input: Write a function to accept a string as an input and return the total number of vowels in the string.
  
  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("AI is transforming the world")==10
  assert count_vowels("Today is a good day")==7
  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