title: matlab find specific VALUES in an array
body: <p>How to find out all array element indices equal to several values (>2)</p>
  
  <p>For example, I have an array a=<code>[1 2 3 4 5 5 4 3 2 2 2 1]</code>, I want to know the indices of all elements equal to b=<code>[2 5]</code></p>
  
  <p>Remember, I cannot use style like <code>a==b(1) | a==b(2)</code> because number of elements in b is arbitrary.</p>
  
  <p>Do I have to use a for loop to do this?</p>