title: how to find the number of dictionaries with a particular value in an array of dictionaries?
body: <p>I have an array of dictionaries that look like</p>
  
  <pre><code>a = [
    {'name': 'a', 'age':10},
    {'name': 'b', 'age':10},
    {'name': 'c', 'age':15},
    {'name': 'd', 'age':10}
  ]
  </code></pre>
  
  <p>I want to find the number of items in this array that have "<code>age</code>" of <code>10</code>. I am looking for an one line solution with <code>len</code> and <code>set</code> (I want to avoid using for loop). Thanks.</p>
  
  <p>=================</p>
  
  <p>What if I want to find the number of unique "age"s?</p>