OpenMP atomically update array values

Question

The problem that I am trying to solve is the following:

I have a big point set which I try to split into smaller ones. I know the sizes of the smaller ones, and I am trying to populate the smaller ones using the bigger one in parallel. In order to do that, I have an array of indices per smaller pointset, which will help me populate my point sets correctly. I have developed the following code. But the omp atomic capture directive does not help me get the same results as I do sequentially.

// points
std::vector<Eigen::Matrix3Xd> points(numberofPointSets);

// initialize arrays
for (auto& pointSetInformation: pointSetsInformation)
{
    int pointSetSize = getPointSetSize();
    int index = getIndexOfPointsSet();

    points[index].resize(3, pointSetSize);
}

// currentIndices to assign specific columns of the matrix in parallel
std::vector<int> currentIndices(numberofPointSets, -1);

// Assign arrays
#pragma omp parallel num_threads(numberOfThreads) shared(pointsArray, points, currentIndices) default(none)
{
    int index;
    int currentIndexInCurrentPointSet;
    double point[3];

    #pragma omp for schedule(static)
    for (int i = 0; i < input->GetNumberOfPoints(); ++i)
    {
        index = getIndexOfPointsSet();

        // update currentIndices (we start from -1, that's why this happens first)
        #pragma omp atomic capture
        currentIndexInCurrentPointSet = ++currentIndices[index];

        pointsArray->GetPoint(i, point);
        points[index](0, currentIndexInCurrentPointSet) = point[0];
        points[index](1, currentIndexInCurrentPointSet) = point[1];
        points[index](2, currentIndexInCurrentPointSet) = point[2];
    }
}

Am I missing something? Can the omp atomic capture directive update array cells, or only variables?

Thanks a lot in advance.

EDIT:

Is there a way to enforce the same order of operation as the i variable?

Answer 1

Is there a way to enforce the same order of operation as the i variable?

You try to use the OpenMP ordered clause

#pragma omp parallel num_threads(numberOfThreads) shared(pointsArray, points, currentIndices) default(none)
{
    int index;
    int currentIndexInCurrentPointSet;
    double point[3];

    #pragma omp for ordered schedule(static, 1)
    for (int i = 0; i < input->GetNumberOfPoints(); ++i)
    {
        #pragma omp ordered
        {
            index = getIndexOfPointsSet();
            currentIndexInCurrentPointSet = ++currentIndices[index];
        }
        pointsArray->GetPoint(i, point);
        points[index](0, currentIndexInCurrentPointSet) = point[0];
        points[index](1, currentIndexInCurrentPointSet) = point[1];
        points[index](2, currentIndexInCurrentPointSet) = point[2];
    }
}