title: Varying Symbols of sign programmatically
body: <p>I have a vector of numbers:</p>
  <pre><code>v1 &lt;- c(1,2,3)
  </code></pre>
  <p>and I want to programmatically analyze the impact of sign change whose
  variants could be:</p>
  <pre><code>v1[1] + v1[2] + v1[3] 
  [1] 6
  v1[1] + v1[2] - v1[3] 
  [1] 0
  v1[1] - v1[2] - v1[3] 
  [1] -4
  v1[1] - v1[2] + v1[3] 
  [1] 2
  </code></pre>
  <p>How can I exchange signs ('+', '-') programatically? I'm thinking this is a silly question, but can't think my way out of my box, though my line of analysis points to evaluating changing signs.</p>