title: subsetting elements of a list by elements of another list
body: <p>Let's assume I have a list of four data.frames containing some NA values:</p>
  
  <pre><code>my.list&lt;-replicate(4,data.frame())
  names(my.list)&lt;-paste0("Frame.Number", c(1:4))
  for (i in 1:4){
    my.list[[i]]&lt;-mapply(rnorm,10,c(1:4))
    my.list[[i]][i+1,3]&lt;-NA
    my.list[[i]][c(i,i*2),4]&lt;-NA
  }
  </code></pre>
  
  <p>For each of the data.frames, I want to select those rows which don't contain NAs in the 4th column. I can, for example, create a list of vectors (?) containing information about the completeliness of the cases in each data.frame:</p>
  
  <pre><code>selector &lt;- lapply(my.list,"[",, 4)
  selector &lt;- lapply(selector,complete.cases)
  </code></pre>
  
  <p>Now this is where I am stuck: how do I apply the <code>selector</code> list to the <code>my.list</code> list in order to select only the complete cases? I thought I could use <code>lapply</code> again, but I cannot figure out some meaningful syntax.</p>