title: How does `if` statement work in Scheme?
body: <p>This is the link that I'm current teaching myself Scheme, <a href="http://www.ccs.neu.edu/home/dorai/t-y-scheme/t-y-scheme-Z-H-1.html" rel="nofollow noreferrer">http://www.ccs.neu.edu/home/dorai/t-y-scheme/t-y-scheme-Z-H-1.html</a></p>
  
  <p>According to the author,
  <img src="https://i.stack.imgur.com/IbnRp.png" alt="enter image description here"></p>
  
  <p>Then I tried a minimal example</p>
  
  <pre><code>(define (check-p x)
    (if (&gt;= x 0)
        (display 1)))
  </code></pre>
  
  <p>, and DrScheme gave me errors: </p>
  
  <pre><code>if: bad syntax (must have an "else" expression) in: (if (&gt;= x 0) (display 1))
  </code></pre>
  
  <p>If I put an extra statement within the <code>if</code> statement, then it works. But I don't get it, why do we need an extra statement there? The code statement above made sense to me. If the number is greater than 0, display a 1, otherwise do nothing. Any idea?</p>
  
  <p>Thanks,</p>