title: Apply a vector of formulas to variables in dataframe
body: <p>I have hundreds of formulas stored in a vector, and need apply those formulas to tree variables (called dbh and ht) in a dataframe. The situation can be reproduced as follows:</p>
  <pre><code>vec_formulas = c('0.32*dbh^1.22*ht^0.99',
                   '0.44 + 3.55*log(dbh) + log(ht)^1.03',
                   '0.40*dbh^1.30 + ht*1.12')  # wrote only 3, but there are many
  
  set.seed(123)
  df_trees = data.frame(tree = 1:200,
                  dbh = rnorm(200, mean=30),
                  ht = rnorm(200, mean=15))
  </code></pre>
  <p>I need some result that apply every formula to every tree.
  If necessary, I can transform 'vec_formulas' into dataframe or datatable.</p>
  <p>Thanks in advance!</p>
  <p>-H.</p>