input: Write a function to capitalize the first character of each word in a given string. If the string is empty, return the empty string.
  
  def capitalize_string(str):
      if not str:
          return ""
      words = 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("hello world")=="Hello World"
  assert capitalize_string("")==""
  assert capitalize_string("python programming")=="Python Programming"
  assert capitalize_string("a b c d e f")=="A B C D E F"
output: def capitalize_string(str):
      if not str:
          return ""
      words = str.split(' ')
      capitalized_words = [word.capitalize() for word in words]
      return " ".join(capitalized_words)