title: Create a new vector formed by a list of vectors
body: <p>Suppose I have m vectors: <code>a_1 = (a_{11}...a_{1n}) ... a_m = (a_{m1}...a_{mn})</code>
  I want a new vector b of length mn such that<br>
  <code>b = (a_{11}...a_{m1} a_{12}...a_{m2}...a_{1n}...a_{mn})</code>
  I can think of a for loop, for example:</p>
  
  <pre><code>&gt;a&lt;-c(1,1,1);b&lt;-c(2,2,2);c&lt;-c(3,3,3)
  &gt;x&lt;-NULL
  &gt;for (i in 1:3) {x&lt;-c(x,c(a[i],b[i],c[i]))}
  &gt;x
  [1] 1 2 3 1 2 3 1 2 3
  </code></pre>
  
  <p>Is there a better way?</p>