[ROOT] GetParameter difficulty

From: Mike R (roedelbr@students.uiuc.edu)
Date: Wed Nov 28 2001 - 23:34:37 MET


Hello,

  I'm am having trouble accessing fit parameters.  I'm fairly new to
ROOT and C++ so the answer may be simple.  I'm running on Red Hat
7.0 or 7.1 and am using ROOT 3.01.  I can fit my histograms to a
Gaussian with no problems from the command prompt or with
a ROOT macro using the predefined function 'gaus'  However, for the
application I'm working on now, I need to do it from compiled C++ code.

  I do get this warning when compiling, but it does compile:

        "`TF1 *gaus' might be used uninitialized in this function"

I don't know what to do to get rid of this.  I don't know if it is related
to the problem I'll explain below...

  I am able to Draw the histogram.  I am able to fit the histogram to a
Gaussian (hadcGCs[k]->Fit("gaus","","",lowGCs[k],highGCs[k])), and the
numbers are reasonable:

  "FCN=301.889 FROM MIGRAD STATUS=CONVERGED  88 CALLS   89 TOTAL
              EDM=4.31488e-09    STRATEGY= 1    ERROR MATRIX ACCURATE
  EXT PARAMETER                             STEP        FIRST
  NO.  NAME       VALUE        ERROR        SIZE      DERIVATIVE
   1  Constant 2.89594e+02  9.22416e+00  5.06654e-02 -1.18601e-05
   2  Mean     1.30441e+02  6.90035e-01  5.85323e-03 -5.57058e-05
   3  Sigma    3.27019e+01  8.30520e-01  3.43433e-05 -5.39811e-03 "

I update the canvas and see a good Gaussian fit, so I know the fit
is being done correctly.  ...BUT... when I try the following lines,
which work fine in my macro, I get a segmentation violation (core
dumped) error:

        "Double_t peakGCs = gaus->GetParameter(1);
         Double_t sigmaGCs = gaus->GetParameter(2);"

  I need to use these numbers for further plots and calculations.  I
searched your archived roottalk correspondence and found a couple
somewhat similar problems, but none of the suggested fixes corrected my
problem.  (e.g. adding the option 'N' in the Fit did not change anything.)
I tried using the GetParameters(par) approach from the FittingDemo example
in the ROOT manual but it gave a segmentation violation as well.


                                                Thanks,
						Mike



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