title: Concatenating Matrices in R
body: <p>How can I concatenate matrices of same columns but different number of rows? For example, I
  want to concatenate <code>a</code> (<code>dim(a) = 15 7000</code>) and <code>b</code> (<code>dim(b) = 16 7000</code>) and I want the result to be a matrix of <code>31</code> rows by <code>7000</code> columns.</p>
  <p>Can I also do this for matrices with a different number of rows <em>and</em> columns? Say I want to combine a matrix of 15 rows and 7000 columns with another of 16 rows and 7500 columns. Can I create one dataset with that?</p>