input: Write a function to extract all the words starting with a specific letter from a given string. If the given string is empty or does not contain any words starting with the specified letter, return an empty list.
  def extract_words(words, letter):
      words_list = []
      for w in words.split():
          if w[0] == letter:
              words_list.append(w)
      return words_list
  assert extract_words('This is a sample sentence to test the program', 's')==['sample', 'sentence', 'test', 'the', 's']
  assert extract_words('Another example sentence with no specific letter', 's')==[]
  assert extract_words('', 'a')==[]
  assert extract_words('All words start with A but there is no S', 's')==[]
  assert extract_words('Simple test with one word starting with S', 's')==['Simple', 'starting', 'with']
output: def extract_words(words, letter):
      words_list = []
      for w in words.split():
          if w[0] == letter:
              words_list.append(w)
      return words_list