I am new in Eigen.

I want to delete a row and a col of a sparse matrix, what is the fastest way to do this?

Please help me, thank you!

I guess that you probably want to remove multiple ones, in that case the best is to build a permutation (PermutationMatrix) moving the unwanted rows/columns to the end, and then use topLeftCorner or conservativeResize to pick the wanted subpart.

If by "suppress" you mean "set to zero", then the prune() method should do the job.