input: Write a function that takes a string as input and returns the total number of vowels in the string. If the input string is empty, the function should return 0.
  
  def count_vowels(string):
      vowels = "aeiou"
      count = 0
      for s in string:
          if s.lower() in vowels:
              count += 1
      return count
  
  assert count_vowels("Hello World")==3
  assert count_vowels("Python Programming")==4
  assert count_vowels("OpenAI")==4
  assert count_vowels("aEiOu")==5
  assert count_vowels("")==0
output: def count_vowels(string):
      vowels = "aeiou"
      count = 0
      for s in string:
          if s.lower() in vowels:
              count += 1
      return count