title: How to divide between groups of rows using dplyr?
body: <p>I have this dataframe:</p>
  
  <pre><code>x &lt;- data.frame(
      name = rep(letters[1:4], each = 2),
      condition = rep(c("A", "B"), times = 4),
      value = c(2,10,4,20,8,40,20,100)
  ) 
  #   name condition value
  # 1    a         A     2
  # 2    a         B    10
  # 3    b         A     4
  # 4    b         B    20
  # 5    c         A     8
  # 6    c         B    40
  # 7    d         A    20
  # 8    d         B   100
  </code></pre>
  
  <p>I want to group by name and divide the value of rows with <code>condition == "B"</code> with those with <code>condition == "A"</code>, to get this:</p>
  
  <pre><code>data.frame(
      name = letters[1:4],
      value = c(5,5,5,5)
  )
  #   name value
  # 1    a     5
  # 2    b     5
  # 3    c     5
  # 4    d     5
  </code></pre>
  
  <p>I know something like this can get me pretty close: </p>
  
  <pre><code>x$value[which(x$condition == "B")]/x$value[which(x$condition == "A")]
  </code></pre>
  
  <p>but I was wondering if there was an easy way to do this with dplyr (My dataframe is a toy example and I got to it by chaining multiple <code>group_by</code> and <code>summarise</code> calls).</p>