title: R: indexes to matrix
body: <p>I have a matrix call res initially like the following:</p>
  
  <pre><code>      [,1] [,2]
  [1,]     0    0
  [2,]     0    0
  [3,]     0    0
  [4,]     0    0
  [5,]     0    0
  [6,]     0    0
  [7,]     0    0
  [8,]     0    0
  [9,]     0    0
  [10,]    0    0 
  </code></pre>
  
  <p>I have a matrix of indexes (indexes) like the following:</p>
  
  <pre><code>     [,1] [,2]
   [1,]   2    3
   [2,]   7    9
  </code></pre>
  
  <p>I want the resulting res matrix to be like the following:</p>
  
  <pre><code>      [,1] [,2]
  [1,]     0    0
  [2,]     1    1
  [3,]     1    1
  [4,]     0    0
  [5,]     0    0
  [6,]     0    0
  [7,]     1    1
  [8,]     1    1
  [9,]     1    1
  [10,]    0    0 
  </code></pre>
  
  <p>I have a big matrix, it takes a long time to loop through the indexes matrix. Please let me know if there is a better approach to do this.  I am hoping to do something like mat[indexes,] &lt;- 1. However, this does not work I wanted.</p>