I encountered the above compile error for my first Eigen program. Really appreciate any help.

The related code is:
a += b * (v.matrix() * X).array();

where the definitions are:
Map<ArrayXf> a, b; // dim: m
ArrayXf v(n); // dim: n
SparseMatrix<float, RowMajor, int> X; // n * m

Should I transpose v?
M is huge, so I'd like to avoid temp object for v.matrix() * X. Can the above code achieve it?

yes you have to transpose v which is a column vector, and a temporary will be required. Nevertheless, your error " 'const Type' has no member named 'array'" seems odd to me. Perhaps an explicit evaluation will solve the issue if it's still exist after transposing:

a += b * (v.matrix() * X).eval().array();