title: Percent operator, string interpolation
body: <p>Here's some code from Zed Shaw book :</p>
  
  <pre><code>formatter = "%{first} %{second} %{third} %{fourth}"
  
  puts formatter % {first: 1, second: 2, third: 3, fourth: 4}
  puts formatter % {first: "one", second: "two", third: "three", fourth: "four"}
  puts formatter % {first: true, second: false, third: true, fourth: false}
  puts formatter % {first: formatter, second: formatter, third: formatter, fourth: formatter}
  
  puts formatter % {
    first: "I had this thing.",
    second: "That you could type up right.",
    third: "But it didn't sing.",
    fourth: "So I said goodnight."
  }
  </code></pre>
  
  <p>I understand that <code>%{}</code> is <code>%Q{}</code> by default, and that is string interpolation. But what is the meaning of <code>%{}</code> inside double quote?</p>
  
  <pre><code>"%{first} %{second} %{third} %{fourth}"
  </code></pre>
  
  <p>And what is the meaning of this line?</p>
  
  <pre><code>puts formatter % {first: 1, second: 2, third: 3, fourth: 4}
  </code></pre>