multifit.py

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## \file
## \ingroup tutorial_pyroot
## \notebook
## Example showing how to fit in a sub-range of an histogram
## An histogram is created and filled with the bin contents and errors
## defined in the table below.
## 3 gaussians are fitted in sub-ranges of this histogram.
## A new function (a sum of 3 gaussians) is fitted on another subrange
## Note that when fitting simple functions, such as gaussians, the initial
## values of parameters are automatically computed by ROOT.
## In the more complicated case of the sum of 3 gaussians, the initial values
## of parameters must be given. In this particular case, the initial values
## are taken from the result of the individual fits.
##
## \macro_output
## \macro_code
##
## \author Wim Lavrijsen

from ROOT import TH1F, TF1
from ROOT import gROOT
from array import array

x = ( 1.913521, 1.953769, 2.347435, 2.883654, 3.493567,
      4.047560, 4.337210, 4.364347, 4.563004, 5.054247,
      5.194183, 5.380521, 5.303213, 5.384578, 5.563983,
      5.728500, 5.685752, 5.080029, 4.251809, 3.372246,
      2.207432, 1.227541, 0.8597788,0.8220503,0.8046592,
      0.7684097,0.7469761,0.8019787,0.8362375,0.8744895,
      0.9143721,0.9462768,0.9285364,0.8954604,0.8410891,
      0.7853871,0.7100883,0.6938808,0.7363682,0.7032954,
      0.6029015,0.5600163,0.7477068,1.188785, 1.938228,
      2.602717, 3.472962, 4.465014, 5.177035 )

np = len(x)
h = TH1F( 'h', 'Example of several fits in subranges', np, 85, 134 )
h.SetMaximum( 7 )

for i in range(np):
   h.SetBinContent( i+1, x[i] )

par = array( 'd', 9*[0.] )
g1 = TF1( 'g1', 'gaus',  85,  95 )
g2 = TF1( 'g2', 'gaus',  98, 108 )
g3 = TF1( 'g3', 'gaus', 110, 121 )

total = TF1( 'total', 'gaus(0)+gaus(3)+gaus(6)', 85, 125 )
total.SetLineColor( 2 )
h.Fit( g1, 'R' )
h.Fit( g2, 'R+' )
h.Fit( g3, 'R+' )

par1 = g1.GetParameters()
par2 = g2.GetParameters()
par3 = g3.GetParameters()

par[0], par[1], par[2] = par1[0], par1[1], par1[2]
par[3], par[4], par[5] = par2[0], par2[1], par2[2]
par[6], par[7], par[8] = par3[0], par3[1], par3[2]

total.SetParameters( par )
h.Fit( total, 'R+' )