("num>>h","1==0");
>
> This will not produce a display with a histogram with 0 entries as one
> rightfully expects but just leaves the canvas unchanged. Now select
> File->'Save as' and the canvas will update, you see the correct
> histogram, and the correct histogram is also saved...
>
> It seems that in general the canvas isn't updated when the selection
> is empty (nor htemp in case no explicit histogram is filled). For my
> taste this isn't correct and is a potential trap leading to wrong
> conclusions when one tries different selections and one of them turns
> out to be empty because cuts are so narrow that there is no event
> matching it. If the selection is empty, an empty histogram is the
> proper visualization of this fact.
>
> Cheers and happy Easter,
>
> Walter
>
This archive was generated by hypermail 2b29 : Thu Jan 01 2004 - 17:50:11 MET