input: Write a function to capitalize the first character of each word in a given string.
  
  
  def capitalize_string(str):
      words = str.split(' ')
      capitalized_words = [word.capitalize() for word in words]
      return " ".join(capitalized_words)
  
  # Example usage:
  assert capitalize_string("hello world") == "Hello World"
  assert capitalize_string("this is an example") == "This Is An Example"
  assert capitalize_string("a b c d e f g h i j k l m n o p q r s t u v w x y z") == "A B C D E F G H I J K L M N O P Q R S T U V W X Y Z"
  
output: def capitalize_string(str):
      words = str.split(' ')
      capitalized_words = [word.capitalize() for word in words]
      return " ".join(capitalized_words)