input: Write a function to extract all the words starting with a specific letter from a given string. If the string is empty then return an empty list.
  
  def extract_words(sentence, letter):
      words_list = []
      if sentence == '':
          return words_list
      for word in sentence.split():
          if word[0] == letter:
              words_list.append(word)
      return words_list
  
  assert extract_words('This is a sample sentence to test the program', 's')==['sample', 'sentence']
  assert extract_words('This is a test', 't')==['test']
  assert extract_words('Hello world', 'w')==['world']
  assert extract_words('This is a test', 'z')==[]
  assert extract_words('', 'a')==[]
output: def extract_words(sentence, letter):
      words_list = []
      if sentence == '':
          return words_list
      for word in sentence.split():
          if word[0] == letter:
              words_list.append(word)
      return words_list