Either read it in with ‘scan’, or just do as.vector() on the matrix. You might want to transpose the matrix first if you want it by rows or columns.
> m=matrix(1:12,3,4)
> m
[,1] [,2] [,3] [,4]
[1,] 1 4 7 10
[2,] 2 5 8 11
[3,] 3 6 9 12
> as.vector(m)
[1] 1 2 3 4 5 6 7 8 9 10 11 12
> as.vector(t(m))
[1] 1 4 7 10 2 5 8 11 3 6 9 12