[R] flatten a matrix and unflatten it

[Bill Simpson]

I want to flatten a matrix and unflatten it again. Please tell me how to 
do it.

1. given a matrix:
x1 y1 z1
x2 y2 z2
...
xk yk zk
convert it to a vector:
x1, y1, z1, x2, y2, z2, ..., xk, yk, zk

2. given a vector:
x1, y1, z1, x2, y2, z2, ..., xk, yk, zk
convert it to a matrix
x1 y1 z1
x2 y2 z2
...
xk yk zk

It is known that the number of dimensions is 3.

Thanks for any help!

Bill