title: data.table: Using with=False and transforming function/summary function?
body: <p>I want to summarise several variables in data.table, output in wide format, output possibly as a list per variable. Since several other approaches did not work, I tried to do an outer lapply, giving the names of the variables as character vectors. I wanted to pass these in, using with=FALSE. </p>
  
  <pre><code>carsx=as.data.table(cars)
  lapply( list(speed="speed",dist= "dist"), #error object 'ansvals' not found
      function(x)  carsx[,list(mean(x), min(x), max(x) ), with=FALSE ] ) 
  </code></pre>
  
  <p>Since this does not work, I tried the more simple approach without lapply. </p>
  
  <pre><code>carsx[,list(mean("speed"), min("speed"), max("speed") ), with=FALSE ] #error object 'ansvals' not found
  </code></pre>
  
  <p>This does not work either. Is there any way to do something like this? Is this behaviour of 'with' wanted? (I am aware that <code>?data.table</code> mentions with only to select columns, but in my case it would be useful to be able to transform them as well)</p>
  
  <blockquote>
    <p>When with=FALSE, j is a vector of names or positions to select, similar to a data.frame. with=FALSE is often useful in data.table to select columns dynamically.</p>
  </blockquote>
  
  <p>EDIT 
  My aim is to get  a summary per group in wide format, for different variables. 
  I tried to extend the following, which works only for one variable, for a list of variables. </p>
  
  <pre><code>carsx[,list(mean(speed), min(speed), max(speed) ) ,by=(dist&gt;50)
  </code></pre>
  
  <p>Lamentably SO doesnt let me post my other question. There I described that I want an output similiar to: </p>
  
  <pre><code>lapply( list(speed="speed",dist= "dist"),
          function(x) do.call("as.data.frame", aggregate(cars[,x], list(class=cars$dist&gt;50), FUN=summary) ) )
  </code></pre>
  
  <p>Expected Output would be something like: </p>
  
  <pre><code>$speed 
           V1       V2 V3
  1: FALSE 12.96970  4 20
  2:  TRUE 20.11765 14 25
  
  $dist
           V1       V2 V3
  1: FALSE 12.96970  4 20
  2:  TRUE 20.11765 14 25
  </code></pre>