title: (re)initialise a vector to a certain length with initial values
body: <p>As a function argument I get a <code>vector&lt;double&gt;&amp; vec</code> (an output vector, hence non-const) with unknown length and values. I want to initialise this vector to a specific length n with all zeroes.</p>
  
  <p>This will work </p>
  
  <pre><code>vec.clear();
  vec.resize( n, 0.0 );
  </code></pre>
  
  <p>And this will work as well:</p>
  
  <pre><code>vec.resize( n );
  vec.assign( n, 0.0 );
  </code></pre>
  
  <p>Is the second more efficient (because no memory deallocation/allocation is involved)? Is there a more efficient way to do this? </p>