title: Find specific value in nested data frames and get position and/or value
body: <p>I have a nested list <code>sampleList</code> that can contain a variable number of data frames. In this example there are 3 data frames:</p>
  <pre><code>df1 &lt;- data.frame(id = as.integer(c(1, 6)), key = c('apple', 'apple.green'), stringsAsFactors=FALSE)
  df2 &lt;- data.frame(id = as.integer(c(1, 3, 5)), key = c('apple', 'apple.red', 'apple.red.rotten'), stringsAsFactors = FALSE)
  df3 &lt;- data.frame(id = as.integer(c(17)), key = c('orange'), stringsAsFactors = FALSE)
  sampleList &lt;- list(df1, df2, df3)
  </code></pre>
  <p>I want to search for specific integers e.g. <code>6</code> in the <code>id</code> column across all data frames contained in the <code>sampleList</code>. As a result, I need the position and if possible the associated value from the <code>key</code> column.</p>
  <p>The closest I got was the position in a specific data frame e.g. <code>1</code>.</p>
  <pre><code>which(sampleList[[1]] == 6)
  [1] 2
  </code></pre>
  <p>Since the number of data frames can be different each time, I need a more dynamic query.</p>
  <p>Thanks a lot for your help.</p>