title: How do I get an item from a list at a given index in racket language?
body: <p>I'm trying to get an item from a list at a given index for a loop statement.</p>
  
  <pre><code>(define decision-tree-learning
    (lambda (examples attribs default)
      (cond
        [(empty? examples) default]
        [(same-classification? examples) (caar examples)] ; returns the classification
        [else (lambda () 
                (let ((best (choose-attribute attributes examples))
                      (tree (make-tree best))
                      (m (majority-value examples))
                      (i 0)
                      (countdown (length best)) ; starts at lengths and will decrease by 1
                    (let loop()
                      (let example-sub ; here, totally stuck now
                        ; more stuff
                        (set! countdown (- countdown 1))
                        ; more stuff
                        )))))])))
  </code></pre>
  
  <p>In this case, <code>best</code> is the list and I need to get its value at the <code>countdown</code> index. Could you help me on that?</p>