title: How to use the list feature in ffload to selectively load an ffdf?
body: <p>How do I use the <code>list</code> feature in <code>ffload</code>properly to selectively load a part of an <code>ffdf</code>? </p>
  
  <p>I have read numerous times that it is possible to selectively load parts of an ff dataframe, but can't seem to get it working myself.</p>
  
  <p>My issue is that I'm trying to perform a merge with a vector that has a list of unique IDs but the ffdf file is too large for my computer to handle this merge.</p>
  
  <p>So far I have been trying to use <code>ffinfo</code> to get a list of the different parts making up my <code>ffdf</code>.</p>
  
  <p>For instance, I get the following list of "file parts" for the ffdf in question:</p>
  
  <pre><code>ffinfo(file=paste(imp_tables_root_loc,table_name,sep=""))
  </code></pre>
  
  <p>where <code>paste(imp_tables_root_loc,table_name,sep="")</code> specifies the ffdf table and its location</p>
  
  <pre><code>[1] "ffdf1590409e251b.ff" "ffdf159047426082.ff" "ffdf159058a426ab.ff" "ffdf1590410708c.ff"  "ffdf159022b49bf.ff" 
   [6] "ffdf1590174e5dec.ff" "ffdf159045d93226.ff" "ffdf159037f87280.ff" "ffdf159044ad3d39.ff" "ffdf15905224601a.ff"
  [11] "ffdf159027936205.ff" "ffdf1590133841c8.ff" "ffdf15902d365cac.ff" "ffdf159065b4259.ff"  "ffdf15904a162908.ff"
  [16] "ffdf15905529c1c.ff"  "ffdf1590eda1092.ff"  "ffdf1590ab65eb.ff"   "ffdf159048e74f82.ff"
  </code></pre>
  
  <p>I then try to ffload by calling:</p>
  
  <pre><code>ffload(file=paste(imp_tables_root_loc,table_name,sep=""),list="ffdf1590409e251b.ff")
  </code></pre>
  
  <p>But this results in this error:</p>
  
  <blockquote>
    <p>Error in ffload(file = paste(imp_tables_root_loc, table_name, sep =
    ""),  :    not in ffarchive: " ffdf1590409e251b.ff",</p>
  </blockquote>