Bad behaviour of TArrayI::Set?

ROOT
1

Dear ROOT friends,

I have encountered a problem with the TArrayI::Set method.
In my code, I have

  TArrayI *fBinPos[2];

defined somewhere. Then I have a method to fill this array, it looks like this (print statements for debugging):

void TXaImage::SetBinPosArray(const EXaCoord crd,
			      const Int_t nPos, 
			      const Int_t binPos[])
{
  cout << "before nPos = " <<  nPos  << endl;
  cout << "before binPos[0] = " <<  binPos[0]  << endl;
  cout << "before binPos[1] = " <<  binPos[1]  << endl;
  cout << "before binPos[2] = " <<  binPos[2]  << endl;
  
  fBinPos[crd]->Set(nPos,binPos);

  cout << "after nPos = " <<  nPos  << endl;
  cout << "after binPos[0] = " <<  binPos[0]  << endl;
  cout << "after binPos[1] = " <<  binPos[1]  << endl;
  cout << "after binPos[2] = " <<  binPos[2]  << endl;

  fNBinPos[crd] = nPos;
}

Now there are situations where I get the following output from the above cout statments:

before nPos = 11
before binPos[0] = 144
before binPos[1] = 171
before binPos[2] = 199
after nPos = 11
after binPos[0] = 0
after binPos[1] = 171
after binPos[2] = 199

In other words, the Set method messes up the array I supply! Why and how can that happen?

Any ideas are welcome.

Cheers,

Thomas

2

Hi,

Set does not modify the array parameter itself. It deletes its own fArray - and if that is the same as the array parameter then you are in trouble as see what you might have seen. As the doc says: “// This function should not be called if the array was declared via Adopt.”

Actually it should be trivial to check whether fArray == array (really whether the two pointers are identical), in which case Set() should simply return. We will let you know. Either way: make sure the array you pass to Set has not been Adopt()ed (or assigned to the public member fArray) before.

Cheers, Axel.

3

Hi Axel,

thanks for the prompt advice, which helped me find my problem quickly.
I was not using Adopt anywhere (I did read the documentation), but I was updating my TArrayI by first reading out the array with

  *binPos = &fBinPos[crd]->GetArray()[0];

then doing something to the array and returning it to the TArrayI with

  fBinPos[crd]->Set(nPos,binPos);

which is a bad thing to do I realize now!

I am surprised at the very subtle manifestation of this mistake: only one element is quietly set to zero, everything else looks just the same. I only found out about this by accident!

Many thanks again,

Thomas