input: Write a function to extract all the words starting with a specific letter from a given string. The function should return a list of these words. If the input string is empty, return an empty list.
  
  def extract_words(word, letter):
      words_list = []
      for w in word.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', 'to']
  assert extract_words('Python is a programming language', 'p')==['Python']
  assert extract_words('', 'a')==[]
  assert extract_words('All letters in this string start with A', 'A')==['All', 'A']
  assert extract_words('No matches here', 'z')==[]
output: def extract_words(word, letter):
      words_list = []
      for w in word.split():
          if w[0] == letter:
              words_list.append(w)
      return words_list