Converting a numeric matrix into a data.table (or data.frame)

Question

Hoping there\'s a simple answer here but I can\'t find it anywhere.

I have a numeric matrix with labelled rows and columns:

     1    2    3    4
a    6          


        

Answer 1

Assuming 'm' is your matrix...

data.frame(col = rep(colnames(m), each = nrow(m)), 
           row = rep(rownames(m), ncol(m)), 
           value = as.vector(m))

This executes extremely fast on a large matrix and also shows you a bit about how a matrix is made, how to access things in it, and how to construct your own vectors.