title: correlation between columns by group
body: <p>How do I calculate correlations between one column and all other columns in a data frame in R without using column names?
  I tried to use ddply and it works if I use just two column names i.e.</p>
  
  <pre><code>library(plyr)
  ddply(iris, ~Species, summarize, cormat=cor(Sepal.Length,Petal.Width)) 
  </code></pre>
  
  <p>But how to get correlations of column 1 with all other columns, broken down by Species, without using the column names?</p>