Eigen :: SparseMatrix <double>的子集

Question

给出

for (timeInWord, timeInNumber) in timeComponents{
    let timeInWordLabel = UILabel()
    timeInWordLabel.text = timeInNumber
    scrollViewElement.addSubview(timeInWordLabel)
    if timeInWord == "twelveAM" {
        timeInWordLabel.edgeTo(scrollViewElement, safeArea: .twelveAm)
    }
    else {
        timeInWordLabel.edgeTo(scrollViewElement, safeArea: .followingHour)
    }
}

是否存在使用向量r和c提取稀疏矩阵B的有效方法？即

SparseMatrix<double> A;
VectorXi r, c;

http://eigen.tuxfamily.org/dox/TopicCustomizing_NullaryExpr.html#title1此处提供的代码似乎不适用于稀疏矩阵？