input: Write a function to count the total number of vowels in a given string. If the string is empty, return 0.
  def count_vowels(string): 
      vowels = "aeiouAEIOU"
      count = 0
      for s in string:
          if s in vowels:
              count += 1
      return count
  assert count_vowels("Hello, World!")==3
  assert count_vowels("Python Programming")==4
  assert count_vowels("Swift")==1
  assert count_vowels("Javascript")==3
  assert count_vowels("")==0
output: def count_vowels(string): 
      vowels = "aeiouAEIOU"
      count = 0
      for s in string:
          if s in vowels:
              count += 1
      return count