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)
  
  assert capitalize_string("this is an example") == "This Is An Example"  
  assert capitalize_string("hello world") == "Hello World"  
  assert capitalize_string("a quick brown fox") == "A Quick Brown Fox"  
  assert capitalize_string("") == ""  
  assert capitalize_string("one") == "One"
output: def capitalize_string(str):
      words = str.split(' ')
      capitalized_words = [word.capitalize() for word in words]
      return " ".join(capitalized_words)