input: Write a function to count the number of vowels in a string. If the string is empty, return 0.
  
  ### Input:
  def count_vowels(string):
      count = 0
      vowels = set("aeiouAEIOU")
      for letter in string:
          if letter in vowels:
              count += 1
      return count
  
  ### Output:
  assert count_vowels("Hello, World!") == 3
  assert count_vowels("AEIOUaeiou") == 10
  assert count_vowels("Python Programming") == 4
  assert count_vowels("12345") == 0
  assert count_vowels("") == 0
output: def count_vowels(string):
      count = 0
      vowels = set("aeiouAEIOU")
      for letter in string:
          if letter in vowels:
              count += 1
      return count