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--- root:ttreedraw [2013/06/14 11:09] – nchiap
+++ root:ttreedraw [2013/06/14 11:41] (current) – nchiap
@@ Line 13: / Line 13: @@
 Alternatively the name of the histogram to be created can be given or the data can be added to an existing histogram.
 <code c++>
-tree->Draw("myvar>>myhist(10,0,100)");    // create a new histogram *myhist* with 10 bins between 0 and 100
+tree->Draw("myvar>>myhist(10,0,100)");     // create a new histogram *myhist* with 10 bins between 0 and 100
-TH1F *hist = gPad->GetPrimitive("myhist") // create a pointer to the new histogram
+TH1F *hist = gPad->GetPrimitive("myhist"); // create a pointer to the new histogram
-tree->Draw("myvar>>+myhist");             // add additional entries to *myhist*
+tree->Draw("myvar>>+myhist");              // add additional entries to *myhist*
 </code>
@@ Line 67: / Line 67: @@
 }
 </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>

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