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 >
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