input: Write a Python function to convert a positive integer into its corresponding Roman numeral in lowercase. The integer should be between 1 and 1000.
  
  assert int_to_lowercase_roman(19) == 'xix'
  assert int_to_lowercase_roman(152) == 'clii'
  assert int_to_lowercase_roman(251) == 'ccli'
  assert int_to_lowercase_roman(426) == 'cdxxvi'
  assert int_to_lowercase_roman(500) == 'd'
  assert int_to_lowercase_roman(1) == 'i'
  assert int_to_lowercase_roman(4) == 'iv'
  assert int_to_lowercase_roman(43) == 'xliii'
  assert int_to_lowercase_roman(90) == 'xc'
  assert int_to_lowercase_roman(94) == 'xciv'
  assert int_to_lowercase_roman(532) == 'dxxxii'
  assert int_to_lowercase_roman(900) == 'cm'
  assert int_to_lowercase_roman(994) == 'cmxciv'
  assert int_to_lowercase_roman(1000) == 'm'
output: def int_to_lowercase_roman(num):
      val = [
          1000, 900, 500, 400,
          100, 90, 50, 40,
          10, 9, 5, 4,
          1
          ]
      syb = [
          "m", "cm", "d", "cd",
          "c", "xc", "l", "xl",
          "x", "ix", "v", "iv",
          "i"
          ]
      roman_num = ''
      i = 0
      while  num > 0:
          for _ in range(num // val[i]):
              roman_num += syb[i]
              num -= val[i]
          i += 1
      return roman_num