title: Compare characters in a dictionary to a string, remove the dic item and return the modified dic as a string
body: <p>I have a function that takes a string argument then it is converted to a histogram dictionary. What the function is supposed to do is compare each key which is a character to a global variable containing all the letters from the alphabet. Return a new string with the alphabet minus the characters in the dictionary. how would I accomplish this in a function using a for loop and not using counter?</p>
  
  <pre><code>alphabet = 'abcdefghi'
  
  def histogram(s):
       d = dict()
       for c in s:
            if c not in d:
                 d[c] = 1
            else:
                 d[c] += 1
       return d
  
  def missing_characters(s):
      h = histogram(s)
      global alphabet
  
      for c in h.keys():
          if c in alphabet:
              del h[c]
  
  missing_characters("abc")
  </code></pre>
  
  <p>I get an error stating that the dictionary has changed. What I need to do is remove the given string characters from the dictionary histogram and return a new string with all the letters in order except for the ones in the string passed as an argument.</p>
  
  <p>thanks in advance. </p>