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root:ttreedraw [2013/06/14 11:10] – [Basic Usage] nchiaproot:ttreedraw [2013/06/14 11:41] (current) nchiap
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 } }
 </file> </file>
 +
 +===== Pitfalls =====
 +
 +==== Unable to add to existing histograms ====
 +Adding data to an existing histogram will fail, if any file was opened after creating the histogram!
 +TTree::Draw() searches for the named histogram in the active directory - and this is changed when opening a file.
 +
 +To avoid this make sure to load the input files before you create any histograms or
 +create a storage file and switch back there after opening any files:
 +<code c++>
 +TFile tmp("tmp.root", "recreate");
 +float bins[] = {0, 10, 20, 40, 80}; 
 +TH1F  myhist("myhist", "myhist", 4, bins)
 +
 +TFile file("myinput.root");
 +TTree *tree = file.Get("mytree");
 +
 +tmp.cd()
 +tree->Draw("myvar>>+myhist");
 +</code>
  
  
root/ttreedraw.txt · Last modified: 2013/06/14 11:41 by nchiap