title: R not returning sum as expected
body: <p>Why does the following code not return zero?</p>
  <pre><code>sum(c(-2.8, -1.8, -0.8, 1.2, 4.2))
  2.220446e-16
  </code></pre>
  <p>It returns an extremely small value, but the answer is clearly zero, not close to zero. Why does this happen and How to deal with this problem?</p>