title: Accessing column names in a data frame
body: <p>I have a data frame with column names z_1, z_2 upto z_200. In the following example, for ease of representation, I am showing only z_1</p>
  
  <pre><code>df &lt;- data.frame(x=1:5, y=2:6, z_1=3:7, u=4:8) 
  df
  i=1
  tmp &lt;- paste("z",i,sep="_")
  subset(df, select=-c(tmp))
  </code></pre>
  
  <p>The above code will be used in a loop i for accessing certain elements that need to be removed from the data frame</p>
  
  <p>While executing the above code, I get the error "Error in -c(tmp) : invalid argument to unary operator"</p>
  
  <p>Thank you for your help</p>