sPHENIX discussion of software

[pinkenburg <pinkenburg AT bnl.gov>]

    Hi folks,
    
    in the tutorial the question came why one should avoid TMath as much
    as possible. Have a look at TMath::ACos(double x):
    
      inline Double_t
    TMath::ACos(Double_t
    x)

{
  if (x < -1.) return TMath::Pi();

  if (x > 1.) return 0;

  return acos(x);

}

You feed it an out of range argument and lo and behold it returns a valid value. Just start ROOT and try
root [0] double d = 1.;
root [1] double c = TMath::ACos(d);
root [2] cout << c << endl;
0
looks reasonable, now try
root [3] d = 2.;
root [4] c = TMath::ACos(d);
root [5] cout << c << endl;
0
then use the regular math acos function:

root [6] c = acos(d);
root [7] cout << c << endl;
nan

Just do not use TMath (blindly - check the source to see if it is doing what you would expect), bad input should lead to recognizably bad output, not valid values which can go unnoticed and create a peak if you have a few of them.

I rest my case

Chris

-- 
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Christopher H. Pinkenburg	;    pinkenburg AT bnl.gov
				;    http://www.phenix.bnl.gov/~pinkenbu

Brookhaven National Laboratory	;    phone: (631) 344-5692
Physics Department Bldg 510 C	;    fax:   (631) 344-3253
Upton, NY 11973-5000

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