Crossprod of two different matrices by a factor/group

[faithQC]

I have two matrices, X is N by p and y is N by 1
In each of the rows of X and y, theres a group associated with it. For example, 1st 1:j-1 elements in both X and y belong to the first group, subsequent j-1 to N elements belong to another group.
Is there anyway I could apply crossprod at the group level (ie give me a p by 1 matrix/vector in this case according to group), something that rowsums is doing
I tried looking up tapply but it does not seem to take up multiply arguments.
Would appreciate any help thanks!

[williaml]

Hi, can you provide a reproducible example of what your dataset looks like and what you're after?

FAQ: How to do a minimal reproducible example ( reprex ) for beginners

A minimal reproducible example consists of the following items: A minimal dataset, necessary to reproduce the issue The minimal runnable code necessary to reproduce the issue, which can be run on the given dataset, and including the necessary information on the used packages. Let's quickly go over each one of these with examples: Minimal Dataset (Sample Data) You need to provide a data frame that is small enough to be (reasonably) pasted on a post, but big enough to reproduce your issue. Let's say, as an example, that you are working with the iris data frame head(iris) #> Sepal.Length Sepal.Width Petal.Length Petal.Width Species #> 1 5.1 3.5 1.4 0.…