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root:pyroot_tfile [2017/06/13 09:37] – created iwnroot:pyroot_tfile [2017/09/05 10:35] – [Handling TFile files] iwn
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-====== Handling TH1 histograms ======+[[root:root|Getting started with ROOT]] -> [[root:pyroot|Useful pyROOT snippets]] → [[root:pyroot_tfile|Handling TFile files]]
  
-===== Filling a histogram =====+====== Handling TFile files ======
  
-Simple enough:+Class references for: 
 +  * ''[[https://root.cern.ch/doc/master/classTFile.html|TDirectory]]'', 
 +  * ''[[https://root.cern.ch/doc/master/classTFile.html|TFile]]''.
  
-<code python> +===== Adding a directory to a TFile =====
-hist = TH1D("hist_name","hist title",100,0,100) +
-for i in xrange(10000): +
-    hist.Fill(gRandom.Gauss()) +
-</code>+
  
-With some weight: +Here is a basic example creating a file with on directory. If you want to create and save an object in this directory, e.g. a tree or histogram, use ''dir.cd()''. Pay attention to the order of creating an object, changing directory (''cd()'') and writing the object!
-<code python> +
-weight = gRandom.Gauss(1,0.1) +
-hist.Fill(gRandom.Gauss(),weight) +
-</code>+
  
-===== Filling a histogram from a tree =====+<file python addDirectory.py> 
 +from ROOT import TFile
  
-The classic way would be +file = TFile("tree.root",'update') 
-<code python> +  
-hist = TH1D("hist_name","hist title",100,0,100) +# make directory if it does not exist 
-for event in tree: +dir = file.GetDirectory("dir") 
-    hist.Fill(tree.px) +if not dir: 
-</code>+    print ">>> created dir" 
 +    dir = file.mkdir("dir") 
 +  
 +# make this directory the current one: everything you write will be saved here 
 +dir.cd()
  
-But the simplest and most powerful way to draw some variable into a histogram, is with the [[https://root.cern.ch/doc/master/classTTree.html#a73450649dc6e54b5b94516c468523e45|Draw]] function: +# make tree, histogram, ... 
-<code python> +         
-hist = TH1D("hist_name","hist title",100,0,100) +file.Write("",TFile.kOverwrite) 
-tree.Draw("px >> hist_name") +file.Close() 
-</code> +</file>
-Note that the histogram has been saved into ''ROOT'''s working memory and is therefore accessible via its name in the ''Draw'' function, ''"hist_name"'' in this example.+
  
-You can also use mathematical expressions: 
-<code python> 
-tree.Draw("2*sqrt(px*px+py*py) >> hist_name") 
-tree.Draw("abs(eta) >> hist_name") 
-</code> 
  
-You can apply some selections using variable, available in the tree's branches: +===== gDirectory =====
-<code python> +
-tree.Draw("px >> hist_name","pt>20 && E>20") +
-</code>+
  
-You can apply some selections and/or a weight, if a weight variable is available in the tree: +Note that ''gDirectory'' is the current directory containing all newly created saved objects, or those saved to this directory. If you just opened a file, this will be your current directory until you close it or use ''cd()'' on some ''TDirectory'' or other ''TFile''. Test the behavior with this macro:
-<code python> +
-tree.Draw("px >> hist_name","(pt>20 && E>20)*weight") +
-tree.Draw("px >> hist_name","weight") +
-</code>+
  
-An equivalent method is creating the histogram in the ''Draw'' method. If you need the ''TH1F'' object, you can still get it with ''gDirectory'' (see [[root:pyroot#gDirectory|below]]): +<file python gDirectory.py> 
-<code python> +from ROOT import TFile, TTree, TH1F, gDirectory
-from ROOT import gDirectory +
-... +
-tree.Draw("pt >> h(100,0,100)") +
-hist = gDirectory.Get("h") # will be a TH1F object +
-</code>+
  
-It's also possible to pass some option you would normally use in "TH1::Draw()". It is passed as the third string. +def printGDirectory(): 
-<code python> +    print ">>> gDirectory.GetName()\n%s" % gDirectory.GetName() 
-tree.Draw("px >> h1(100,0,100)","","E2") +    print ">>> gDirectory.pwd()" 
-tree.Draw("py >> h2(100,0,100)","","E2 SAME")+    gDirectory.pwd() 
 +    print ">>> gDirectory.ls()" 
 +    gDirectory.ls() 
 +    print
  
-tree.Draw("px >> h1(100,0,100)","abs(eta)>2","E2") +print "\ndefault" 
-tree.Draw("px >> h2(100,0,100)","abs(eta)<2","E2 SAME") +printGDirectory() 
-</code>+ 
 +print ">>> creating a file with some contents" 
 +file = TFile("test.root","recreate") 
 +tree = TTree("tree","tree") 
 +hist = TH1F("hist","hist",100,0,100) 
 +dir1 = file.mkdir("dir1") 
 +printGDirectory() 
 + 
 +print ">>> gDirectory.Delete(\"hist\")" 
 +gDirectory.Delete("hist") 
 +printGDirectory() 
 + 
 +print ">>> dir1.cd()" 
 +dir1.cd() 
 +printGDirectory() 
 + 
 +print ">>> file.Close()" 
 +file.Close() 
 +printGDirectory() 
 +</file>
  
-__Protip__: This is very useful for quick drawing and comparing, for example in the ''ROOT'' command line. 
-<code C++> 
-$ root -l tree.root 
-[1] tree->Draw("px >> h1(100,0,100)","","E2") 
-[2] tree->Draw("py >> h2(100,0,100)","","E2 SAME") 
-</code> 
root/pyroot_tfile.txt · Last modified: 2017/09/05 10:37 by iwn

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