title: Convert matrix to three defined columns in R
body: <p>Given <code>m</code>:</p>
  
  <pre><code>m &lt;- structure(c(5, 1, 3, 2, 1, 4, 5, 2, 5, 1, 1, 5, 1, 4, 0, 4, 5, 
  5, 3, 2, 0, 0, 3, 0, 3, 2, 3, 0, 0, 0, 0, 0, 0, 0, 0), .Dim = c(7L, 
  5L))
  
       # [,1] [,2] [,3] [,4] [,5]
  # [1,]    5    2    0    0    0
  # [2,]    1    5    4    3    0
  # [3,]    3    1    5    0    0
  # [4,]    2    1    5    3    0
  # [5,]    1    5    3    2    0
  # [6,]    4    1    2    3    0
  # [7,]    5    4    0    0    0
  </code></pre>
  
  <p>Consider the element <code>1</code>, it appears in 5 rows <code>(2, 3, 4, ,5, 6)</code> and the respective column-wise indices are <code>(1, 2, 2, 1, 2)</code>. I would like to have the following:</p>
  
  <pre><code>1 2 1
  1 3 2
  1 4 2
  1 5 1
  1 6 2
  </code></pre>
  
  <p>As another example, consider the element <code>2</code>, it appears in 4 rows <code>(1, 4, 5, 6)</code> and the respective column-wise indices are <code>(2, 1, 4, 3)</code> and we have:</p>
  
  <pre><code>1 2 1
  1 3 2
  1 4 2
  1 5 1
  1 6 2
  2 1 2
  2 4 1
  2 5 4
  2 6 3
  </code></pre>
  
  <p>What I want is a <code>n*3</code> matrix for all 1-5. <strong>Preferably in base R</strong></p>