title: Matrix multiplication in R: requires numeric/complex matrix/vector arguments
body: <p>I'm using the dataset <code>BreastCancer</code> in the <code>mlbench</code> package, and I am trying to do the following matrix multiplication as a part of logistic regression.</p>
  
  <p>I got the features in the first 10 columns, and create a vector of parameters called theta:</p>
  
  <pre><code>X &lt;- BreastCancer[, 1:10]
  theta &lt;- data.frame(rep(1, 10))
  </code></pre>
  
  <p>Then I did the following matrix multiplication:</p>
  
  <pre><code>constant &lt;- as.matrix(X) %*% as.vector(theta[, 1])
  </code></pre>
  
  <p>However, I got the following error:</p>
  
  <pre><code>Error in as.matrix(X) %*% as.vector(theta[, 1]) : 
    requires numeric/complex matrix/vector arguments
  </code></pre>
  
  <p>Do I need to cast the matrix to double using <code>as.numeric(X)</code> first? Values in <code>X</code> look like strings as they have double quotes.</p>