Remove all the entries = 0 from a TVectorD

ROOT
1

Hello,

I have a TVectorD with many entries equal to 0 and I’d like to create a new TVectorD which contains all the entries non equal to 0. Since my TVectorD contains a lot of entries (more than 1 million), I’d like to avoid to loop on it. Is there a way to do it ?
thanks !

Marie

2

Hi Marie,

You will have to loop across the vector elements somewhere .
Try something like:

Int_t n1 = 1000;
TVectorD v1(n1);

Double_t *pV1 = v1.GetMatrixArray();
TVectorD v2(v1);
Double_t *pV2 = v2.GetMatrixArray();
Int n2 = 0;
for (Int_t i = 0; i < n1; i++)
{
  if (pV1[i] != 0.)
    pV2[n++] = pV1[i];
}
if (n1 != n2)
  v2.ResizeTo(n2);

Eddy

3

Is there any reason to make a copy?

Int_t n1 = 1000;
TVectorD v1(n1);
// fill v1 magically

Double_t *pV1 = v1.GetMatrixArray();
Int n2 = 0;
for (Int_t i = 0; i < n1; ++i)
{
   if (pV1[i] != 0.)
   {
       if (n != i)
       { 
           pV1[n2++] = pV1[i];
       } // if n different from i
       ++n;
   } // if non-zero value
} // for i
if (n1 != n2)
{
  v1.ResizeTo(n2);
}
4

Yup, the user asked for a new vector :wink:

5

Note to self: Try reading before responding. :blush:

Note to Marie: If you have a really large vector and you don’t need the original, you may be better off with shrinking the one in place.

Cheers,
Charles

6

Hello,

thanks for your answers. Actually, I convinced myself that I have to do a loop anyway, and I want to keep the original vector also ! I did a loop on the vector entries : is it more efficient to create first an array containing all the entries of the vector (using GetMatrixArray function, as suggested by Eddy) and then to loop on that array ?

Marie

7

Hi Marie,

I am not sure I understand your question .

If you are asking whether

TVectorD v2(v1);

is necesarry ,the answer is : no it is enough to do

TVectorD v2(v1.GetNrows());

Eddy