title: How to ddply a nested list
body: <p>I have a nested list and need to calculate the correlation of frt and srt</p>
  
  <pre><code>   $`bs. bs`
      fapp  frt sapp  srt
  1   bs 2280   bs 0.25
  2   bs 2287   bs 0.25
  3   bs 2288   bs 0.25
  4   bs 2289   bs 0.25
  
  $`bs. lhc`
      fapp  frt sapp      srt
  5   bs 2320  lhc 0.250000
  6   bs 2333  lhc 0.250214
  7   bs 2524  lhc 0.316449
  </code></pre>
  
  <p>source:</p>
  
  <pre><code>    structure(list(`bs. bs` = structure(list(fapp = structure(c(1L,
      1L, 1L, 1L), .Label = "bs", class = "factor"), frt = c(2280L,
      2287L, 2288L, 2289L), sapp = structure(c(1L, 1L, 1L, 1L), .Label = c(" bs",
      " lhc"), class = "factor"), srt = c(0.25, 0.25, 0.25, 0.25)), .Names = c("fapp",
      "frt", "sapp", "srt"), row.names = c(NA, 4L), class = "data.frame"),
     `bs. lhc` = structure(list(fapp = structure(c(1L, 1L, 1L), .Label = "bs", class=         "factor"),
      frt = c(2320L, 2333L, 2524L), sapp = structure(c(2L,
      2L, 2L), .Label = c(" bs", " lhc"), class = "factor"),
      srt = c(0.25, 0.250214, 0.316449)), .Names = c("fapp",
  "frt", "sapp", "srt"), row.names = 5:7, class = "data.frame")), .Names = c("bs. bs",
     "bs. lhc"))
  </code></pre>
  
  <p>something like</p>
  
  <pre><code>    ddply(y,.(fapp + sapp),cor)
  </code></pre>
  
  <p>or</p>
  
  <pre><code>    ddply(y,.(fapp,sapp),cor)
  </code></pre>
  
  <p>doesn't work</p>