title: How to check existence of an input argument for R functions
body: <p>I have a function defined as</p>
  
  <pre><code>myFun &lt;- function(x, y, ...) {
    # using exists
    if (exists("z")) { print("exists z!") }
    # using missing
    try(if (!missing("z")) { print("z is not missing!") }, silent = TRUE)
    # using get
    try(if (get("z")) { print("get z!") }, silent = TRUE)
  
    # anotherFun(...)
  }
  </code></pre>
  
  <p>In this function, I want to check whether user input "z" in the argument list. How can I do that? I tried <code>exists("z")</code>, <code>missing("z")</code>, and <code>get("z")</code> and none of them works.</p>