How to create Eigen matrix out of 2 vectors [closed]

Question

I have 2 vectors and a matrix:

VectorXd A;
VectorXd B;
MatrixXd C; 

How should I efficiently (without explicit loops and working fast) compute matrix C values so that

C(i,k) = A(i) * B(k);

Assume that matrix C already has appropriate dimensions.

IMPORTANT: I only need help in using built-in Eigen syntax. Please no CUDA/MKL/BLAS suggestions. Thank you.

Answer 1

You are looking for an outer product which is just a standard matrix product:

C = A * B.transpose();

Since the destination c does not alias with the operand of the product you can save one temporary with:

C.noalias() = A * B.transpose();

noalias makes sense for matrix products only.