title: R- multiply columns that match a naming pattern
body: <p>I have multiple columns in a data frame that follow the same naming convention. Lets say I have:  </p>
  
  <pre><code>    a_1 a_2 b_1 b_2
  1    0    0   0   0
  2    0    0   0   0
  </code></pre>
  
  <p>I want to multiply the values in a_1 by the values in b_1, and the values in a_2 by the values in b_2, hypothetically a_3 * b_3, ....., and so on. </p>
  
  <p>What are some ways I can accomplish this and insert the information into a new column? </p>