input: Write a function that takes a string and capitalizes the first character of each word. If the input string is empty, the function should return an empty string.
  
  def capitalize_string(input_str):
      if not input_str:
          return ""
      words = input_str.split(' ')
      capitalized_words = [word.capitalize() for word in words]
      return " ".join(capitalized_words)
  
  assert capitalize_string("this is an example") == "This Is An Example"
  assert capitalize_string("") == ""
  assert capitalize_string("hello world") == "Hello World"
  assert capitalize_string("PYTHON PROGRAMMING") == "Python Programming"
  assert capitalize_string("a b c d e f g h i j k") == "A B C D E F G H I J K"
output: def capitalize_string(input_str):
      if not input_str:
          return ""
      words = input_str.split(' ')
      capitalized_words = [word.capitalize() for word in words]
      return " ".join(capitalized_words)