title: How to properly address covariates in functions with their names listed into strings?
body: <p>I have a long list of covariates that I want to store as a string in order to work with it more conveniently in several functions. </p>
  
  <p>I think the issue is related to <a href="https://stackoverflow.com/q/46615693/6574038">this question</a>, but I can't apply it to cases different to regression formulas.</p>
  
  <pre><code>xvar &lt;- c("Sepal.Length", "Sepal.Width", "Petal.Length", "Petal.Width")
  
  with(iris, iris[order(Sepal.Length, Sepal.Width, 
                        Petal.Length, Petal.Width), ])  # working
  
  with(iris, iris[order(xvar), ])  # not working
  </code></pre>
  
  <p>Or an other case</p>
  
  <pre><code>with(iris, ave(Sepal.Length, Sepal.Width, 
             Petal.Length, Petal.Width))  # working
  
  with(iris, ave(xvar))  # not working
  with(iris, ave(noquote(paste(xvar, collapse = ", "))))  # not working as well
  </code></pre>
  
  <p>Especially the paste-version I don't understand why it's not working, though it yields exactly the same string as the one in the parentheses:</p>
  
  <pre><code>&gt; noquote(paste(xvar, collapse = ", "))
  [1] Sepal.Length, Sepal.Width, Petal.Length, Petal.Width
  </code></pre>
  
  <p>What am I missing?</p>