title: how to count the lower triangle elements in a matrix
body: <p>Suppose I have a matrix. Suppose I have a list of a lower triangular matrix. How can I count the number of the elements in the matrices?</p>
  
  <p>For example, </p>
  
  <pre><code>Matrix &lt;- c(0, 4, 3, 1, 2,
              0, 0, 3, 2, 1,
              0, 0, 0, 2, 1,
              0, 0, 0, 0, 1,
              0, 0, 0, 0, 0)
  Matrix &lt;- matrix(Matrix, 5, 5)
  
  &gt; Matrix
       [,1] [,2] [,3] [,4] [,5]
  [1,]    0    0    0    0    0
  [2,]    4    0    0    0    0
  [3,]    3    3    0    0    0
  [4,]    1    2    2    0    0
  [5,]    2    1    1    1    0
  </code></pre>
  
  <p>How to count the number of the elements in this matrix? 
  This matrix contains 10 elements. How to count it in R? </p>