CODEWITHSUNDEEP
c++loopsvectorindexing
LCFactorization
LCFactorization

Reputation: 1722

How to efficiently manuplate very long vector?

I have a very long vector as below in Eigen:

MatrixXi iIndex(1000000);

which is initialized into zeros, with only a short continuous part(less than 100) filled with 1 's, the position of which is randomized.

I need to do the following things in a long loop:

int count;
int position;
for(int i=0;i<99999;i++){
  //... randomly fill iIndex will short `1` s
  // e.g. index = (someVectorXi.array() == i).cast<int>();
  count = iIndex.sum(); // count all the nonzero elements
  //position the first nonzero element index:
  for (int j=0; j<1000000;j++){
    if(iIndex(j))
    position = j;
  }
}

But it is really very slow.

Is there any way to expedite?

Upvotes: 0

Views: 75

Answers (1)

stefaanv
stefaanv

Reputation: 14392

my 2 cents: group the bits in e.g. uint32_t, so you can check whether an i32 differs from 0. When it is different, then you may take longer to find out which bits are 1.

Assuming the number of bits is a multitude of 32 (making it easier):

for (int i = 0; i < max / sizeof(uint32_t); ++i)
{
  if (wordGrp[i] != 0)
  {
    uint32_t grp = wordGrp[i];
    for (j = 0; j < BITS_PER_UINT32; j++)
    {
      if ((grp & 1) == 1) std::cout << "idx " << (i*32 + j) << " is 1\n";
      grp >>= 1;
    }
  }
}

Upvotes: 1

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