title: Finding the number of matching letters in two different string at the same indices
body: <p>I am having trouble finish python code.</p>
  
  <p>overlap('','hello') → 0.</p>
  
  <p>I have managed to get the number back when the length of the strings match but if one of the strings has a smaller length than the other. I keep getting index out of range. Can someone help me finish this.</p>
  
  <pre><code>def overlap(string1,string2):
      count = 0
      for i in range(len(string1)):
          for j in range(len(string2)):
              if string1[i] == string2[j]:
                  count = count + 1
                  i+=1
              else:
                  i+=1
          return count
  </code></pre>
  
  <p>When running this with a function call. if both strings are equal it gives me the correct number, but if one is smaller or longer then its index out of range. 
  Thanks</p>